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<link title="Weak" rel="Chapter" href="Weak.html"><title>Gc.Memprof</title>
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<h1>Module <a href="type_Gc.Memprof.html">Gc.Memprof</a></h1>
<pre><span id="MODULEMemprof"><span class="keyword">module</span> Memprof</span>: <code class="code"><span class="keyword">sig</span></code> <a href="Gc.Memprof.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info module top">
<div class="info-desc">
<p><code class="code"><span class="constructor">Memprof</span></code> is a sampling engine for allocated memory words. Every
allocated word has a probability of being sampled equal to a
configurable sampling rate. Once a block is sampled, it becomes
tracked. A tracked block triggers a user-defined callback as soon
as it is allocated, promoted or deallocated.</p>
<p>Since blocks are composed of several words, a block can potentially
be sampled several times. If a block is sampled several times, then
each of the callback is called once for each event of this block:
the multiplicity is given in the <code class="code">n_samples</code> field of the
<code class="code">allocation</code> structure.</p>
<p>This engine makes it possible to implement a low-overhead memory
profiler as an OCaml library.</p>
<p>Note: this API is EXPERIMENTAL. It may change without prior
notice.</p>
</div>
</div>
<hr width="100%">
<pre><code><span id="TYPEallocation"><span class="keyword">type</span> <code class="type"></code>allocation</span> = private {</code></pre><table class="typetable">
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTallocation.n_samples">n_samples</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>The number of samples in this block (>= 1).</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTallocation.size">size</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>The size of the block, in words, excluding the header.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTallocation.unmarshalled">unmarshalled</span> : <code class="type">bool</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Whether the block comes from unmarshalling.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTallocation.callstack">callstack</span> : <code class="type"><a href="Printexc.html#TYPEraw_backtrace">Printexc.raw_backtrace</a></code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>The callstack for the allocation.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr></table>
<code>}</code>
<div class="info ">
<div class="info-desc">
<p>The type of metadata associated with allocations. This is the
type of records passed to the callback triggered by the
sampling of an allocation.</p>
</div>
</div>
<pre><code><span id="TYPEtracker"><span class="keyword">type</span> <code class="type">('minor, 'major)</code> tracker</span> = {</code></pre><table class="typetable">
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTtracker.alloc_minor">alloc_minor</span> : <code class="type"><a href="Gc.Memprof.html#TYPEallocation">allocation</a> -> 'minor option</code>;</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTtracker.alloc_major">alloc_major</span> : <code class="type"><a href="Gc.Memprof.html#TYPEallocation">allocation</a> -> 'major option</code>;</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTtracker.promote">promote</span> : <code class="type">'minor -> 'major option</code>;</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTtracker.dealloc_minor">dealloc_minor</span> : <code class="type">'minor -> unit</code>;</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTtracker.dealloc_major">dealloc_major</span> : <code class="type">'major -> unit</code>;</code></td>
</tr></table>
<code>}</code>
<div class="info ">
<div class="info-desc">
<p>A <code class="code">(<span class="keywordsign">'</span>minor, <span class="keywordsign">'</span>major) tracker</code> describes how memprof should track
sampled blocks over their lifetime, keeping a user-defined piece
of metadata for each of them: <code class="code"><span class="keywordsign">'</span>minor</code> is the type of metadata
to keep for minor blocks, and <code class="code"><span class="keywordsign">'</span>major</code> the type of metadata
for major blocks.</p>
<p>If an allocation-tracking or promotion-tracking function returns <code class="code"><span class="constructor">None</span></code>,
memprof stops tracking the corresponding value.</p>
</div>
</div>
<pre><span id="VALnull_tracker"><span class="keyword">val</span> null_tracker</span> : <code class="type">('minor, 'major) <a href="Gc.Memprof.html#TYPEtracker">tracker</a></code></pre><div class="info ">
<div class="info-desc">
<p>Default callbacks simply return <code class="code"><span class="constructor">None</span></code> or <code class="code">()</code></p>
</div>
</div>
<pre><span id="VALstart"><span class="keyword">val</span> start</span> : <code class="type">sampling_rate:float -><br> ?callstack_size:int -> ('minor, 'major) <a href="Gc.Memprof.html#TYPEtracker">tracker</a> -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>Start the sampling with the given parameters. Fails if
sampling is already active.</p>
<p>The parameter <code class="code">sampling_rate</code> is the sampling rate in samples
per word (including headers). Usually, with cheap callbacks, a
rate of 1e-4 has no visible effect on performance, and 1e-3
causes the program to run a few percent slower</p>
<p>The parameter <code class="code">callstack_size</code> is the length of the callstack
recorded at every sample. Its default is <code class="code">max_int</code>.</p>
<p>The parameter <code class="code">tracker</code> determines how to track sampled blocks
over their lifetime in the minor and major heap.</p>
<p>Sampling is temporarily disabled when calling a callback
for the current thread. So they do not need to be reentrant if
the program is single-threaded. However, if threads are used,
it is possible that a context switch occurs during a callback,
in this case the callback functions must be reentrant.</p>
<p>Note that the callback can be postponed slightly after the
actual event. The callstack passed to the callback is always
accurate, but the program state may have evolved.</p>
<p>Calling <code class="code"><span class="constructor">Thread</span>.exit</code> in a callback is currently unsafe and can
result in undefined behavior.</p>
</div>
</div>
<pre><span id="VALstop"><span class="keyword">val</span> stop</span> : <code class="type">unit -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>Stop the sampling. Fails if sampling is not active.</p>
<p>This function does not allocate memory, but tries to run the
postponed callbacks for already allocated memory blocks (of
course, these callbacks may allocate).</p>
<p>All the already tracked blocks are discarded.</p>
<p>Calling <code class="code">stop</code> when a callback is running can lead to
callbacks not being called even though some events happened.</p>
</div>
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