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<h1 class="menu-title">Notes</h1>
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<h1 id="callgrind"><a class="header" href="#callgrind">callgrind</a></h1>
<p>Callgrind is a tracing profiler which records the function call history of a
target program and collects the number of executed instructions. It is part of
the <a href="https://valgrind.org/docs/manual/cl-manual.html">valgrind</a> tool suite.</p>
<p>Profiling data is collected by instrumentation rather than sampling of the
target program.</p>
<p>Callgrind does not capture the actual time spent in a function but computes the
<em>inclusive</em> & <em>exclusive</em> cost of a function based on the instructions fetched
(<code>Ir = Instruction read</code>). This provides reproducibility and high-precision and
is a major difference to sampling profilers like <code>perf</code> or <code>vtune</code>.
Therefore effects like slow IO are not reflected, which should be kept in mind
when analyzing callgrind results.</p>
<p>By default the profiler data is dumped when the target process is terminating,
but <a href="https://valgrind.org/docs/manual/cl-manual.html#cl-manual.callgrind_control-options">callgrind_control</a> allows for interactive control of callgrind.</p>
<pre><code class="language-bash"># Run a program under callgrind.
valgrind --tool=callgrind -- <prog> [<args>]
# Interactive control of callgrind.
callgrind_control [opts]
opts:
-b ............. show current backtrace
-e ............. show current event counters
-s ............. show current stats
--dump[=file] .. dump current collection
-i=on|off ...... turn instrumentation on|off
</code></pre>
<p>Results can be analyzed by using one of the following tools</p>
<ul>
<li><a href="https://valgrind.org/docs/manual/cl-manual.html#cl-manual.callgrind_annotate-options">callgrind_annotate</a> (cli)
<pre><code class="language-sh"># Show only specific trace events (default is all).
callgrind_annotate --show=Ir,Dr,Dw [callgrind_out_file]
</code></pre>
</li>
<li><a href="https://kcachegrind.github.io/html/Home.html">kcachegrind</a> (ui)</li>
</ul>
<p>The following is a collection of frequently used callgrind options.</p>
<pre><code class="language-bash">valgrind --tool=callgrind [opts] -- <prog>
opts:
--callgrind-out-file=<file> .... output file, rather than callgrind.out.<pid>
--dump-instr=<yes|no> .......... annotation on instrucion level,
allows for asm annotations
--instr-atstart=<yes|no> ....... control if instrumentation is enabled from
beginning of the program
--separate-threads=<yes|no> .... create separate output files per thread,
appends -<thread_id> to the output file
--cache-sim=<yes|no> ........... control if cache simulation is enabled
</code></pre>
<h2 id="trace-events"><a class="header" href="#trace-events">Trace events</a></h2>
<p>By default callgrind collects following events:</p>
<ul>
<li><code>Ir</code>: Instruction read</li>
</ul>
<p>Callgrind also provides a functional cache simulation with their own model,
which is enabled by passing <code>--cache-sim=yes</code>.
This simulates a 2-level cache hierarchy with separate L1 <em>instruction</em> and
<em>data</em> caches (<code>L1i</code>/ <code>L1d</code>) and a <em>unified</em> last level (<code>LL</code>) cache.
When enabled, this collects the following additional events:</p>
<ul>
<li><code>I1mr</code>: L1 cache miss on instruction read</li>
<li><code>ILmr</code>: LL cache miss on instruction read</li>
<li><code>Dr</code>: Data reads access</li>
<li><code>D1mr</code>: L1 cache miss on data read</li>
<li><code>DLmr</code>: LL cache miss on data read</li>
<li><code>Dw</code>: Data write access</li>
<li><code>D1mw</code>: L1 cache miss on data write</li>
<li><code>DLmw</code>: LL cache miss on data write</li>
</ul>
<h2 id="profile-specific-part-of-the-target"><a class="header" href="#profile-specific-part-of-the-target">Profile specific part of the target</a></h2>
<p>Programmatically enable/disable instrumentation using the macros defined in
the callgrind header.</p>
<pre><code class="language-c">#include <valgrind/callgrind.h>
int main() {
// init ..
CALLGRIND_START_INSTRUMENTATION;
compute();
CALLGRIND_STOP_INSTRUMENTATION;
// shutdown ..
}
</code></pre>
<blockquote>
<p>In this case, callgrind should be launched with <code>--instr-atstart=no</code>.</p>
</blockquote>
<p>Alternatively instrumentation can be controlled with <code>callgrind_control -i on/off</code>.</p>
<p>The files <a href="callgrind/cg_example.cc">cg_example.cc</a> and
<a href="callgrind/Makefile">Makefile</a> provide a full example.</p>
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