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  <div class="section" id="torch-utils-bottleneck">
<h1>torch.utils.bottleneck<a class="headerlink" href="#torch-utils-bottleneck" title="Permalink to this headline">¶</a></h1>
<p><cite>torch.utils.bottleneck</cite> is a tool that can be used as an initial step for
debugging bottlenecks in your program. It summarizes runs of your script with
the Python profiler and PyTorch’s autograd profiler.</p>
<p>Run it on the command line with</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">python</span> <span class="o">-</span><span class="n">m</span> <span class="n">torch</span><span class="o">.</span><span class="n">utils</span><span class="o">.</span><span class="n">bottleneck</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">source</span><span class="o">/</span><span class="n">script</span><span class="o">.</span><span class="n">py</span> <span class="p">[</span><span class="n">args</span><span class="p">]</span>
</pre></div>
</div>
<p>where [args] are any number of arguments to <cite>script.py</cite>, or run
<code class="docutils literal notranslate"><span class="pre">python</span> <span class="pre">-m</span> <span class="pre">torch.utils.bottleneck</span> <span class="pre">-h</span></code> for more usage instructions.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>Because your script will be profiled, please ensure that it exits in a
finite amount of time.</p>
</div>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>Due to the asynchronous nature of CUDA kernels, when running against
CUDA code, the cProfile output and CPU-mode autograd profilers may
not show correct timings: the reported CPU time reports the amount of time
used to launch the kernels but does not include the time the kernel
spent executing on a GPU unless the operation does a synchronize.
Ops that do synchronize appear to be extremely expensive under regular
CPU-mode profilers.
In these case where timings are incorrect, the CUDA-mode autograd profiler
may be helpful.</p>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>To decide which (CPU-only-mode or CUDA-mode) autograd profiler output to
look at, you should first check if your script is CPU-bound
(“CPU total time is much greater than CUDA total time”).
If it is CPU-bound, looking at the results of the CPU-mode autograd
profiler will help. If on the other hand your script spends most of its
time executing on the GPU, then it makes sense to start
looking for responsible CUDA operators in the output of the CUDA-mode
autograd profiler.</p>
<p>Of course the reality is much more complicated and your script might not be
in one of those two extremes depending on the part of the model you’re
evaluating. If the profiler outputs don’t help, you could try looking at
the result of <a class="reference internal" href="autograd.html#torch.autograd.profiler.emit_nvtx" title="torch.autograd.profiler.emit_nvtx"><code class="xref py py-func docutils literal notranslate"><span class="pre">torch.autograd.profiler.emit_nvtx()</span></code></a> with <code class="docutils literal notranslate"><span class="pre">nvprof</span></code>.
However, please take into account that the NVTX overhead is very high and
often gives a heavily skewed timeline.</p>
</div>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>If you are profiling CUDA code, the first profiler that <code class="docutils literal notranslate"><span class="pre">bottleneck</span></code> runs
(cProfile) will include the CUDA startup time (CUDA buffer allocation cost)
in its time reporting. This should not matter if your bottlenecks result
in code much slower than the CUDA startup time.</p>
</div>
<p>For more complicated uses of the profilers (like in a multi-GPU case),
please see <a class="reference external" href="https://docs.python.org/3/library/profile.html">https://docs.python.org/3/library/profile.html</a>
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