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<p>This is the CPU profiler we use at Google.  There are three parts
to using it: linking the library into an application, running the
code, and analyzing the output.</p>

<p>On the off-chance that you should need to understand it, the CPU
profiler data file format is documented separately,
<a href="cpuprofile-fileformat.html">here</a>.


<H1>Linking in the Library</H1>

<p>To install the CPU profiler into your executable, add
<code>-lprofiler</code> to the link-time step for your executable.
(It's also probably possible to add in the profiler at run-time using
<code>LD_PRELOAD</code>, e.g.
<code>% env LD_PRELOAD="/usr/lib/libprofiler.so" &lt;binary&gt;</code>,
but this isn't necessarily recommended.)</p>

<p>This does <i>not</i> turn on CPU profiling; it just inserts the
code.  For that reason, it's practical to just always link
<code>-lprofiler</code> into a binary while developing; that's what we
do at Google.  (However, since any user can turn on the profiler by
setting an environment variable, it's not necessarily recommended to
install profiler-linked binaries into a production, running
system.)</p>


<H1>Running the Code</H1>

<p>There are several alternatives to actually turn on CPU profiling
for a given run of an executable:</p>

<ol>
  <li> <p>Define the environment variable CPUPROFILE to the filename
       to dump the profile to.  For instance, if you had a version of
       <code>/bin/ls</code> that had been linked against libprofiler,
       you could run:</p>
       <pre>% env CPUPROFILE=ls.prof /bin/ls</pre>

  <li> <p>In your code, bracket the code you want profiled in calls to
       <code>ProfilerStart()</code> and <code>ProfilerStop()</code>.
       (These functions are declared in <code>&lt;gperftools/profiler.h&gt;</code>.)
       <code>ProfilerStart()</code> will take
       the profile-filename as an argument.</p>
</ol>

<p>In Linux 2.6 and above, profiling works correctly with threads,
automatically profiling all threads.  In Linux 2.4, profiling only
profiles the main thread (due to a kernel bug involving itimers and
threads).  Profiling works correctly with sub-processes: each child
process gets its own profile with its own name (generated by combining
CPUPROFILE with the child's process id).</p>

<p>For security reasons, CPU profiling will not write to a file -- and
is thus not usable -- for setuid programs.</p>

<p>See the include-file <code>gperftools/profiler.h</code> for
advanced-use functions, including <code>ProfilerFlush()</code> and
<code>ProfilerStartWithOptions()</code>.</p>


<H2>Modifying Runtime Behavior</H2>

<p>You can more finely control the behavior of the CPU profiler via
environment variables.</p>

<table frame=box rules=sides cellpadding=5 width=100%>

<tr valign=top>
  <td><code>CPUPROFILE_FREQUENCY=<i>x</i></code></td>
  <td>default: 100</td>
  <td>
    How many interrupts/second the cpu-profiler samples.
  </td>
</tr>

<tr valign=top>
  <td><code>CPUPROFILE_REALTIME=1</code></td>
  <td>default: [not set]</td>
  <td>
    If set to any value (including 0 or the empty string), use
    ITIMER_REAL instead of ITIMER_PROF to gather profiles.  In
    general, ITIMER_REAL is not as accurate as ITIMER_PROF, and also
    interacts badly with use of alarm(), so prefer ITIMER_PROF unless
    you have a reason prefer ITIMER_REAL.
  </td>
</tr>

</table>


<h1><a name="pprof">Analyzing the Output</a></h1>

<p><code>pprof</code> is the script used to analyze a profile.  It has
many output modes, both textual and graphical.  Some give just raw
numbers, much like the <code>-pg</code> output of <code>gcc</code>,
and others show the data in the form of a dependency graph.</p>

<p>pprof <b>requires</b> <code>perl5</code> to be installed to run.
It also requires <code>dot</code> to be installed for any of the
graphical output routines, and <code>gv</code> to be installed for
<code>--gv</code> mode (described below).
</p>

<p>Here are some ways to call pprof.  These are described in more
detail below.</p>

<pre>
% pprof /bin/ls ls.prof
                       Enters "interactive" mode
% pprof --text /bin/ls ls.prof
                       Outputs one line per procedure
% pprof --gv /bin/ls ls.prof
                       Displays annotated call-graph via 'gv'
% pprof --gv --focus=Mutex /bin/ls ls.prof
                       Restricts to code paths including a .*Mutex.* entry
% pprof --gv --focus=Mutex --ignore=string /bin/ls ls.prof
                       Code paths including Mutex but not string
% pprof --list=getdir /bin/ls ls.prof
                       (Per-line) annotated source listing for getdir()
% pprof --disasm=getdir /bin/ls ls.prof
                       (Per-PC) annotated disassembly for getdir()
% pprof --text localhost:1234
                       Outputs one line per procedure for localhost:1234
% pprof --callgrind /bin/ls ls.prof
                       Outputs the call information in callgrind format
</pre>


<h3>Analyzing Text Output</h3>

<p>Text mode has lines of output that look like this:</p>
<pre>
       14   2.1%  17.2%       58   8.7% std::_Rb_tree::find
</pre>

<p>Here is how to interpret the columns:</p>
<ol>
  <li> Number of profiling samples in this function
  <li> Percentage of profiling samples in this function
  <li> Percentage of profiling samples in the functions printed so far
  <li> Number of profiling samples in this function and its callees
  <li> Percentage of profiling samples in this function and its callees
  <li> Function name
</ol>

<h3>Analyzing Callgrind Output</h3>

<p>Use <a href="http://kcachegrind.sourceforge.net">kcachegrind</a> to
analyze your callgrind output:</p>
<pre>
% pprof --callgrind /bin/ls ls.prof > ls.callgrind
% kcachegrind ls.callgrind
</pre>

<p>The cost is specified in 'hits', i.e. how many times a function
appears in the recorded call stack information. The 'calls' from
function a to b record how many times function b was found in the
stack traces directly below function a.</p>

<p>Tip: if you use a debug build the output will include file and line
number information and kcachegrind will show an annotated source
code view.</p>

<h3>Node Information</h3>

<p>In the various graphical modes of pprof, the output is a call graph
annotated with timing information, like so:</p>

<A HREF="pprof-test-big.gif">
<center><table><tr><td>
   <img src="pprof-test.gif">
</td></tr></table></center>
</A>

<p>Each node represents a procedure.  The directed edges indicate
caller to callee relations.  Each node is formatted as follows:</p>

<center><pre>
Class Name
Method Name
local (percentage)
<b>of</b> cumulative (percentage)
</pre></center>

<p>The last one or two lines contains the timing information.  (The
profiling is done via a sampling method, where by default we take 100
samples a second.  Therefor one unit of time in the output corresponds
to about 10 milliseconds of execution time.) The "local" time is the
time spent executing the instructions directly contained in the
procedure (and in any other procedures that were inlined into the
procedure).  The "cumulative" time is the sum of the "local" time and
the time spent in any callees.  If the cumulative time is the same as
the local time, it is not printed.</p>

<p>For instance, the timing information for test_main_thread()
indicates that 155 units (about 1.55 seconds) were spent executing the
code in <code>test_main_thread()</code> and 200 units were spent while
executing <code>test_main_thread()</code> and its callees such as
<code>snprintf()</code>.</p>

<p>The size of the node is proportional to the local count.  The
percentage displayed in the node corresponds to the count divided by
the total run time of the program (that is, the cumulative count for
<code>main()</code>).</p>

<h3>Edge Information</h3>

<p>An edge from one node to another indicates a caller to callee
relationship.  Each edge is labelled with the time spent by the callee
on behalf of the caller.  E.g, the edge from
<code>test_main_thread()</code> to <code>snprintf()</code> indicates
that of the 200 samples in <code>test_main_thread()</code>, 37 are
because of calls to <code>snprintf()</code>.</p>

<p>Note that <code>test_main_thread()</code> has an edge to
<code>vsnprintf()</code>, even though <code>test_main_thread()</code>
doesn't call that function directly.  This is because the code was
compiled with <code>-O2</code>; the profile reflects the optimized
control flow.</p>

<h3>Meta Information</h3>

<p>The top of the display should contain some meta information
like:</p>
<pre>
      /tmp/profiler2_unittest
      Total samples: 202
      Focusing on: 202
      Dropped nodes with &lt;= 1 abs(samples)
      Dropped edges with &lt;= 0 samples
</pre>

<p>This section contains the name of the program, and the total
samples collected during the profiling run.  If the
<code>--focus</code> option is on (see the <a href="#focus">Focus</a>
section below), the legend also contains the number of samples being
shown in the focused display.  Furthermore, some unimportant nodes and
edges are dropped to reduce clutter.  The characteristics of the
dropped nodes and edges are also displayed in the legend.</p>

<h3><a name=focus>Focus and Ignore</a></h3>

<p>You can ask pprof to generate a display focused on a particular
piece of the program.  You specify a regular expression.  Any portion
of the call-graph that is on a path which contains at least one node
matching the regular expression is preserved.  The rest of the
call-graph is dropped on the floor.  For example, you can focus on the
<code>vsnprintf()</code> libc call in <code>profiler2_unittest</code>
as follows:</p>

<pre>
% pprof --gv --focus=vsnprintf /tmp/profiler2_unittest test.prof
</pre>
<A HREF="pprof-vsnprintf-big.gif">
<center><table><tr><td>
   <img src="pprof-vsnprintf.gif">
</td></tr></table></center>
</A>

<p>Similarly, you can supply the <code>--ignore</code> option to
ignore samples that match a specified regular expression.  E.g., if
you are interested in everything except calls to
<code>snprintf()</code>, you can say:</p>
<pre>
% pprof --gv --ignore=snprintf /tmp/profiler2_unittest test.prof
</pre>


<h3>Interactive mode</a></h3>

<p>By default -- if you don't specify any flags to the contrary --
pprof runs in interactive mode.  At the <code>(pprof)</code> prompt,
you can run many of the commands described above.  You can type
<code>help</code> for a list of what commands are available in
interactive mode.</p>

<h3><a name=options>pprof Options</a></h3>

For a complete list of pprof options, you can run <code>pprof
--help</code>.

<h4>Output Type</h4>

<p>
<center>
<table frame=box rules=sides cellpadding=5 width=100%>
<tr valign=top>
  <td><code>--text</code></td>
  <td>
    Produces a textual listing.  (Note: If you have an X display, and
    <code>dot</code> and <code>gv</code> installed, you will probably
    be happier with the <code>--gv</code> output.)
  </td>
</tr>
<tr valign=top>
  <td><code>--gv</code></td>
  <td>
    Generates annotated call-graph, converts to postscript, and
    displays via gv (requres <code>dot</code> and <code>gv</code> be
    installed).
  </td>
</tr>
<tr valign=top>
  <td><code>--dot</code></td>
  <td>
    Generates the annotated call-graph in dot format and
    emits to stdout (requres <code>dot</code> be installed).
  </td>
</tr>
<tr valign=top>
  <td><code>--ps</code></td>
  <td>
    Generates the annotated call-graph in Postscript format and
    emits to stdout (requres <code>dot</code> be installed).
  </td>
</tr>
<tr valign=top>
  <td><code>--pdf</code></td>
  <td>
    Generates the annotated call-graph in PDF format and emits to
    stdout (requires <code>dot</code> and <code>ps2pdf</code> be
    installed).
  </td>
</tr>
<tr valign=top>
  <td><code>--gif</code></td>
  <td>
    Generates the annotated call-graph in GIF format and
    emits to stdout (requres <code>dot</code> be installed).
  </td>
</tr>
<tr valign=top>
  <td><code>--list=&lt;<i>regexp</i>&gt;</code></td>
  <td>
    <p>Outputs source-code listing of routines whose
    name matches &lt;regexp&gt;.  Each line
    in the listing is annotated with flat and cumulative
    sample counts.</p>

    <p>In the presence of inlined calls, the samples
    associated with inlined code tend to get assigned
    to a line that follows the location of the
    inlined call.  A more precise accounting can be
    obtained by disassembling the routine using the
    --disasm flag.</p>
  </td>
</tr>
<tr valign=top>
  <td><code>--disasm=&lt;<i>regexp</i>&gt;</code></td>
  <td>
    Generates disassembly of routines that match
    &lt;regexp&gt;, annotated with flat and
    cumulative sample counts and emits to stdout.
  </td>
</tr>
</table>
</center>

<h4>Reporting Granularity</h4>

<p>By default, pprof produces one entry per procedure.  However you can
use one of the following options to change the granularity of the
output.  The <code>--files</code> option seems to be particularly
useless, and may be removed eventually.</p>

<center>
<table frame=box rules=sides cellpadding=5 width=100%>
<tr valign=top>
  <td><code>--addresses</code></td>
  <td>
     Produce one node per program address.
  </td>
</tr>
  <td><code>--lines</code></td>
  <td>
     Produce one node per source line.
  </td>
</tr>
  <td><code>--functions</code></td>
  <td>
     Produce one node per function (this is the default).
  </td>
</tr>
  <td><code>--files</code></td>
  <td>
     Produce one node per source file.
  </td>
</tr>
</table>
</center>

<h4>Controlling the Call Graph Display</h4>

<p>Some nodes and edges are dropped to reduce clutter in the output
display.  The following options control this effect:</p>

<center>
<table frame=box rules=sides cellpadding=5 width=100%>
<tr valign=top>
  <td><code>--nodecount=&lt;n&gt;</code></td>
  <td>
    This option controls the number of displayed nodes.  The nodes
    are first sorted by decreasing cumulative count, and then only
    the top N nodes are kept.  The default value is 80.
  </td>
</tr>
<tr valign=top>
  <td><code>--nodefraction=&lt;f&gt;</code></td>
  <td>
    This option provides another mechanism for discarding nodes
    from the display.  If the cumulative count for a node is
    less than this option's value multiplied by the total count
    for the profile, the node is dropped.  The default value
    is 0.005; i.e. nodes that account for less than
    half a percent of the total time are dropped.  A node
    is dropped if either this condition is satisfied, or the
    --nodecount condition is satisfied.
  </td>
</tr>
<tr valign=top>
  <td><code>--edgefraction=&lt;f&gt;</code></td>
  <td>
    This option controls the number of displayed edges.  First of all,
    an edge is dropped if either its source or destination node is
    dropped.  Otherwise, the edge is dropped if the sample
    count along the edge is less than this option's value multiplied
    by the total count for the profile.  The default value is
    0.001; i.e., edges that account for less than
    0.1% of the total time are dropped.
  </td>
</tr>
<tr valign=top>
  <td><code>--focus=&lt;re&gt;</code></td>
  <td>
    This option controls what region of the graph is displayed
    based on the regular expression supplied with the option.
    For any path in the callgraph, we check all nodes in the path
    against the supplied regular expression.  If none of the nodes
    match, the path is dropped from the output.
  </td>
</tr>
<tr valign=top>
  <td><code>--ignore=&lt;re&gt;</code></td>
  <td>
    This option controls what region of the graph is displayed
    based on the regular expression supplied with the option.
    For any path in the callgraph, we check all nodes in the path
    against the supplied regular expression.  If any of the nodes
    match, the path is dropped from the output.
  </td>
</tr>
</table>
</center>

<p>The dropped edges and nodes account for some count mismatches in
the display.  For example, the cumulative count for
<code>snprintf()</code> in the first diagram above was 41.  However
the local count (1) and the count along the outgoing edges (12+1+20+6)
add up to only 40.</p>


<h1>Caveats</h1>

<ul>
  <li> If the program exits because of a signal, the generated profile
       will be <font color=red>incomplete, and may perhaps be
       completely empty</font>.
  <li> The displayed graph may have disconnected regions because
       of the edge-dropping heuristics described above.
  <li> If the program linked in a library that was not compiled
       with enough symbolic information, all samples associated
       with the library may be charged to the last symbol found
       in the program before the library.  This will artificially
       inflate the count for that symbol.
  <li> If you run the program on one machine, and profile it on
       another, and the shared libraries are different on the two
       machines, the profiling output may be confusing: samples that
       fall within  shared libaries may be assigned to arbitrary
       procedures.
  <li> If your program forks, the children will also be profiled
       (since they inherit the same CPUPROFILE setting).  Each process
       is profiled separately; to distinguish the child profiles from
       the parent profile and from each other, all children will have
       their process-id appended to the CPUPROFILE name.
  <li> Due to a hack we make to work around a possible gcc bug, your
       profiles may end up named strangely if the first character of
       your CPUPROFILE variable has ascii value greater than 127.
       This should be exceedingly rare, but if you need to use such a
       name, just set prepend <code>./</code> to your filename:
       <code>CPUPROFILE=./&Auml;gypten</code>.
</ul>


<hr>
<address>Sanjay Ghemawat<br>
<!-- Created: Tue Dec 19 10:43:14 PST 2000 -->
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Last modified: Fri May  9 14:41:29 PDT 2008
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