mirror of
https://github.com/Zenithsiz/ftmemsim-valgrind.git
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3a5719678f
The number of sets, ie. number of cache lines divided by associativity,
and the cache line size still have to be powers of two.
This change is needed for default cache parameters used on some Intel
Core 2 and Atom processors.
Includes cachegrind manual update and explicit tests with 24KB D1/3MB L2
Reverts addition of 6MB warning to {cachegrind,callgrind}/tests/filter_stderr
Backporting to VALGRIND_3_4_BRANCH needs r8912
git-svn-id: svn://svn.valgrind.org/valgrind/trunk@9080
1350 lines
53 KiB
XML
1350 lines
53 KiB
XML
<?xml version="1.0"?> <!-- -*- sgml -*- -->
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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
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[ <!ENTITY % vg-entities SYSTEM "../../docs/xml/vg-entities.xml"> %vg-entities; ]>
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<chapter id="cg-manual" xreflabel="Cachegrind: a cache-miss profiler">
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<title>Cachegrind: a cache and branch profiler</title>
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<sect1 id="cg-manual.cache" xreflabel="Cache profiling">
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<title>Cache and branch profiling</title>
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<para>To use this tool, you must specify
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<computeroutput>--tool=cachegrind</computeroutput> on the
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Valgrind command line.</para>
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<para>Cachegrind is a tool for finding places where programs
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interact badly with typical modern superscalar processors
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and run slowly as a result.
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In particular, it will do a cache simulation of your program,
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and optionally a branch-predictor simulation, and can
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then annotate your source line-by-line with the number of cache
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misses and branch mispredictions. The following statistics are
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collected:</para>
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<itemizedlist>
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<listitem>
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<para>L1 instruction cache reads and misses;</para>
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</listitem>
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<listitem>
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<para>L1 data cache reads and read misses, writes and write
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misses;</para>
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</listitem>
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<listitem>
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<para>L2 unified cache reads and read misses, writes and
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writes misses.</para>
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</listitem>
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<listitem>
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<para>Conditional branches and mispredicted conditional branches.</para>
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</listitem>
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<listitem>
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<para>Indirect branches and mispredicted indirect branches. An
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indirect branch is a jump or call to a destination only known at
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run time.</para>
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</listitem>
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</itemizedlist>
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<para>On a modern machine, an L1 miss will typically cost
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around 10 cycles, an L2 miss can cost as much as 200
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cycles, and a mispredicted branch costs in the region of 10
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to 30 cycles. Detailed cache and branch profiling can be very useful
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for improving the performance of your program.</para>
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<para>Also, since one instruction cache read is performed per
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instruction executed, you can find out how many instructions are
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executed per line, which can be useful for traditional profiling
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and test coverage.</para>
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<para>Branch profiling is not enabled by default. To use it, you must
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additionally specify <computeroutput>--branch-sim=yes</computeroutput>
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on the command line.</para>
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<sect2 id="cg-manual.overview" xreflabel="Overview">
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<title>Overview</title>
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<para>First off, as for normal Valgrind use, you probably want to
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compile with debugging info (the
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<computeroutput>-g</computeroutput> flag). But by contrast with
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normal Valgrind use, you probably <command>do</command> want to turn
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optimisation on, since you should profile your program as it will
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be normally run.</para>
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<para>The two steps are:</para>
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<orderedlist>
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<listitem>
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<para>Run your program with <computeroutput>valgrind
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--tool=cachegrind</computeroutput> in front of the normal
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command line invocation. When the program finishes,
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Cachegrind will print summary cache statistics. It also
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collects line-by-line information in a file
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<computeroutput>cachegrind.out.<pid></computeroutput>, where
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<computeroutput><pid></computeroutput> is the program's process
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ID.</para>
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<para>Branch prediction statistics are not collected by default.
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To do so, add the flag
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<computeroutput>--branch-sim=yes</computeroutput>.
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</para>
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<para>This step should be done every time you want to collect
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information about a new program, a changed program, or about
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the same program with different input.</para>
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</listitem>
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<listitem>
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<para>Generate a function-by-function summary, and possibly
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annotate source files, using the supplied
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cg_annotate program. Source
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files to annotate can be specified manually, or manually on
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the command line, or "interesting" source files can be
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annotated automatically with the
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<computeroutput>--auto=yes</computeroutput> option. You can
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annotate C/C++ files or assembly language files equally
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easily.</para>
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<para>This step can be performed as many times as you like
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for each Step 2. You may want to do multiple annotations
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showing different information each time.</para>
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</listitem>
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</orderedlist>
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<para>As an optional intermediate step, you can use the supplied
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cg_merge program to sum together the
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outputs of multiple Cachegrind runs, into a single file which you then
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use as the input for cg_annotate.</para>
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<para>These steps are described in detail in the following
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sections.</para>
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</sect2>
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<sect2 id="cache-sim" xreflabel="Cache simulation specifics">
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<title>Cache simulation specifics</title>
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<para>Cachegrind simulates a machine with independent
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first level instruction and data caches (I1 and D1), backed by a
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unified second level cache (L2). This configuration is used by almost
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all modern machines. Some old Cyrix CPUs had a unified I and D L1
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cache, but they are ancient history now.</para>
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<para>Specific characteristics of the simulation are as
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follows:</para>
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<itemizedlist>
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<listitem>
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<para>Write-allocate: when a write miss occurs, the block
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written to is brought into the D1 cache. Most modern caches
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have this property.</para>
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</listitem>
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<listitem>
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<para>Bit-selection hash function: the set of line(s) in the cache
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to which a memory block maps is chosen by the middle bits
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M--(M+N-1) of the byte address, where:</para>
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<itemizedlist>
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<listitem>
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<para>line size = 2^M bytes</para>
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</listitem>
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<listitem>
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<para>(cache size / line size / associativity) = 2^N bytes</para>
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</listitem>
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</itemizedlist>
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</listitem>
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<listitem>
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<para>Inclusive L2 cache: the L2 cache typically replicates all
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the entries of the L1 caches, because fetching into L1 involves
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fetching into L2 first (this does not guarantee strict inclusiveness,
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as lines evicted from L2 still could reside in L1). This is
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standard on Pentium chips, but AMD Opterons, Athlons and Durons
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use an exclusive L2 cache that only holds
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blocks evicted from L1. Ditto most modern VIA CPUs.</para>
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</listitem>
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</itemizedlist>
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<para>The cache configuration simulated (cache size,
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associativity and line size) is determined automagically using
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the CPUID instruction. If you have an old machine that (a)
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doesn't support the CPUID instruction, or (b) supports it in an
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early incarnation that doesn't give any cache information, then
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Cachegrind will fall back to using a default configuration (that
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of a model 3/4 Athlon). Cachegrind will tell you if this
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happens. You can manually specify one, two or all three levels
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(I1/D1/L2) of the cache from the command line using the
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<computeroutput>--I1</computeroutput>,
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<computeroutput>--D1</computeroutput> and
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<computeroutput>--L2</computeroutput> options.
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For cache parameters to be valid for simulation, the number
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of sets (with associativity being the number of cache lines in
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each set) has to be a power of two.</para>
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<para>On PowerPC platforms
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Cachegrind cannot automatically
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determine the cache configuration, so you will
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need to specify it with the
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<computeroutput>--I1</computeroutput>,
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<computeroutput>--D1</computeroutput> and
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<computeroutput>--L2</computeroutput> options.</para>
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<para>Other noteworthy behaviour:</para>
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<itemizedlist>
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<listitem>
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<para>References that straddle two cache lines are treated as
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follows:</para>
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<itemizedlist>
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<listitem>
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<para>If both blocks hit --> counted as one hit</para>
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</listitem>
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<listitem>
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<para>If one block hits, the other misses --> counted
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as one miss.</para>
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</listitem>
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<listitem>
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<para>If both blocks miss --> counted as one miss (not
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two)</para>
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</listitem>
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</itemizedlist>
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</listitem>
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<listitem>
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<para>Instructions that modify a memory location
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(eg. <computeroutput>inc</computeroutput> and
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<computeroutput>dec</computeroutput>) are counted as doing
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just a read, ie. a single data reference. This may seem
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strange, but since the write can never cause a miss (the read
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guarantees the block is in the cache) it's not very
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interesting.</para>
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<para>Thus it measures not the number of times the data cache
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is accessed, but the number of times a data cache miss could
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occur.</para>
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</listitem>
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</itemizedlist>
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<para>If you are interested in simulating a cache with different
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properties, it is not particularly hard to write your own cache
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simulator, or to modify the existing ones in
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<computeroutput>cg_sim.c</computeroutput>. We'd be
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interested to hear from anyone who does.</para>
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</sect2>
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<sect2 id="branch-sim" xreflabel="Branch simulation specifics">
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<title>Branch simulation specifics</title>
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<para>Cachegrind simulates branch predictors intended to be
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typical of mainstream desktop/server processors of around 2004.</para>
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<para>Conditional branches are predicted using an array of 16384 2-bit
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saturating counters. The array index used for a branch instruction is
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computed partly from the low-order bits of the branch instruction's
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address and partly using the taken/not-taken behaviour of the last few
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conditional branches. As a result the predictions for any specific
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branch depend both on its own history and the behaviour of previous
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branches. This is a standard technique for improving prediction
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accuracy.</para>
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<para>For indirect branches (that is, jumps to unknown destinations)
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Cachegrind uses a simple branch target address predictor. Targets are
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predicted using an array of 512 entries indexed by the low order 9
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bits of the branch instruction's address. Each branch is predicted to
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jump to the same address it did last time. Any other behaviour causes
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a mispredict.</para>
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<para>More recent processors have better branch predictors, in
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particular better indirect branch predictors. Cachegrind's predictor
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design is deliberately conservative so as to be representative of the
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large installed base of processors which pre-date widespread
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deployment of more sophisticated indirect branch predictors. In
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particular, late model Pentium 4s (Prescott), Pentium M, Core and Core
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2 have more sophisticated indirect branch predictors than modelled by
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Cachegrind. </para>
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<para>Cachegrind does not simulate a return stack predictor. It
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assumes that processors perfectly predict function return addresses,
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an assumption which is probably close to being true.</para>
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<para>See Hennessy and Patterson's classic text "Computer
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Architecture: A Quantitative Approach", 4th edition (2007), Section
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2.3 (pages 80-89) for background on modern branch predictors.</para>
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</sect2>
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</sect1>
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<sect1 id="cg-manual.profile" xreflabel="Profiling programs">
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<title>Profiling programs</title>
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<para>To gather cache profiling information about the program
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<computeroutput>ls -l</computeroutput>, invoke Cachegrind like
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this:</para>
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<programlisting><![CDATA[
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valgrind --tool=cachegrind ls -l]]></programlisting>
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<para>The program will execute (slowly). Upon completion,
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summary statistics that look like this will be printed:</para>
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<programlisting><![CDATA[
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==31751== I refs: 27,742,716
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==31751== I1 misses: 276
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==31751== L2 misses: 275
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==31751== I1 miss rate: 0.0%
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==31751== L2i miss rate: 0.0%
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==31751==
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==31751== D refs: 15,430,290 (10,955,517 rd + 4,474,773 wr)
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==31751== D1 misses: 41,185 ( 21,905 rd + 19,280 wr)
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==31751== L2 misses: 23,085 ( 3,987 rd + 19,098 wr)
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==31751== D1 miss rate: 0.2% ( 0.1% + 0.4%)
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==31751== L2d miss rate: 0.1% ( 0.0% + 0.4%)
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==31751==
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==31751== L2 misses: 23,360 ( 4,262 rd + 19,098 wr)
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==31751== L2 miss rate: 0.0% ( 0.0% + 0.4%)]]></programlisting>
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<para>Cache accesses for instruction fetches are summarised
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first, giving the number of fetches made (this is the number of
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instructions executed, which can be useful to know in its own
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right), the number of I1 misses, and the number of L2 instruction
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(<computeroutput>L2i</computeroutput>) misses.</para>
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<para>Cache accesses for data follow. The information is similar
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to that of the instruction fetches, except that the values are
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also shown split between reads and writes (note each row's
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<computeroutput>rd</computeroutput> and
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<computeroutput>wr</computeroutput> values add up to the row's
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total).</para>
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<para>Combined instruction and data figures for the L2 cache
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follow that.</para>
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<sect2 id="cg-manual.outputfile" xreflabel="Output file">
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<title>Output file</title>
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<para>As well as printing summary information, Cachegrind also
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writes line-by-line cache profiling information to a user-specified
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file. By default this file is named
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<computeroutput>cachegrind.out.<pid></computeroutput>. This file
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is human-readable, but is intended to be interpreted by the accompanying
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program cg_annotate, described in the next section.</para>
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<para>Things to note about the
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<computeroutput>cachegrind.out.<pid></computeroutput>
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file:</para>
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<itemizedlist>
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<listitem>
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<para>It is written every time Cachegrind is run, and will
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overwrite any existing
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<computeroutput>cachegrind.out.<pid></computeroutput>
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in the current directory (but that won't happen very often
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because it takes some time for process ids to be
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recycled).</para>
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</listitem>
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<listitem>
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<para>To use an output file name other than the default
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<computeroutput>cachegrind.out</computeroutput>,
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use the <computeroutput>--cachegrind-out-file</computeroutput>
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switch.</para>
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</listitem>
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<listitem>
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<para>It can be big: <computeroutput>ls -l</computeroutput>
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generates a file of about 350KB. Browsing a few files and
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web pages with a Konqueror built with full debugging
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information generates a file of around 15 MB.</para>
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</listitem>
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</itemizedlist>
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<para>The default <computeroutput>.<pid></computeroutput> suffix
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on the output file name serves two purposes. Firstly, it means you
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don't have to rename old log files that you don't want to overwrite.
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Secondly, and more importantly, it allows correct profiling with the
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<computeroutput>--trace-children=yes</computeroutput> option of
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programs that spawn child processes.</para>
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</sect2>
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<sect2 id="cg-manual.cgopts" xreflabel="Cachegrind options">
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<title>Cachegrind options</title>
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<!-- start of xi:include in the manpage -->
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<para id="cg.opts.para">Using command line options, you can
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manually specify the I1/D1/L2 cache
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configuration to simulate. For each cache, you can specify the
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size, associativity and line size. The size and line size
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are measured in bytes. The three items
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must be comma-separated, but with no spaces, eg:
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<literallayout> valgrind --tool=cachegrind --I1=65535,2,64</literallayout>
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You can specify one, two or three of the I1/D1/L2 caches. Any level not
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manually specified will be simulated using the configuration found in
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the normal way (via the CPUID instruction for automagic cache
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configuration, or failing that, via defaults).</para>
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<para>Cache-simulation specific options are:</para>
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<variablelist id="cg.opts.list">
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<varlistentry id="opt.I1" xreflabel="--I1">
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<term>
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<option><![CDATA[--I1=<size>,<associativity>,<line size> ]]></option>
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</term>
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<listitem>
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<para>Specify the size, associativity and line size of the level 1
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instruction cache. </para>
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</listitem>
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</varlistentry>
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<varlistentry id="opt.D1" xreflabel="--D1">
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<term>
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<option><![CDATA[--D1=<size>,<associativity>,<line size> ]]></option>
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</term>
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<listitem>
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<para>Specify the size, associativity and line size of the level 1
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data cache.</para>
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</listitem>
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</varlistentry>
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<varlistentry id="opt.L2" xreflabel="--L2">
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<term>
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<option><![CDATA[--L2=<size>,<associativity>,<line size> ]]></option>
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</term>
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<listitem>
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<para>Specify the size, associativity and line size of the level 2
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cache.</para>
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</listitem>
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</varlistentry>
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<varlistentry id="opt.cachegrind-out-file" xreflabel="--cachegrind-out-file">
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<term>
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<option><![CDATA[--cachegrind-out-file=<file> ]]></option>
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</term>
|
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<listitem>
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<para>Write the profile data to
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<computeroutput>file</computeroutput> rather than to the default
|
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output file,
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|
<computeroutput>cachegrind.out.<pid></computeroutput>. The
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|
<option>%p</option> and <option>%q</option> format specifiers
|
|
can be used to embed the process ID and/or the contents of an
|
|
environment variable in the name, as is the case for the core
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|
option <option>--log-file</option>. See <link
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|
linkend="manual-core.basicopts">here</link> for details.
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|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry id="opt.cache-sim" xreflabel="--cache-sim">
|
|
<term>
|
|
<option><![CDATA[--cache-sim=no|yes [yes] ]]></option>
|
|
</term>
|
|
<listitem>
|
|
<para>Enables or disables collection of cache access and miss
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counts.</para>
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|
</listitem>
|
|
</varlistentry>
|
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|
|
<varlistentry id="opt.branch-sim" xreflabel="--branch-sim">
|
|
<term>
|
|
<option><![CDATA[--branch-sim=no|yes [no] ]]></option>
|
|
</term>
|
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<listitem>
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|
<para>Enables or disables collection of branch instruction and
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misprediction counts. By default this is disabled as it
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|
slows Cachegrind down by approximately 25%. Note that you
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|
cannot specify <computeroutput>--cache-sim=no</computeroutput>
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|
and <computeroutput>--branch-sim=no</computeroutput>
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|
together, as that would leave Cachegrind with no
|
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information to collect.</para>
|
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</listitem>
|
|
</varlistentry>
|
|
|
|
</variablelist>
|
|
<!-- end of xi:include in the manpage -->
|
|
|
|
</sect2>
|
|
|
|
|
|
|
|
<sect2 id="cg-manual.annotate" xreflabel="Annotating C/C++ programs">
|
|
<title>Annotating C/C++ programs</title>
|
|
|
|
<para>Before using cg_annotate,
|
|
it is worth widening your window to be at least 120-characters
|
|
wide if possible, as the output lines can be quite long.</para>
|
|
|
|
<para>To get a function-by-function summary, run <computeroutput>cg_annotate
|
|
<filename></computeroutput> on a Cachegrind output file.</para>
|
|
|
|
<para>The output looks like this:</para>
|
|
|
|
<programlisting><![CDATA[
|
|
--------------------------------------------------------------------------------
|
|
I1 cache: 65536 B, 64 B, 2-way associative
|
|
D1 cache: 65536 B, 64 B, 2-way associative
|
|
L2 cache: 262144 B, 64 B, 8-way associative
|
|
Command: concord vg_to_ucode.c
|
|
Events recorded: Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
|
|
Events shown: Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
|
|
Event sort order: Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
|
|
Threshold: 99%
|
|
Chosen for annotation:
|
|
Auto-annotation: on
|
|
|
|
--------------------------------------------------------------------------------
|
|
Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
|
|
--------------------------------------------------------------------------------
|
|
27,742,716 276 275 10,955,517 21,905 3,987 4,474,773 19,280 19,098 PROGRAM TOTALS
|
|
|
|
--------------------------------------------------------------------------------
|
|
Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw file:function
|
|
--------------------------------------------------------------------------------
|
|
8,821,482 5 5 2,242,702 1,621 73 1,794,230 0 0 getc.c:_IO_getc
|
|
5,222,023 4 4 2,276,334 16 12 875,959 1 1 concord.c:get_word
|
|
2,649,248 2 2 1,344,810 7,326 1,385 . . . vg_main.c:strcmp
|
|
2,521,927 2 2 591,215 0 0 179,398 0 0 concord.c:hash
|
|
2,242,740 2 2 1,046,612 568 22 448,548 0 0 ctype.c:tolower
|
|
1,496,937 4 4 630,874 9,000 1,400 279,388 0 0 concord.c:insert
|
|
897,991 51 51 897,831 95 30 62 1 1 ???:???
|
|
598,068 1 1 299,034 0 0 149,517 0 0 ../sysdeps/generic/lockfile.c:__flockfile
|
|
598,068 0 0 299,034 0 0 149,517 0 0 ../sysdeps/generic/lockfile.c:__funlockfile
|
|
598,024 4 4 213,580 35 16 149,506 0 0 vg_clientmalloc.c:malloc
|
|
446,587 1 1 215,973 2,167 430 129,948 14,057 13,957 concord.c:add_existing
|
|
341,760 2 2 128,160 0 0 128,160 0 0 vg_clientmalloc.c:vg_trap_here_WRAPPER
|
|
320,782 4 4 150,711 276 0 56,027 53 53 concord.c:init_hash_table
|
|
298,998 1 1 106,785 0 0 64,071 1 1 concord.c:create
|
|
149,518 0 0 149,516 0 0 1 0 0 ???:tolower@@GLIBC_2.0
|
|
149,518 0 0 149,516 0 0 1 0 0 ???:fgetc@@GLIBC_2.0
|
|
95,983 4 4 38,031 0 0 34,409 3,152 3,150 concord.c:new_word_node
|
|
85,440 0 0 42,720 0 0 21,360 0 0 vg_clientmalloc.c:vg_bogus_epilogue]]></programlisting>
|
|
|
|
|
|
<para>First up is a summary of the annotation options:</para>
|
|
|
|
<itemizedlist>
|
|
|
|
<listitem>
|
|
<para>I1 cache, D1 cache, L2 cache: cache configuration. So
|
|
you know the configuration with which these results were
|
|
obtained.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Command: the command line invocation of the program
|
|
under examination.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Events recorded: event abbreviations are:</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para><computeroutput>Ir</computeroutput>: I cache reads
|
|
(ie. instructions executed)</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>I1mr</computeroutput>: I1 cache read
|
|
misses</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>I2mr</computeroutput>: L2 cache
|
|
instruction read misses</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>Dr</computeroutput>: D cache reads
|
|
(ie. memory reads)</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>D1mr</computeroutput>: D1 cache read
|
|
misses</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>D2mr</computeroutput>: L2 cache data
|
|
read misses</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>Dw</computeroutput>: D cache writes
|
|
(ie. memory writes)</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>D1mw</computeroutput>: D1 cache write
|
|
misses</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>D2mw</computeroutput>: L2 cache data
|
|
write misses</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>Bc</computeroutput>: Conditional branches
|
|
executed</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>Bcm</computeroutput>: Conditional branches
|
|
mispredicted</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>Bi</computeroutput>: Indirect branches
|
|
executed</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>Bim</computeroutput>: Conditional branches
|
|
mispredicted</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
<para>Note that D1 total accesses is given by
|
|
<computeroutput>D1mr</computeroutput> +
|
|
<computeroutput>D1mw</computeroutput>, and that L2 total
|
|
accesses is given by <computeroutput>I2mr</computeroutput> +
|
|
<computeroutput>D2mr</computeroutput> +
|
|
<computeroutput>D2mw</computeroutput>.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Events shown: the events shown, which is a subset of the events
|
|
gathered. This can be adjusted with the
|
|
<computeroutput>--show</computeroutput> option.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Event sort order: the sort order in which functions are
|
|
shown. For example, in this case the functions are sorted
|
|
from highest <computeroutput>Ir</computeroutput> counts to
|
|
lowest. If two functions have identical
|
|
<computeroutput>Ir</computeroutput> counts, they will then be
|
|
sorted by <computeroutput>I1mr</computeroutput> counts, and
|
|
so on. This order can be adjusted with the
|
|
<computeroutput>--sort</computeroutput> option.</para>
|
|
|
|
<para>Note that this dictates the order the functions appear.
|
|
It is <command>not</command> the order in which the columns
|
|
appear; that is dictated by the "events shown" line (and can
|
|
be changed with the <computeroutput>--show</computeroutput>
|
|
option).</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Threshold: cg_annotate
|
|
by default omits functions that cause very low counts
|
|
to avoid drowning you in information. In this case,
|
|
cg_annotate shows summaries the functions that account for
|
|
99% of the <computeroutput>Ir</computeroutput> counts;
|
|
<computeroutput>Ir</computeroutput> is chosen as the
|
|
threshold event since it is the primary sort event. The
|
|
threshold can be adjusted with the
|
|
<computeroutput>--threshold</computeroutput>
|
|
option.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Chosen for annotation: names of files specified
|
|
manually for annotation; in this case none.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Auto-annotation: whether auto-annotation was requested
|
|
via the <computeroutput>--auto=yes</computeroutput>
|
|
option. In this case no.</para>
|
|
</listitem>
|
|
|
|
</itemizedlist>
|
|
|
|
<para>Then follows summary statistics for the whole
|
|
program. These are similar to the summary provided when running
|
|
<computeroutput>valgrind --tool=cachegrind</computeroutput>.</para>
|
|
|
|
<para>Then follows function-by-function statistics. Each function
|
|
is identified by a
|
|
<computeroutput>file_name:function_name</computeroutput> pair. If
|
|
a column contains only a dot it means the function never performs
|
|
that event (eg. the third row shows that
|
|
<computeroutput>strcmp()</computeroutput> contains no
|
|
instructions that write to memory). The name
|
|
<computeroutput>???</computeroutput> is used if the the file name
|
|
and/or function name could not be determined from debugging
|
|
information. If most of the entries have the form
|
|
<computeroutput>???:???</computeroutput> the program probably
|
|
wasn't compiled with <computeroutput>-g</computeroutput>. If any
|
|
code was invalidated (either due to self-modifying code or
|
|
unloading of shared objects) its counts are aggregated into a
|
|
single cost centre written as
|
|
<computeroutput>(discarded):(discarded)</computeroutput>.</para>
|
|
|
|
<para>It is worth noting that functions will come both from
|
|
the profiled program (eg. <filename>concord.c</filename>)
|
|
and from libraries (eg. <filename>getc.c</filename>)</para>
|
|
|
|
<para>There are two ways to annotate source files -- by choosing
|
|
them manually, or with the
|
|
<computeroutput>--auto=yes</computeroutput> option. To do it
|
|
manually, just specify the filenames as additional arguments to
|
|
cg_annotate. For example, the
|
|
output from running <filename>cg_annotate <filename>
|
|
concord.c</filename> for our example produces the same output as above
|
|
followed by an annotated version of <filename>concord.c</filename>, a
|
|
section of which looks like:</para>
|
|
|
|
<programlisting><![CDATA[
|
|
--------------------------------------------------------------------------------
|
|
-- User-annotated source: concord.c
|
|
--------------------------------------------------------------------------------
|
|
Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
|
|
|
|
[snip]
|
|
|
|
. . . . . . . . . void init_hash_table(char *file_name, Word_Node *table[])
|
|
3 1 1 . . . 1 0 0 {
|
|
. . . . . . . . . FILE *file_ptr;
|
|
. . . . . . . . . Word_Info *data;
|
|
1 0 0 . . . 1 1 1 int line = 1, i;
|
|
. . . . . . . . .
|
|
5 0 0 . . . 3 0 0 data = (Word_Info *) create(sizeof(Word_Info));
|
|
. . . . . . . . .
|
|
4,991 0 0 1,995 0 0 998 0 0 for (i = 0; i < TABLE_SIZE; i++)
|
|
3,988 1 1 1,994 0 0 997 53 52 table[i] = NULL;
|
|
. . . . . . . . .
|
|
. . . . . . . . . /* Open file, check it. */
|
|
6 0 0 1 0 0 4 0 0 file_ptr = fopen(file_name, "r");
|
|
2 0 0 1 0 0 . . . if (!(file_ptr)) {
|
|
. . . . . . . . . fprintf(stderr, "Couldn't open '%s'.\n", file_name);
|
|
1 1 1 . . . . . . exit(EXIT_FAILURE);
|
|
. . . . . . . . . }
|
|
. . . . . . . . .
|
|
165,062 1 1 73,360 0 0 91,700 0 0 while ((line = get_word(data, line, file_ptr)) != EOF)
|
|
146,712 0 0 73,356 0 0 73,356 0 0 insert(data->;word, data->line, table);
|
|
. . . . . . . . .
|
|
4 0 0 1 0 0 2 0 0 free(data);
|
|
4 0 0 1 0 0 2 0 0 fclose(file_ptr);
|
|
3 0 0 2 0 0 . . . }]]></programlisting>
|
|
|
|
<para>(Although column widths are automatically minimised, a wide
|
|
terminal is clearly useful.)</para>
|
|
|
|
<para>Each source file is clearly marked
|
|
(<computeroutput>User-annotated source</computeroutput>) as
|
|
having been chosen manually for annotation. If the file was
|
|
found in one of the directories specified with the
|
|
<computeroutput>-I / --include</computeroutput> option, the directory
|
|
and file are both given.</para>
|
|
|
|
<para>Each line is annotated with its event counts. Events not
|
|
applicable for a line are represented by a dot. This is useful
|
|
for distinguishing between an event which cannot happen, and one
|
|
which can but did not.</para>
|
|
|
|
<para>Sometimes only a small section of a source file is
|
|
executed. To minimise uninteresting output, Cachegrind only shows
|
|
annotated lines and lines within a small distance of annotated
|
|
lines. Gaps are marked with the line numbers so you know which
|
|
part of a file the shown code comes from, eg:</para>
|
|
|
|
<programlisting><![CDATA[
|
|
(figures and code for line 704)
|
|
-- line 704 ----------------------------------------
|
|
-- line 878 ----------------------------------------
|
|
(figures and code for line 878)]]></programlisting>
|
|
|
|
<para>The amount of context to show around annotated lines is
|
|
controlled by the <computeroutput>--context</computeroutput>
|
|
option.</para>
|
|
|
|
<para>To get automatic annotation, run
|
|
<computeroutput>cg_annotate <filename> --auto=yes</computeroutput>.
|
|
cg_annotate will automatically annotate every source file it can
|
|
find that is mentioned in the function-by-function summary.
|
|
Therefore, the files chosen for auto-annotation are affected by
|
|
the <computeroutput>--sort</computeroutput> and
|
|
<computeroutput>--threshold</computeroutput> options. Each
|
|
source file is clearly marked (<computeroutput>Auto-annotated
|
|
source</computeroutput>) as being chosen automatically. Any
|
|
files that could not be found are mentioned at the end of the
|
|
output, eg:</para>
|
|
|
|
<programlisting><![CDATA[
|
|
------------------------------------------------------------------
|
|
The following files chosen for auto-annotation could not be found:
|
|
------------------------------------------------------------------
|
|
getc.c
|
|
ctype.c
|
|
../sysdeps/generic/lockfile.c]]></programlisting>
|
|
|
|
<para>This is quite common for library files, since libraries are
|
|
usually compiled with debugging information, but the source files
|
|
are often not present on a system. If a file is chosen for
|
|
annotation <command>both</command> manually and automatically, it
|
|
is marked as <computeroutput>User-annotated
|
|
source</computeroutput>. Use the <computeroutput>-I /
|
|
--include</computeroutput> option to tell Valgrind where to look
|
|
for source files if the filenames found from the debugging
|
|
information aren't specific enough.</para>
|
|
|
|
<para>Beware that cg_annotate can take some time to digest large
|
|
<computeroutput>cachegrind.out.<pid></computeroutput> files,
|
|
e.g. 30 seconds or more. Also beware that auto-annotation can
|
|
produce a lot of output if your program is large!</para>
|
|
|
|
</sect2>
|
|
|
|
|
|
<sect2 id="cg-manual.assembler" xreflabel="Annotating assembler programs">
|
|
<title>Annotating assembly code programs</title>
|
|
|
|
<para>Valgrind can annotate assembly code programs too, or annotate
|
|
the assembly code generated for your C program. Sometimes this is
|
|
useful for understanding what is really happening when an
|
|
interesting line of C code is translated into multiple
|
|
instructions.</para>
|
|
|
|
<para>To do this, you just need to assemble your
|
|
<computeroutput>.s</computeroutput> files with assembly-level debug
|
|
information. You can use <computeroutput>gcc
|
|
-S</computeroutput> to compile C/C++ programs to assembly code, and then
|
|
<computeroutput>gcc -g</computeroutput> on the assembly code files to
|
|
achieve this. You can then profile and annotate the assembly code source
|
|
files in the same way as C/C++ source files.</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="ms-manual.forkingprograms" xreflabel="Forking Programs">
|
|
<title>Forking Programs</title>
|
|
<para>If your program forks, the child will inherit all the profiling data that
|
|
has been gathered for the parent.</para>
|
|
|
|
<para>If the output file format string (controlled by
|
|
<option>--cachegrind-out-file</option>) does not contain <option>%p</option>,
|
|
then the outputs from the parent and child will be intermingled in a single
|
|
output file, which will almost certainly make it unreadable by
|
|
cg_annotate.</para>
|
|
</sect2>
|
|
|
|
|
|
</sect1>
|
|
|
|
|
|
<sect1 id="cg-manual.annopts" xreflabel="cg_annotate options">
|
|
<title>cg_annotate options</title>
|
|
|
|
<itemizedlist>
|
|
|
|
<listitem>
|
|
<para><computeroutput>-h, --help</computeroutput></para>
|
|
<para><computeroutput>-v, --version</computeroutput></para>
|
|
<para>Help and version, as usual.</para>
|
|
</listitem>
|
|
|
|
<listitem id="sort">
|
|
<para><computeroutput>--sort=A,B,C</computeroutput> [default:
|
|
order in
|
|
<computeroutput>cachegrind.out.<pid></computeroutput>]</para>
|
|
<para>Specifies the events upon which the sorting of the
|
|
function-by-function entries will be based. Useful if you
|
|
want to concentrate on eg. I cache misses
|
|
(<computeroutput>--sort=I1mr,I2mr</computeroutput>), or D
|
|
cache misses
|
|
(<computeroutput>--sort=D1mr,D2mr</computeroutput>), or L2
|
|
misses
|
|
(<computeroutput>--sort=D2mr,I2mr</computeroutput>).</para>
|
|
</listitem>
|
|
|
|
<listitem id="show">
|
|
<para><computeroutput>--show=A,B,C</computeroutput> [default:
|
|
all, using order in
|
|
<computeroutput>cachegrind.out.<pid></computeroutput>]</para>
|
|
<para>Specifies which events to show (and the column
|
|
order). Default is to use all present in the
|
|
<computeroutput>cachegrind.out.<pid></computeroutput> file (and
|
|
use the order in the file).</para>
|
|
</listitem>
|
|
|
|
<listitem id="threshold">
|
|
<para><computeroutput>--threshold=X</computeroutput>
|
|
[default: 99%]</para>
|
|
<para>Sets the threshold for the function-by-function
|
|
summary. Functions are shown that account for more than X%
|
|
of the primary sort event. If auto-annotating, also affects
|
|
which files are annotated.</para>
|
|
|
|
<para>Note: thresholds can be set for more than one of the
|
|
events by appending any events for the
|
|
<computeroutput>--sort</computeroutput> option with a colon
|
|
and a number (no spaces, though). E.g. if you want to see
|
|
the functions that cover 99% of L2 read misses and 99% of L2
|
|
write misses, use this option:</para>
|
|
<para><computeroutput>--sort=D2mr:99,D2mw:99</computeroutput></para>
|
|
</listitem>
|
|
|
|
<listitem id="auto">
|
|
<para><computeroutput>--auto=no</computeroutput> [default]</para>
|
|
<para><computeroutput>--auto=yes</computeroutput></para>
|
|
<para>When enabled, automatically annotates every file that
|
|
is mentioned in the function-by-function summary that can be
|
|
found. Also gives a list of those that couldn't be found.</para>
|
|
</listitem>
|
|
|
|
<listitem id="context">
|
|
<para><computeroutput>--context=N</computeroutput> [default:
|
|
8]</para>
|
|
<para>Print N lines of context before and after each
|
|
annotated line. Avoids printing large sections of source
|
|
files that were not executed. Use a large number
|
|
(eg. 10,000) to show all source lines.</para>
|
|
</listitem>
|
|
|
|
<listitem id="include">
|
|
<para><computeroutput>-I<dir>,
|
|
--include=<dir></computeroutput> [default: empty
|
|
string]</para>
|
|
<para>Adds a directory to the list in which to search for
|
|
files. Multiple -I/--include options can be given to add
|
|
multiple directories.</para>
|
|
</listitem>
|
|
|
|
</itemizedlist>
|
|
|
|
|
|
|
|
<sect2 id="cg-manual.annopts.warnings" xreflabel="Warnings">
|
|
<title>Warnings</title>
|
|
|
|
<para>There are a couple of situations in which
|
|
cg_annotate issues warnings.</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>If a source file is more recent than the
|
|
<computeroutput>cachegrind.out.<pid></computeroutput> file.
|
|
This is because the information in
|
|
<computeroutput>cachegrind.out.<pid></computeroutput> is only
|
|
recorded with line numbers, so if the line numbers change at
|
|
all in the source (eg. lines added, deleted, swapped), any
|
|
annotations will be incorrect.</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>If information is recorded about line numbers past the
|
|
end of a file. This can be caused by the above problem,
|
|
ie. shortening the source file while using an old
|
|
<computeroutput>cachegrind.out.<pid></computeroutput> file. If
|
|
this happens, the figures for the bogus lines are printed
|
|
anyway (clearly marked as bogus) in case they are
|
|
important.</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
</sect2>
|
|
|
|
|
|
|
|
<sect2 id="cg-manual.annopts.things-to-watch-out-for"
|
|
xreflabel="Things to watch out for">
|
|
<title>Things to watch out for</title>
|
|
|
|
<para>Some odd things that can occur during annotation:</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>If annotating at the assembler level, you might see
|
|
something like this:</para>
|
|
<programlisting><![CDATA[
|
|
1 0 0 . . . . . . leal -12(%ebp),%eax
|
|
1 0 0 . . . 1 0 0 movl %eax,84(%ebx)
|
|
2 0 0 0 0 0 1 0 0 movl $1,-20(%ebp)
|
|
. . . . . . . . . .align 4,0x90
|
|
1 0 0 . . . . . . movl $.LnrB,%eax
|
|
1 0 0 . . . 1 0 0 movl %eax,-16(%ebp)]]></programlisting>
|
|
|
|
<para>How can the third instruction be executed twice when
|
|
the others are executed only once? As it turns out, it
|
|
isn't. Here's a dump of the executable, using
|
|
<computeroutput>objdump -d</computeroutput>:</para>
|
|
<programlisting><![CDATA[
|
|
8048f25: 8d 45 f4 lea 0xfffffff4(%ebp),%eax
|
|
8048f28: 89 43 54 mov %eax,0x54(%ebx)
|
|
8048f2b: c7 45 ec 01 00 00 00 movl $0x1,0xffffffec(%ebp)
|
|
8048f32: 89 f6 mov %esi,%esi
|
|
8048f34: b8 08 8b 07 08 mov $0x8078b08,%eax
|
|
8048f39: 89 45 f0 mov %eax,0xfffffff0(%ebp)]]></programlisting>
|
|
|
|
<para>Notice the extra <computeroutput>mov
|
|
%esi,%esi</computeroutput> instruction. Where did this come
|
|
from? The GNU assembler inserted it to serve as the two
|
|
bytes of padding needed to align the <computeroutput>movl
|
|
$.LnrB,%eax</computeroutput> instruction on a four-byte
|
|
boundary, but pretended it didn't exist when adding debug
|
|
information. Thus when Valgrind reads the debug info it
|
|
thinks that the <computeroutput>movl
|
|
$0x1,0xffffffec(%ebp)</computeroutput> instruction covers the
|
|
address range 0x8048f2b--0x804833 by itself, and attributes
|
|
the counts for the <computeroutput>mov
|
|
%esi,%esi</computeroutput> to it.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Inlined functions can cause strange results in the
|
|
function-by-function summary. If a function
|
|
<computeroutput>inline_me()</computeroutput> is defined in
|
|
<filename>foo.h</filename> and inlined in the functions
|
|
<computeroutput>f1()</computeroutput>,
|
|
<computeroutput>f2()</computeroutput> and
|
|
<computeroutput>f3()</computeroutput> in
|
|
<filename>bar.c</filename>, there will not be a
|
|
<computeroutput>foo.h:inline_me()</computeroutput> function
|
|
entry. Instead, there will be separate function entries for
|
|
each inlining site, ie.
|
|
<computeroutput>foo.h:f1()</computeroutput>,
|
|
<computeroutput>foo.h:f2()</computeroutput> and
|
|
<computeroutput>foo.h:f3()</computeroutput>. To find the
|
|
total counts for
|
|
<computeroutput>foo.h:inline_me()</computeroutput>, add up
|
|
the counts from each entry.</para>
|
|
|
|
<para>The reason for this is that although the debug info
|
|
output by gcc indicates the switch from
|
|
<filename>bar.c</filename> to <filename>foo.h</filename>, it
|
|
doesn't indicate the name of the function in
|
|
<filename>foo.h</filename>, so Valgrind keeps using the old
|
|
one.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Sometimes, the same filename might be represented with
|
|
a relative name and with an absolute name in different parts
|
|
of the debug info, eg:
|
|
<filename>/home/user/proj/proj.h</filename> and
|
|
<filename>../proj.h</filename>. In this case, if you use
|
|
auto-annotation, the file will be annotated twice with the
|
|
counts split between the two.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Files with more than 65,535 lines cause difficulties
|
|
for the Stabs-format debug info reader. This is because the line
|
|
number in the <computeroutput>struct nlist</computeroutput>
|
|
defined in <filename>a.out.h</filename> under Linux is only a
|
|
16-bit value. Valgrind can handle some files with more than
|
|
65,535 lines correctly by making some guesses to identify
|
|
line number overflows. But some cases are beyond it, in
|
|
which case you'll get a warning message explaining that
|
|
annotations for the file might be incorrect.</para>
|
|
|
|
<para>If you are using gcc 3.1 or later, this is most likely
|
|
irrelevant, since gcc switched to using the more modern DWARF2
|
|
format by default at version 3.1. DWARF2 does not have any such
|
|
limitations on line numbers.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>If you compile some files with
|
|
<computeroutput>-g</computeroutput> and some without, some
|
|
events that take place in a file without debug info could be
|
|
attributed to the last line of a file with debug info
|
|
(whichever one gets placed before the non-debug-info file in
|
|
the executable).</para>
|
|
</listitem>
|
|
|
|
</itemizedlist>
|
|
|
|
<para>This list looks long, but these cases should be fairly
|
|
rare.</para>
|
|
|
|
</sect2>
|
|
|
|
|
|
|
|
<sect2 id="cg-manual.annopts.accuracy" xreflabel="Accuracy">
|
|
<title>Accuracy</title>
|
|
|
|
<para>Valgrind's cache profiling has a number of
|
|
shortcomings:</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>It doesn't account for kernel activity -- the effect of
|
|
system calls on the cache contents is ignored.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>It doesn't account for other process activity.
|
|
This is probably desirable when considering a single
|
|
program.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>It doesn't account for virtual-to-physical address
|
|
mappings. Hence the simulation is not a true
|
|
representation of what's happening in the
|
|
cache. Most caches are physically indexed, but Cachegrind
|
|
simulates caches using virtual addresses.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>It doesn't account for cache misses not visible at the
|
|
instruction level, eg. those arising from TLB misses, or
|
|
speculative execution.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Valgrind will schedule
|
|
threads differently from how they would be when running natively.
|
|
This could warp the results for threaded programs.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>The x86/amd64 instructions <computeroutput>bts</computeroutput>,
|
|
<computeroutput>btr</computeroutput> and
|
|
<computeroutput>btc</computeroutput> will incorrectly be
|
|
counted as doing a data read if both the arguments are
|
|
registers, eg:</para>
|
|
<programlisting><![CDATA[
|
|
btsl %eax, %edx]]></programlisting>
|
|
|
|
<para>This should only happen rarely.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>x86/amd64 FPU instructions with data sizes of 28 and 108 bytes
|
|
(e.g. <computeroutput>fsave</computeroutput>) are treated as
|
|
though they only access 16 bytes. These instructions seem to
|
|
be rare so hopefully this won't affect accuracy much.</para>
|
|
</listitem>
|
|
|
|
</itemizedlist>
|
|
|
|
<para>Another thing worth noting is that results are very sensitive.
|
|
Changing the size of the the executable being profiled, or the sizes
|
|
of any of the shared libraries it uses, or even the length of their
|
|
file names, can perturb the results. Variations will be small, but
|
|
don't expect perfectly repeatable results if your program changes at
|
|
all.</para>
|
|
|
|
<para>More recent GNU/Linux distributions do address space
|
|
randomisation, in which identical runs of the same program have their
|
|
shared libraries loaded at different locations, as a security measure.
|
|
This also perturbs the results.</para>
|
|
|
|
<para>While these factors mean you shouldn't trust the results to
|
|
be super-accurate, hopefully they should be close enough to be
|
|
useful.</para>
|
|
|
|
</sect2>
|
|
|
|
</sect1>
|
|
|
|
|
|
|
|
<sect1 id="cg-manual.cg_merge" xreflabel="cg_merge">
|
|
<title>Merging profiles with cg_merge</title>
|
|
|
|
<para>
|
|
cg_merge is a simple program which
|
|
reads multiple profile files, as created by cachegrind, merges them
|
|
together, and writes the results into another file in the same format.
|
|
You can then examine the merged results using
|
|
<computeroutput>cg_annotate <filename></computeroutput>, as
|
|
described above. The merging functionality might be useful if you
|
|
want to aggregate costs over multiple runs of the same program, or
|
|
from a single parallel run with multiple instances of the same
|
|
program.</para>
|
|
|
|
<para>
|
|
cg_merge is invoked as follows:
|
|
</para>
|
|
|
|
<programlisting><![CDATA[
|
|
cg_merge -o outputfile file1 file2 file3 ...]]></programlisting>
|
|
|
|
<para>
|
|
It reads and checks <computeroutput>file1</computeroutput>, then read
|
|
and checks <computeroutput>file2</computeroutput> and merges it into
|
|
the running totals, then the same with
|
|
<computeroutput>file3</computeroutput>, etc. The final results are
|
|
written to <computeroutput>outputfile</computeroutput>, or to standard
|
|
out if no output file is specified.</para>
|
|
|
|
<para>
|
|
Costs are summed on a per-function, per-line and per-instruction
|
|
basis. Because of this, the order in which the input files does not
|
|
matter, although you should take care to only mention each file once,
|
|
since any file mentioned twice will be added in twice.</para>
|
|
|
|
<para>
|
|
cg_merge does not attempt to check
|
|
that the input files come from runs of the same executable. It will
|
|
happily merge together profile files from completely unrelated
|
|
programs. It does however check that the
|
|
<computeroutput>Events:</computeroutput> lines of all the inputs are
|
|
identical, so as to ensure that the addition of costs makes sense.
|
|
For example, it would be nonsensical for it to add a number indicating
|
|
D1 read references to a number from a different file indicating L2
|
|
write misses.</para>
|
|
|
|
<para>
|
|
A number of other syntax and sanity checks are done whilst reading the
|
|
inputs. cg_merge will stop and
|
|
attempt to print a helpful error message if any of the input files
|
|
fail these checks.</para>
|
|
|
|
</sect1>
|
|
|
|
|
|
<sect1 id="cg-manual.acting-on"
|
|
xreflabel="Acting on Cachegrind's information">
|
|
<title>Acting on Cachegrind's information</title>
|
|
<para>
|
|
So, you've managed to profile your program with Cachegrind. Now what?
|
|
What's the best way to actually act on the information it provides to speed
|
|
up your program? Here are some rules of thumb that we have found to be
|
|
useful.</para>
|
|
|
|
<para>
|
|
First of all, the global hit/miss rate numbers are not that useful. If you
|
|
have multiple programs or multiple runs of a program, comparing the numbers
|
|
might identify if any are outliers and worthy of closer investigation.
|
|
Otherwise, they're not enough to act on.</para>
|
|
|
|
<para>
|
|
The line-by-line source code annotations are much more useful. In our
|
|
experience, the best place to start is by looking at the
|
|
<computeroutput>Ir</computeroutput> numbers. They simply measure how many
|
|
instructions were executed for each line, and don't include any cache
|
|
information, but they can still be very useful for identifying
|
|
bottlenecks.</para>
|
|
|
|
<para>
|
|
After that, we have found that L2 misses are typically a much bigger source
|
|
of slow-downs than L1 misses. So it's worth looking for any snippets of
|
|
code that cause a high proportion of the L2 misses. If you find any, it's
|
|
still not always easy to work out how to improve things. You need to have a
|
|
reasonable understanding of how caches work, the principles of locality, and
|
|
your program's data access patterns. Improving things may require
|
|
redesigning a data structure, for example.</para>
|
|
|
|
<para>
|
|
In short, Cachegrind can tell you where some of the bottlenecks in your code
|
|
are, but it can't tell you how to fix them. You have to work that out for
|
|
yourself. But at least you have the information!
|
|
</para>
|
|
|
|
</sect1>
|
|
|
|
<sect1 id="cg-manual.impl-details"
|
|
xreflabel="Implementation details">
|
|
<title>Implementation details</title>
|
|
<para>
|
|
This section talks about details you don't need to know about in order to
|
|
use Cachegrind, but may be of interest to some people.
|
|
</para>
|
|
|
|
<sect2 id="cg-manual.impl-details.how-cg-works"
|
|
xreflabel="How Cachegrind works">
|
|
<title>How Cachegrind works</title>
|
|
<para>The best reference for understanding how Cachegrind works is chapter 3 of
|
|
"Dynamic Binary Analysis and Instrumentation", by Nicholas Nethercote. It
|
|
is available on the <ulink url="&vg-pubs;">Valgrind publications
|
|
page</ulink>.</para>
|
|
</sect2>
|
|
|
|
<sect2 id="cg-manual.impl-details.file-format"
|
|
xreflabel="Cachegrind output file format">
|
|
<title>Cachegrind output file format</title>
|
|
<para>The file format is fairly straightforward, basically giving the
|
|
cost centre for every line, grouped by files and
|
|
functions. Total counts (eg. total cache accesses, total L1
|
|
misses) are calculated when traversing this structure rather than
|
|
during execution, to save time; the cache simulation functions
|
|
are called so often that even one or two extra adds can make a
|
|
sizeable difference.</para>
|
|
|
|
<para>The file format:</para>
|
|
<programlisting><![CDATA[
|
|
file ::= desc_line* cmd_line events_line data_line+ summary_line
|
|
desc_line ::= "desc:" ws? non_nl_string
|
|
cmd_line ::= "cmd:" ws? cmd
|
|
events_line ::= "events:" ws? (event ws)+
|
|
data_line ::= file_line | fn_line | count_line
|
|
file_line ::= "fl=" filename
|
|
fn_line ::= "fn=" fn_name
|
|
count_line ::= line_num ws? (count ws)+
|
|
summary_line ::= "summary:" ws? (count ws)+
|
|
count ::= num | "."]]></programlisting>
|
|
|
|
<para>Where:</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para><computeroutput>non_nl_string</computeroutput> is any
|
|
string not containing a newline.</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>cmd</computeroutput> is a string holding the
|
|
command line of the profiled program.</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>event</computeroutput> is a string containing
|
|
no whitespace.</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>filename</computeroutput> and
|
|
<computeroutput>fn_name</computeroutput> are strings.</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>num</computeroutput> and
|
|
<computeroutput>line_num</computeroutput> are decimal
|
|
numbers.</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para><computeroutput>ws</computeroutput> is whitespace.</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
<para>The contents of the "desc:" lines are printed out at the top
|
|
of the summary. This is a generic way of providing simulation
|
|
specific information, eg. for giving the cache configuration for
|
|
cache simulation.</para>
|
|
|
|
<para>More than one line of info can be presented for each file/fn/line number.
|
|
In such cases, the counts for the named events will be accumulated.</para>
|
|
|
|
<para>Counts can be "." to represent zero. This makes the files easier for
|
|
humans to read.</para>
|
|
|
|
<para>The number of counts in each
|
|
<computeroutput>line</computeroutput> and the
|
|
<computeroutput>summary_line</computeroutput> should not exceed
|
|
the number of events in the
|
|
<computeroutput>event_line</computeroutput>. If the number in
|
|
each <computeroutput>line</computeroutput> is less, cg_annotate
|
|
treats those missing as though they were a "." entry. This saves space.
|
|
</para>
|
|
|
|
<para>A <computeroutput>file_line</computeroutput> changes the
|
|
current file name. A <computeroutput>fn_line</computeroutput>
|
|
changes the current function name. A
|
|
<computeroutput>count_line</computeroutput> contains counts that
|
|
pertain to the current filename/fn_name. A "fn="
|
|
<computeroutput>file_line</computeroutput> and a
|
|
<computeroutput>fn_line</computeroutput> must appear before any
|
|
<computeroutput>count_line</computeroutput>s to give the context
|
|
of the first <computeroutput>count_line</computeroutput>s.</para>
|
|
|
|
<para>Each <computeroutput>file_line</computeroutput> will normally be
|
|
immediately followed by a <computeroutput>fn_line</computeroutput>. But it
|
|
doesn't have to be.</para>
|
|
|
|
|
|
</sect2>
|
|
|
|
</sect1>
|
|
</chapter>
|