Gitiles
Code Review Sign In
gerrit-public.fairphone.software / platform / external / valgrind / ed39800a83baf5bffbe391f3974eb2af0f415f80 / . / docs / html / cg-manual.html
blob: 99ec6cd8352cc5a433d8308d42a55b5b0ea2e1d7 [file] [log] [blame]
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>5. Cachegrind: a cache and branch-prediction profiler</title>
<link rel="stylesheet" type="text/css" href="vg_basic.css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="index.html" title="Valgrind Documentation">
<link rel="up" href="manual.html" title="Valgrind User Manual">
<link rel="prev" href="mc-manual.html" title="4. Memcheck: a memory error detector">
<link rel="next" href="cl-manual.html" title="6. Callgrind: a call-graph generating cache and branch prediction profiler">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<div><table class="nav" width="100%" cellspacing="3" cellpadding="3" border="0" summary="Navigation header"><tr>
<td width="22px" align="center" valign="middle"><a accesskey="p" href="mc-manual.html"><img src="images/prev.png" width="18" height="21" border="0" alt="Prev"></a></td>
<td width="25px" align="center" valign="middle"><a accesskey="u" href="manual.html"><img src="images/up.png" width="21" height="18" border="0" alt="Up"></a></td>
<td width="31px" align="center" valign="middle"><a accesskey="h" href="index.html"><img src="images/home.png" width="27" height="20" border="0" alt="Up"></a></td>
<th align="center" valign="middle">Valgrind User Manual</th>
<td width="22px" align="center" valign="middle"><a accesskey="n" href="cl-manual.html"><img src="images/next.png" width="18" height="21" border="0" alt="Next"></a></td>
</tr></table></div>
<div class="chapter">
<div class="titlepage"><div><div><h1 class="title">
<a name="cg-manual"></a>5. Cachegrind: a cache and branch-prediction profiler</h1></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="sect1"><a href="cg-manual.html#cg-manual.overview">5.1. Overview</a></span></dt>
<dt><span class="sect1"><a href="cg-manual.html#cg-manual.profile">5.2. Using Cachegrind, cg_annotate and cg_merge</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.running-cachegrind">5.2.1. Running Cachegrind</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.outputfile">5.2.2. Output File</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.running-cg_annotate">5.2.3. Running cg_annotate</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.the-output-preamble">5.2.4. The Output Preamble</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.the-global">5.2.5. The Global and Function-level Counts</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.line-by-line">5.2.6. Line-by-line Counts</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.assembler">5.2.7. Annotating Assembly Code Programs</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#ms-manual.forkingprograms">5.2.8. Forking Programs</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.annopts.warnings">5.2.9. cg_annotate Warnings</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.annopts.things-to-watch-out-for">5.2.10. Unusual Annotation Cases</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.cg_merge">5.2.11. Merging Profiles with cg_merge</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.cg_diff">5.2.12. Differencing Profiles with cg_diff</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="cg-manual.html#cg-manual.cgopts">5.3. Cachegrind Command-line Options</a></span></dt>
<dt><span class="sect1"><a href="cg-manual.html#cg-manual.annopts">5.4. cg_annotate Command-line Options</a></span></dt>
<dt><span class="sect1"><a href="cg-manual.html#cg-manual.mergeopts">5.5. cg_merge Command-line Options</a></span></dt>
<dt><span class="sect1"><a href="cg-manual.html#cg-manual.diffopts">5.6. cg_diff Command-line Options</a></span></dt>
<dt><span class="sect1"><a href="cg-manual.html#cg-manual.acting-on">5.7. Acting on Cachegrind's Information</a></span></dt>
<dt><span class="sect1"><a href="cg-manual.html#cg-manual.sim-details">5.8. Simulation Details</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="cg-manual.html#cache-sim">5.8.1. Cache Simulation Specifics</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#branch-sim">5.8.2. Branch Simulation Specifics</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.annopts.accuracy">5.8.3. Accuracy</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="cg-manual.html#cg-manual.impl-details">5.9. Implementation Details</a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.impl-details.how-cg-works">5.9.1. How Cachegrind Works</a></span></dt>
<dt><span class="sect2"><a href="cg-manual.html#cg-manual.impl-details.file-format">5.9.2. Cachegrind Output File Format</a></span></dt>
</dl></dd>
</dl>
</div>
<p>To use this tool, you must specify
<code class="option">--tool=cachegrind</code> on the
Valgrind command line.</p>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="cg-manual.overview"></a>5.1. Overview</h2></div></div></div>
<p>Cachegrind simulates how your program interacts with a machine's cache
hierarchy and (optionally) branch predictor. It simulates a machine with
independent first-level instruction and data caches (I1 and D1), backed by a
unified second-level cache (L2). This exactly matches the configuration of
many modern machines.</p>
<p>However, some modern machines have three or four levels of cache. For these
machines (in the cases where Cachegrind can auto-detect the cache
configuration) Cachegrind simulates the first-level and last-level caches.
The reason for this choice is that the last-level cache has the most influence on
runtime, as it masks accesses to main memory. Furthermore, the L1 caches
often have low associativity, so simulating them can detect cases where the
code interacts badly with this cache (eg. traversing a matrix column-wise
with the row length being a power of 2).</p>
<p>Therefore, Cachegrind always refers to the I1, D1 and LL (last-level)
caches.</p>
<p>
Cachegrind gathers the following statistics (abbreviations used for each statistic
is given in parentheses):</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>I cache reads (<code class="computeroutput">Ir</code>,
which equals the number of instructions executed),
I1 cache read misses (<code class="computeroutput">I1mr</code>) and
LL cache instruction read misses (<code class="computeroutput">ILmr</code>).
</p></li>
<li class="listitem"><p>D cache reads (<code class="computeroutput">Dr</code>, which
equals the number of memory reads),
D1 cache read misses (<code class="computeroutput">D1mr</code>), and
LL cache data read misses (<code class="computeroutput">DLmr</code>).
</p></li>
<li class="listitem"><p>D cache writes (<code class="computeroutput">Dw</code>, which equals
the number of memory writes),
D1 cache write misses (<code class="computeroutput">D1mw</code>), and
LL cache data write misses (<code class="computeroutput">DLmw</code>).
</p></li>
<li class="listitem"><p>Conditional branches executed (<code class="computeroutput">Bc</code>) and
conditional branches mispredicted (<code class="computeroutput">Bcm</code>).
</p></li>
<li class="listitem"><p>Indirect branches executed (<code class="computeroutput">Bi</code>) and
indirect branches mispredicted (<code class="computeroutput">Bim</code>).
</p></li>
</ul></div>
<p>Note that D1 total accesses is given by
<code class="computeroutput">D1mr</code> +
<code class="computeroutput">D1mw</code>, and that LL total
accesses is given by <code class="computeroutput">ILmr</code> +
<code class="computeroutput">DLmr</code> +
<code class="computeroutput">DLmw</code>.
</p>
<p>These statistics are presented for the entire program and for each
function in the program. You can also annotate each line of source code in
the program with the counts that were caused directly by it.</p>
<p>On a modern machine, an L1 miss will typically cost
around 10 cycles, an LL miss can cost as much as 200
cycles, and a mispredicted branch costs in the region of 10
to 30 cycles. Detailed cache and branch profiling can be very useful
for understanding how your program interacts with the machine and thus how
to make it faster.</p>
<p>Also, since one instruction cache read is performed per
instruction executed, you can find out how many instructions are
executed per line, which can be useful for traditional profiling.</p>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="cg-manual.profile"></a>5.2. Using Cachegrind, cg_annotate and cg_merge</h2></div></div></div>
<p>First off, as for normal Valgrind use, you probably want to
compile with debugging info (the
<code class="option">-g</code> option). But by contrast with
normal Valgrind use, you probably do want to turn
optimisation on, since you should profile your program as it will
be normally run.</p>
<p>Then, you need to run Cachegrind itself to gather the profiling
information, and then run cg_annotate to get a detailed presentation of that
information. As an optional intermediate step, you can use cg_merge to sum
together the outputs of multiple Cachegrind runs into a single file which
you then use as the input for cg_annotate. Alternatively, you can use
cg_diff to difference the outputs of two Cachegrind runs into a single file
which you then use as the input for cg_annotate.</p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.running-cachegrind"></a>5.2.1. Running Cachegrind</h3></div></div></div>
<p>To run Cachegrind on a program <code class="filename">prog</code>, run:</p>
<pre class="screen">
valgrind --tool=cachegrind prog
</pre>
<p>The program will execute (slowly). Upon completion,
summary statistics that look like this will be printed:</p>
<pre class="programlisting">
==31751== I refs: 27,742,716
==31751== I1 misses: 276
==31751== LLi misses: 275
==31751== I1 miss rate: 0.0%
==31751== LLi miss rate: 0.0%
==31751==
==31751== D refs: 15,430,290 (10,955,517 rd + 4,474,773 wr)
==31751== D1 misses: 41,185 ( 21,905 rd + 19,280 wr)
==31751== LLd misses: 23,085 ( 3,987 rd + 19,098 wr)
==31751== D1 miss rate: 0.2% ( 0.1% + 0.4%)
==31751== LLd miss rate: 0.1% ( 0.0% + 0.4%)
==31751==
==31751== LL misses: 23,360 ( 4,262 rd + 19,098 wr)
==31751== LL miss rate: 0.0% ( 0.0% + 0.4%)</pre>
<p>Cache accesses for instruction fetches are summarised
first, giving the number of fetches made (this is the number of
instructions executed, which can be useful to know in its own
right), the number of I1 misses, and the number of LL instruction
(<code class="computeroutput">LLi</code>) misses.</p>
<p>Cache accesses for data follow. The information is similar
to that of the instruction fetches, except that the values are
also shown split between reads and writes (note each row's
<code class="computeroutput">rd</code> and
<code class="computeroutput">wr</code> values add up to the row's
total).</p>
<p>Combined instruction and data figures for the LL cache
follow that. Note that the LL miss rate is computed relative to the total
number of memory accesses, not the number of L1 misses. I.e. it is
<code class="computeroutput">(ILmr + DLmr + DLmw) / (Ir + Dr + Dw)</code>
not
<code class="computeroutput">(ILmr + DLmr + DLmw) / (I1mr + D1mr + D1mw)</code>
</p>
<p>Branch prediction statistics are not collected by default.
To do so, add the option <code class="option">--branch-sim=yes</code>.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.outputfile"></a>5.2.2. Output File</h3></div></div></div>
<p>As well as printing summary information, Cachegrind also writes
more detailed profiling information to a file. By default this file is named
<code class="filename">cachegrind.out.&lt;pid&gt;</code> (where
<code class="filename">&lt;pid&gt;</code> is the program's process ID), but its name
can be changed with the <code class="option">--cachegrind-out-file</code> option. This
file is human-readable, but is intended to be interpreted by the
accompanying program cg_annotate, described in the next section.</p>
<p>The default <code class="computeroutput">.&lt;pid&gt;</code> suffix
on the output file name serves two purposes. Firstly, it means you
don't have to rename old log files that you don't want to overwrite.
Secondly, and more importantly, it allows correct profiling with the
<code class="option">--trace-children=yes</code> option of
programs that spawn child processes.</p>
<p>The output file can be big, many megabytes for large applications
built with full debugging information.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.running-cg_annotate"></a>5.2.3. Running cg_annotate</h3></div></div></div>
<p>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.</p>
<p>To get a function-by-function summary, run:</p>
<pre class="screen">cg_annotate &lt;filename&gt;</pre>
<p>on a Cachegrind output file.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.the-output-preamble"></a>5.2.4. The Output Preamble</h3></div></div></div>
<p>The first part of the output looks like this:</p>
<pre class="programlisting">
--------------------------------------------------------------------------------
I1 cache: 65536 B, 64 B, 2-way associative
D1 cache: 65536 B, 64 B, 2-way associative
LL cache: 262144 B, 64 B, 8-way associative
Command: concord vg_to_ucode.c
Events recorded: Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
Events shown: Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
Event sort order: Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
Threshold: 99%
Chosen for annotation:
Auto-annotation: off
</pre>
<p>This is a summary of the annotation options:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>I1 cache, D1 cache, LL cache: cache configuration. So
you know the configuration with which these results were
obtained.</p></li>
<li class="listitem"><p>Command: the command line invocation of the program
under examination.</p></li>
<li class="listitem"><p>Events recorded: which events were recorded.</p></li>
<li class="listitem"><p>Events shown: the events shown, which is a subset of the events
gathered. This can be adjusted with the
<code class="option">--show</code> option.</p></li>
<li class="listitem">
<p>Event sort order: the sort order in which functions are
shown. For example, in this case the functions are sorted
from highest <code class="computeroutput">Ir</code> counts to
lowest. If two functions have identical
<code class="computeroutput">Ir</code> counts, they will then be
sorted by <code class="computeroutput">I1mr</code> counts, and
so on. This order can be adjusted with the
<code class="option">--sort</code> option.</p>
<p>Note that this dictates the order the functions appear.
It is <span class="emphasis"><em>not</em></span> the order in which the columns
appear; that is dictated by the "events shown" line (and can
be changed with the <code class="option">--show</code>
option).</p>
</li>
<li class="listitem"><p>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 <code class="computeroutput">Ir</code> counts;
<code class="computeroutput">Ir</code> is chosen as the
threshold event since it is the primary sort event. The
threshold can be adjusted with the
<code class="option">--threshold</code>
option.</p></li>
<li class="listitem"><p>Chosen for annotation: names of files specified
manually for annotation; in this case none.</p></li>
<li class="listitem"><p>Auto-annotation: whether auto-annotation was requested
via the <code class="option">--auto=yes</code>
option. In this case no.</p></li>
</ul></div>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.the-global"></a>5.2.5. The Global and Function-level Counts</h3></div></div></div>
<p>Then follows summary statistics for the whole
program:</p>
<pre class="programlisting">
--------------------------------------------------------------------------------
Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
--------------------------------------------------------------------------------
27,742,716 276 275 10,955,517 21,905 3,987 4,474,773 19,280 19,098 PROGRAM TOTALS</pre>
<p>
These are similar to the summary provided when Cachegrind finishes running.
</p>
<p>Then comes function-by-function statistics:</p>
<pre class="programlisting">
--------------------------------------------------------------------------------
Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw 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</pre>
<p>Each function
is identified by a
<code class="computeroutput">file_name:function_name</code> pair. If
a column contains only a dot it means the function never performs
that event (e.g. the third row shows that
<code class="computeroutput">strcmp()</code> contains no
instructions that write to memory). The name
<code class="computeroutput">???</code> is used if the file name
and/or function name could not be determined from debugging
information. If most of the entries have the form
<code class="computeroutput">???:???</code> the program probably
wasn't compiled with <code class="option">-g</code>.</p>
<p>It is worth noting that functions will come both from
the profiled program (e.g. <code class="filename">concord.c</code>)
and from libraries (e.g. <code class="filename">getc.c</code>)</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.line-by-line"></a>5.2.6. Line-by-line Counts</h3></div></div></div>
<p>There are two ways to annotate source files -- by specifying them
manually as arguments to cg_annotate, or with the
<code class="option">--auto=yes</code> option. For example, the output from running
<code class="filename">cg_annotate &lt;filename&gt; concord.c</code> for our example
produces the same output as above followed by an annotated version of
<code class="filename">concord.c</code>, a section of which looks like:</p>
<pre class="programlisting">
--------------------------------------------------------------------------------
-- User-annotated source: concord.c
--------------------------------------------------------------------------------
Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
. . . . . . . . . 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 &lt; 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-&gt;;word, data-&gt;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 . . . }</pre>
<p>(Although column widths are automatically minimised, a wide
terminal is clearly useful.)</p>
<p>Each source file is clearly marked
(<code class="computeroutput">User-annotated source</code>) as
having been chosen manually for annotation. If the file was
found in one of the directories specified with the
<code class="option">-I</code>/<code class="option">--include</code> option, the directory
and file are both given.</p>
<p>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.</p>
<p>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:</p>
<pre class="programlisting">
(figures and code for line 704)
-- line 704 ----------------------------------------
-- line 878 ----------------------------------------
(figures and code for line 878)</pre>
<p>The amount of context to show around annotated lines is
controlled by the <code class="option">--context</code>
option.</p>
<p>To get automatic annotation, use the <code class="option">--auto=yes</code> option.
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 <code class="option">--sort</code> and
<code class="option">--threshold</code> options. Each
source file is clearly marked (<code class="computeroutput">Auto-annotated
source</code>) as being chosen automatically. Any
files that could not be found are mentioned at the end of the
output, eg:</p>
<pre class="programlisting">
------------------------------------------------------------------
The following files chosen for auto-annotation could not be found:
------------------------------------------------------------------
getc.c
ctype.c
../sysdeps/generic/lockfile.c</pre>
<p>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 both manually and automatically, it
is marked as <code class="computeroutput">User-annotated
source</code>. Use the
<code class="option">-I</code>/<code class="option">--include</code> option to tell Valgrind where
to look for source files if the filenames found from the debugging
information aren't specific enough.</p>
<p>Beware that cg_annotate can take some time to digest large
<code class="filename">cachegrind.out.&lt;pid&gt;</code> files,
e.g. 30 seconds or more. Also beware that auto-annotation can
produce a lot of output if your program is large!</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.assembler"></a>5.2.7. Annotating Assembly Code Programs</h3></div></div></div>
<p>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.</p>
<p>To do this, you just need to assemble your
<code class="computeroutput">.s</code> files with assembly-level debug
information. You can use compile with the <code class="option">-S</code> to compile C/C++
programs to assembly code, and then assemble the assembly code files with
<code class="option">-g</code> to achieve this. You can then profile and annotate the
assembly code source files in the same way as C/C++ source files.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="ms-manual.forkingprograms"></a>5.2.8. Forking Programs</h3></div></div></div>
<p>If your program forks, the child will inherit all the profiling data that
has been gathered for the parent.</p>
<p>If the output file format string (controlled by
<code class="option">--cachegrind-out-file</code>) does not contain <code class="option">%p</code>,
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.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.annopts.warnings"></a>5.2.9. cg_annotate Warnings</h3></div></div></div>
<p>There are a couple of situations in which
cg_annotate issues warnings.</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>If a source file is more recent than the
<code class="filename">cachegrind.out.&lt;pid&gt;</code> file.
This is because the information in
<code class="filename">cachegrind.out.&lt;pid&gt;</code> is only
recorded with line numbers, so if the line numbers change at
all in the source (e.g. lines added, deleted, swapped), any
annotations will be incorrect.</p></li>
<li class="listitem"><p>If information is recorded about line numbers past the
end of a file. This can be caused by the above problem,
i.e. shortening the source file while using an old
<code class="filename">cachegrind.out.&lt;pid&gt;</code> file. If
this happens, the figures for the bogus lines are printed
anyway (clearly marked as bogus) in case they are
important.</p></li>
</ul></div>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.annopts.things-to-watch-out-for"></a>5.2.10. Unusual Annotation Cases</h3></div></div></div>
<p>Some odd things that can occur during annotation:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p>If annotating at the assembler level, you might see
something like this:</p>
<pre class="programlisting">
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)</pre>
<p>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
<code class="computeroutput">objdump -d</code>:</p>
<pre class="programlisting">
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)</pre>
<p>Notice the extra <code class="computeroutput">mov
%esi,%esi</code> instruction. Where did this come
from? The GNU assembler inserted it to serve as the two
bytes of padding needed to align the <code class="computeroutput">movl
$.LnrB,%eax</code> 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 <code class="computeroutput">movl
$0x1,0xffffffec(%ebp)</code> instruction covers the
address range 0x8048f2b--0x804833 by itself, and attributes
the counts for the <code class="computeroutput">mov
%esi,%esi</code> to it.</p>
</li>
<li class="listitem"><p>Sometimes, the same filename might be represented with
a relative name and with an absolute name in different parts
of the debug info, eg:
<code class="filename">/home/user/proj/proj.h</code> and
<code class="filename">../proj.h</code>. In this case, if you use
auto-annotation, the file will be annotated twice with the
counts split between the two.</p></li>
<li class="listitem"><p>If you compile some files with
<code class="option">-g</code> 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).</p></li>
</ul></div>
<p>This list looks long, but these cases should be fairly
rare.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.cg_merge"></a>5.2.11. Merging Profiles with cg_merge</h3></div></div></div>
<p>
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
<code class="computeroutput">cg_annotate &lt;filename&gt;</code>, 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.</p>
<p>
cg_merge is invoked as follows:
</p>
<pre class="programlisting">
cg_merge -o outputfile file1 file2 file3 ...</pre>
<p>
It reads and checks <code class="computeroutput">file1</code>, then read
and checks <code class="computeroutput">file2</code> and merges it into
the running totals, then the same with
<code class="computeroutput">file3</code>, etc. The final results are
written to <code class="computeroutput">outputfile</code>, or to standard
out if no output file is specified.</p>
<p>
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.</p>
<p>
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
<code class="computeroutput">Events:</code> 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 LL
write misses.</p>
<p>
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.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.cg_diff"></a>5.2.12. Differencing Profiles with cg_diff</h3></div></div></div>
<p>
cg_diff is a simple program which
reads two profile files, as created by Cachegrind, finds the difference
between them, and writes the results into another file in the same format.
You can then examine the merged results using
<code class="computeroutput">cg_annotate &lt;filename&gt;</code>, as
described above. This is very useful if you want to measure how a change to
a program affected its performance.
</p>
<p>
cg_diff is invoked as follows:
</p>
<pre class="programlisting">
cg_diff file1 file2</pre>
<p>
It reads and checks <code class="computeroutput">file1</code>, then read
and checks <code class="computeroutput">file2</code>, then computes the
difference (effectively <code class="computeroutput">file1</code> -
<code class="computeroutput">file2</code>). The final results are written to
standard output.</p>
<p>
Costs are summed on a per-function basis. Per-line costs are not summed,
because doing so is too difficult. For example, consider differencing two
profiles, one from a single-file program A, and one from the same program A
where a single blank line was inserted at the top of the file. Every single
per-line count has changed. In comparison, the per-function counts have not
changed. The per-function count differences are still very useful for
determining differences between programs. Note that because the result is
the difference of two profiles, many of the counts will be negative; this
indicates that the counts for the relevant function are fewer in the second
version than those in the first version.</p>
<p>
cg_diff 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
<code class="computeroutput">Events:</code> 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 LL
write misses.</p>
<p>
A number of other syntax and sanity checks are done whilst reading the
inputs. cg_diff will stop and
attempt to print a helpful error message if any of the input files
fail these checks.</p>
<p>
Sometimes you will want to compare Cachegrind profiles of two versions of a
program that you have sitting side-by-side. For example, you might have
<code class="computeroutput">version1/prog.c</code> and
<code class="computeroutput">version2/prog.c</code>, where the second is
slightly different to the first. A straight comparison of the two will not
be useful -- because functions are qualified with filenames, a function
<code class="function">f</code> will be listed as
<code class="computeroutput">version1/prog.c:f</code> for the first version but
<code class="computeroutput">version2/prog.c:f</code> for the second
version.</p>
<p>
When this happens, you can use the <code class="option">--mod-filename</code> option.
Its argument is a Perl search-and-replace expression that will be applied
to all the filenames in both Cachegrind output files. It can be used to
remove minor differences in filenames. For example, the option
<code class="option">--mod-filename='s/version[0-9]/versionN/'</code> will suffice for
this case.</p>
<p>
Similarly, sometimes compilers auto-generate certain functions and give them
randomized names. For example, GCC sometimes auto-generates functions with
names like <code class="function">T.1234</code>, and the suffixes vary from build to
build. You can use the <code class="option">--mod-funcname</code> option to remove
small differences like these; it works in the same way as
<code class="option">--mod-filename</code>.</p>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="cg-manual.cgopts"></a>5.3. Cachegrind Command-line Options</h2></div></div></div>
<p>Cachegrind-specific options are:</p>
<div class="variablelist">
<a name="cg.opts.list"></a><dl class="variablelist">
<dt>
<a name="opt.I1"></a><span class="term">
<code class="option">--I1=&lt;size&gt;,&lt;associativity&gt;,&lt;line size&gt; </code>
</span>
</dt>
<dd><p>Specify the size, associativity and line size of the level 1
instruction cache. </p></dd>
<dt>
<a name="opt.D1"></a><span class="term">
<code class="option">--D1=&lt;size&gt;,&lt;associativity&gt;,&lt;line size&gt; </code>
</span>
</dt>
<dd><p>Specify the size, associativity and line size of the level 1
data cache.</p></dd>
<dt>
<a name="opt.LL"></a><span class="term">
<code class="option">--LL=&lt;size&gt;,&lt;associativity&gt;,&lt;line size&gt; </code>
</span>
</dt>
<dd><p>Specify the size, associativity and line size of the last-level
cache.</p></dd>
<dt>
<a name="opt.cache-sim"></a><span class="term">
<code class="option">--cache-sim=no|yes [yes] </code>
</span>
</dt>
<dd><p>Enables or disables collection of cache access and miss
counts.</p></dd>
<dt>
<a name="opt.branch-sim"></a><span class="term">
<code class="option">--branch-sim=no|yes [no] </code>
</span>
</dt>
<dd><p>Enables or disables collection of branch instruction and
misprediction counts. By default this is disabled as it
slows Cachegrind down by approximately 25%. Note that you
cannot specify <code class="option">--cache-sim=no</code>
and <code class="option">--branch-sim=no</code>
together, as that would leave Cachegrind with no
information to collect.</p></dd>
<dt>
<a name="opt.cachegrind-out-file"></a><span class="term">
<code class="option">--cachegrind-out-file=&lt;file&gt; </code>
</span>
</dt>
<dd><p>Write the profile data to
<code class="computeroutput">file</code> rather than to the default
output file,
<code class="filename">cachegrind.out.&lt;pid&gt;</code>. The
<code class="option">%p</code> and <code class="option">%q</code> 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
option <code class="option"><a class="xref" href="manual-core.html#opt.log-file">--log-file</a></code>.
</p></dd>
</dl>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="cg-manual.annopts"></a>5.4. cg_annotate Command-line Options</h2></div></div></div>
<div class="variablelist">
<a name="cg_annotate.opts.list"></a><dl class="variablelist">
<dt><span class="term">
<code class="option">-h --help </code>
</span></dt>
<dd><p>Show the help message.</p></dd>
<dt><span class="term">
<code class="option">--version </code>
</span></dt>
<dd><p>Show the version number.</p></dd>
<dt><span class="term">
<code class="option">--show=A,B,C [default: all, using order in
cachegrind.out.&lt;pid&gt;] </code>
</span></dt>
<dd><p>Specifies which events to show (and the column
order). Default is to use all present in the
<code class="filename">cachegrind.out.&lt;pid&gt;</code> file (and
use the order in the file). Useful if you want to concentrate on, for
example, I cache misses (<code class="option">--show=I1mr,ILmr</code>), or data
read misses (<code class="option">--show=D1mr,DLmr</code>), or LL data misses
(<code class="option">--show=DLmr,DLmw</code>). Best used in conjunction with
<code class="option">--sort</code>.</p></dd>
<dt><span class="term">
<code class="option">--sort=A,B,C [default: order in
cachegrind.out.&lt;pid&gt;] </code>
</span></dt>
<dd><p>Specifies the events upon which the sorting of the
function-by-function entries will be based.</p></dd>
<dt><span class="term">
<code class="option">--threshold=X [default: 0.1%] </code>
</span></dt>
<dd>
<p>Sets the threshold for the function-by-function
summary. A function is shown if it accounts for more than X%
of the counts for the primary sort event. If auto-annotating, also
affects which files are annotated.</p>
<p>Note: thresholds can be set for more than one of the
events by appending any events for the
<code class="option">--sort</code> option with a colon
and a number (no spaces, though). E.g. if you want to see
each function that covers more than 1% of LL read misses or 1% of LL
write misses, use this option:</p>
<p><code class="option">--sort=DLmr:1,DLmw:1</code></p>
</dd>
<dt><span class="term">
<code class="option">--auto=&lt;no|yes&gt; [default: no] </code>
</span></dt>
<dd><p>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.</p></dd>
<dt><span class="term">
<code class="option">--context=N [default: 8] </code>
</span></dt>
<dd><p>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
(e.g. 100000) to show all source lines.</p></dd>
<dt><span class="term">
<code class="option">-I&lt;dir&gt; --include=&lt;dir&gt; [default: none] </code>
</span></dt>
<dd><p>Adds a directory to the list in which to search for
files. Multiple <code class="option">-I</code>/<code class="option">--include</code>
options can be given to add multiple directories.</p></dd>
</dl>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="cg-manual.mergeopts"></a>5.5. cg_merge Command-line Options</h2></div></div></div>
<div class="variablelist">
<a name="cg_merge.opts.list"></a><dl class="variablelist">
<dt><span class="term">
<code class="option">-o outfile</code>
</span></dt>
<dd><p>Write the profile data to <code class="computeroutput">outfile</code>
rather than to standard output.
</p></dd>
</dl>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="cg-manual.diffopts"></a>5.6. cg_diff Command-line Options</h2></div></div></div>
<div class="variablelist">
<a name="cg_diff.opts.list"></a><dl class="variablelist">
<dt><span class="term">
<code class="option">-h --help </code>
</span></dt>
<dd><p>Show the help message.</p></dd>
<dt><span class="term">
<code class="option">--version </code>
</span></dt>
<dd><p>Show the version number.</p></dd>
<dt><span class="term">
<code class="option">--mod-filename=&lt;expr&gt; [default: none]</code>
</span></dt>
<dd><p>Specifies a Perl search-and-replace expression that is applied
to all filenames. Useful for removing minor differences in paths
between two different versions of a program that are sitting in
different directories.</p></dd>
<dt><span class="term">
<code class="option">--mod-funcname=&lt;expr&gt; [default: none]</code>
</span></dt>
<dd><p>Like <code class="option">--mod-filename</code>, but for filenames.
Useful for removing minor differences in randomized names of
auto-generated functions generated by some compilers.</p></dd>
</dl>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="cg-manual.acting-on"></a>5.7. Acting on Cachegrind's Information</h2></div></div></div>
<p>
Cachegrind gives you lots of information, but acting on that information
isn't always easy. Here are some rules of thumb that we have found to be
useful.</p>
<p>
First of all, the global hit/miss counts and miss rates 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.</p>
<p>
The function-by-function counts are more useful to look at, as they pinpoint
which functions are causing large numbers of counts. However, beware that
inlining can make these counts misleading. If a function
<code class="function">f</code> is always inlined, counts will be attributed to the
functions it is inlined into, rather than itself. However, if you look at
the line-by-line annotations for <code class="function">f</code> you'll see the
counts that belong to <code class="function">f</code>. (This is hard to avoid, it's
how the debug info is structured.) So it's worth looking for large numbers
in the line-by-line annotations.</p>
<p>
The line-by-line source code annotations are much more useful. In our
experience, the best place to start is by looking at the
<code class="computeroutput">Ir</code> 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.</p>
<p>
After that, we have found that LL misses are typically a much bigger source
of slow-downs than L1 misses. So it's worth looking for any snippets of
code with high <code class="computeroutput">DLmr</code> or
<code class="computeroutput">DLmw</code> counts. (You can use
<code class="option">--show=DLmr
--sort=DLmr</code> with cg_annotate to focus just on
<code class="literal">DLmr</code> counts, for example.) 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.</p>
<p>
Looking at the <code class="computeroutput">Bcm</code> and
<code class="computeroutput">Bim</code> misses can also be helpful.
In particular, <code class="computeroutput">Bim</code> misses are often caused
by <code class="literal">switch</code> statements, and in some cases these
<code class="literal">switch</code> statements can be replaced with table-driven code.
For example, you might replace code like this:</p>
<pre class="programlisting">
enum E { A, B, C };
enum E e;
int i;
...
switch (e)
{
case A: i += 1; break;
case B: i += 2; break;
case C: i += 3; break;
}
</pre>
<p>with code like this:</p>
<pre class="programlisting">
enum E { A, B, C };
enum E e;
enum E table[] = { 1, 2, 3 };
int i;
...
i += table[e];
</pre>
<p>
This is obviously a contrived example, but the basic principle applies in a
wide variety of situations.</p>
<p>
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!
</p>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="cg-manual.sim-details"></a>5.8. Simulation Details</h2></div></div></div>
<p>
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.
</p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cache-sim"></a>5.8.1. Cache Simulation Specifics</h3></div></div></div>
<p>Specific characteristics of the cache simulation are as
follows:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>Write-allocate: when a write miss occurs, the block
written to is brought into the D1 cache. Most modern caches
have this property.</p></li>
<li class="listitem">
<p>Bit-selection hash function: the set of line(s) in the cache
to which a memory block maps is chosen by the middle bits
M--(M+N-1) of the byte address, where:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem"><p>line size = 2^M bytes</p></li>
<li class="listitem"><p>(cache size / line size / associativity) = 2^N bytes</p></li>
</ul></div>
</li>
<li class="listitem"><p>Inclusive LL cache: the LL cache typically replicates all
the entries of the L1 caches, because fetching into L1 involves
fetching into LL first (this does not guarantee strict inclusiveness,
as lines evicted from LL still could reside in L1). This is
standard on Pentium chips, but AMD Opterons, Athlons and Durons
use an exclusive LL cache that only holds
blocks evicted from L1. Ditto most modern VIA CPUs.</p></li>
</ul></div>
<p>The cache configuration simulated (cache size,
associativity and line size) is determined automatically using
the x86 CPUID instruction. If you have a machine that (a)
doesn't support the CPUID instruction, or (b) supports it in an
early incarnation that doesn't give any cache information, then
Cachegrind will fall back to using a default configuration (that
of a model 3/4 Athlon). Cachegrind will tell you if this
happens. You can manually specify one, two or all three levels
(I1/D1/LL) of the cache from the command line using the
<code class="option">--I1</code>,
<code class="option">--D1</code> and
<code class="option">--LL</code> options.
For cache parameters to be valid for simulation, the number
of sets (with associativity being the number of cache lines in
each set) has to be a power of two.</p>
<p>On PowerPC platforms
Cachegrind cannot automatically
determine the cache configuration, so you will
need to specify it with the
<code class="option">--I1</code>,
<code class="option">--D1</code> and
<code class="option">--LL</code> options.</p>
<p>Other noteworthy behaviour:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
<p>References that straddle two cache lines are treated as
follows:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
<li class="listitem"><p>If both blocks hit --&gt; counted as one hit</p></li>
<li class="listitem"><p>If one block hits, the other misses --&gt; counted
as one miss.</p></li>
<li class="listitem"><p>If both blocks miss --&gt; counted as one miss (not
two)</p></li>
</ul></div>
</li>
<li class="listitem">
<p>Instructions that modify a memory location
(e.g. <code class="computeroutput">inc</code> and
<code class="computeroutput">dec</code>) are counted as doing
just a read, i.e. a single data reference. This may seem
strange, but since the write can never cause a miss (the read
guarantees the block is in the cache) it's not very
interesting.</p>
<p>Thus it measures not the number of times the data cache
is accessed, but the number of times a data cache miss could
occur.</p>
</li>
</ul></div>
<p>If you are interested in simulating a cache with different
properties, it is not particularly hard to write your own cache
simulator, or to modify the existing ones in
<code class="computeroutput">cg_sim.c</code>. We'd be
interested to hear from anyone who does.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="branch-sim"></a>5.8.2. Branch Simulation Specifics</h3></div></div></div>
<p>Cachegrind simulates branch predictors intended to be
typical of mainstream desktop/server processors of around 2004.</p>
<p>Conditional branches are predicted using an array of 16384 2-bit
saturating counters. The array index used for a branch instruction is
computed partly from the low-order bits of the branch instruction's
address and partly using the taken/not-taken behaviour of the last few
conditional branches. As a result the predictions for any specific
branch depend both on its own history and the behaviour of previous
branches. This is a standard technique for improving prediction
accuracy.</p>
<p>For indirect branches (that is, jumps to unknown destinations)
Cachegrind uses a simple branch target address predictor. Targets are
predicted using an array of 512 entries indexed by the low order 9
bits of the branch instruction's address. Each branch is predicted to
jump to the same address it did last time. Any other behaviour causes
a mispredict.</p>
<p>More recent processors have better branch predictors, in
particular better indirect branch predictors. Cachegrind's predictor
design is deliberately conservative so as to be representative of the
large installed base of processors which pre-date widespread
deployment of more sophisticated indirect branch predictors. In
particular, late model Pentium 4s (Prescott), Pentium M, Core and Core
2 have more sophisticated indirect branch predictors than modelled by
Cachegrind. </p>
<p>Cachegrind does not simulate a return stack predictor. It
assumes that processors perfectly predict function return addresses,
an assumption which is probably close to being true.</p>
<p>See Hennessy and Patterson's classic text "Computer
Architecture: A Quantitative Approach", 4th edition (2007), Section
2.3 (pages 80-89) for background on modern branch predictors.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.annopts.accuracy"></a>5.8.3. Accuracy</h3></div></div></div>
<p>Valgrind's cache profiling has a number of
shortcomings:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>It doesn't account for kernel activity -- the effect of system
calls on the cache and branch predictor contents is ignored.</p></li>
<li class="listitem"><p>It doesn't account for other process activity.
This is probably desirable when considering a single
program.</p></li>
<li class="listitem"><p>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 and branch predictors are physically indexed, but
Cachegrind simulates caches using virtual addresses.</p></li>
<li class="listitem"><p>It doesn't account for cache misses not visible at the
instruction level, e.g. those arising from TLB misses, or
speculative execution.</p></li>
<li class="listitem"><p>Valgrind will schedule
threads differently from how they would be when running natively.
This could warp the results for threaded programs.</p></li>
<li class="listitem">
<p>The x86/amd64 instructions <code class="computeroutput">bts</code>,
<code class="computeroutput">btr</code> and
<code class="computeroutput">btc</code> will incorrectly be
counted as doing a data read if both the arguments are
registers, eg:</p>
<pre class="programlisting">
btsl %eax, %edx</pre>
<p>This should only happen rarely.</p>
</li>
<li class="listitem"><p>x86/amd64 FPU instructions with data sizes of 28 and 108 bytes
(e.g. <code class="computeroutput">fsave</code>) are treated as
though they only access 16 bytes. These instructions seem to
be rare so hopefully this won't affect accuracy much.</p></li>
</ul></div>
<p>Another thing worth noting is that results are very sensitive.
Changing the size of 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.</p>
<p>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.</p>
<p>While these factors mean you shouldn't trust the results to
be super-accurate, they should be close enough to be useful.</p>
</div>
</div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="cg-manual.impl-details"></a>5.9. Implementation Details</h2></div></div></div>
<p>
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.
</p>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.impl-details.how-cg-works"></a>5.9.1. How Cachegrind Works</h3></div></div></div>
<p>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 <a class="ulink" href="http://www.valgrind.org/docs/pubs.html" target="_top">Valgrind publications
page</a>.</p>
</div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="cg-manual.impl-details.file-format"></a>5.9.2. Cachegrind Output File Format</h3></div></div></div>
<p>The file format is fairly straightforward, basically giving the
cost centre for every line, grouped by files and
functions. It's also totally generic and self-describing, in the sense that
it can be used for any events that can be counted on a line-by-line basis,
not just cache and branch predictor events. For example, earlier versions
of Cachegrind didn't have a branch predictor simulation. When this was
added, the file format didn't need to change at all. So the format (and
consequently, cg_annotate) could be used by other tools.</p>
<p>The file format:</p>
<pre class="programlisting">
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 | "."</pre>
<p>Where:</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><code class="computeroutput">non_nl_string</code> is any
string not containing a newline.</p></li>
<li class="listitem"><p><code class="computeroutput">cmd</code> is a string holding the
command line of the profiled program.</p></li>
<li class="listitem"><p><code class="computeroutput">event</code> is a string containing
no whitespace.</p></li>
<li class="listitem"><p><code class="computeroutput">filename</code> and
<code class="computeroutput">fn_name</code> are strings.</p></li>
<li class="listitem"><p><code class="computeroutput">num</code> and
<code class="computeroutput">line_num</code> are decimal
numbers.</p></li>
<li class="listitem"><p><code class="computeroutput">ws</code> is whitespace.</p></li>
</ul></div>
<p>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, e.g. for giving the cache configuration for
cache simulation.</p>
<p>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.</p>
<p>Counts can be "." to represent zero. This makes the files easier for
humans to read.</p>
<p>The number of counts in each
<code class="computeroutput">line</code> and the
<code class="computeroutput">summary_line</code> should not exceed
the number of events in the
<code class="computeroutput">event_line</code>. If the number in
each <code class="computeroutput">line</code> is less, cg_annotate
treats those missing as though they were a "." entry. This saves space.
</p>
<p>A <code class="computeroutput">file_line</code> changes the
current file name. A <code class="computeroutput">fn_line</code>
changes the current function name. A
<code class="computeroutput">count_line</code> contains counts that
pertain to the current filename/fn_name. A "fn="
<code class="computeroutput">file_line</code> and a
<code class="computeroutput">fn_line</code> must appear before any
<code class="computeroutput">count_line</code>s to give the context
of the first <code class="computeroutput">count_line</code>s.</p>
<p>Each <code class="computeroutput">file_line</code> will normally be
immediately followed by a <code class="computeroutput">fn_line</code>. But it
doesn't have to be.</p>
<p>The summary line is redundant, because it just holds the total counts
for each event. But this serves as a useful sanity check of the data; if
the totals for each event don't match the summary line, something has gone
wrong.</p>
</div>
</div>
</div>
<div>
<br><table class="nav" width="100%" cellspacing="3" cellpadding="2" border="0" summary="Navigation footer">
<tr>
<td rowspan="2" width="40%" align="left">
<a accesskey="p" href="mc-manual.html">&lt;&lt; 4. Memcheck: a memory error detector</a> </td>
<td width="20%" align="center"><a accesskey="u" href="manual.html">Up</a></td>
<td rowspan="2" width="40%" align="right"> <a accesskey="n" href="cl-manual.html">6. Callgrind: a call-graph generating cache and branch prediction profiler &gt;&gt;</a>
</td>
</tr>
<tr><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td></tr>
</table>
</div>
</body>
</html>