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<div class="section" id="automatic-mixed-precision-examples">
<span id="amp-examples"></span><h1>Automatic Mixed Precision examples<a class="headerlink" href="#automatic-mixed-precision-examples" title="Permalink to this headline">¶</a></h1>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p><a class="reference internal" href="../amp.html#torch.cuda.amp.GradScaler" title="torch.cuda.amp.GradScaler"><code class="xref py py-class docutils literal notranslate"><span class="pre">torch.cuda.amp.GradScaler</span></code></a> is not a complete implementation of automatic mixed precision.
<a class="reference internal" href="../amp.html#torch.cuda.amp.GradScaler" title="torch.cuda.amp.GradScaler"><code class="xref py py-class docutils literal notranslate"><span class="pre">GradScaler</span></code></a> is only useful if you manually run regions of your model in <code class="docutils literal notranslate"><span class="pre">float16</span></code>.
If you aren’t sure how to choose op precision manually, the master branch and nightly pip/conda
builds include a context manager that chooses op precision automatically wherever it’s enabled.
See the <a class="reference external" href="https://pytorch.org/docs/master/amp.html">master documentation</a> for details.</p>
</div>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><p><a class="reference internal" href="#gradient-scaling" id="id2">Gradient Scaling</a></p>
<ul>
<li><p><a class="reference internal" href="#typical-use" id="id3">Typical Use</a></p></li>
<li><p><a class="reference internal" href="#working-with-unscaled-gradients" id="id4">Working with Unscaled Gradients</a></p>
<ul>
<li><p><a class="reference internal" href="#gradient-clipping" id="id5">Gradient clipping</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#working-with-scaled-gradients" id="id6">Working with Scaled Gradients</a></p>
<ul>
<li><p><a class="reference internal" href="#gradient-penalty" id="id7">Gradient penalty</a></p></li>
</ul>
</li>
<li><p><a class="reference internal" href="#working-with-multiple-losses-and-optimizers" id="id8">Working with Multiple Losses and Optimizers</a></p></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="gradient-scaling">
<span id="gradient-scaling-examples"></span><h2><a class="toc-backref" href="#id2">Gradient Scaling</a><a class="headerlink" href="#gradient-scaling" title="Permalink to this headline">¶</a></h2>
<p>Gradient scaling helps prevent gradient underflow when training with mixed precision,
as explained <a class="reference internal" href="../amp.html#gradient-scaling"><span class="std std-ref">here</span></a>.</p>
<p>Instances of <a class="reference internal" href="../amp.html#torch.cuda.amp.GradScaler" title="torch.cuda.amp.GradScaler"><code class="xref py py-class docutils literal notranslate"><span class="pre">torch.cuda.amp.GradScaler</span></code></a> help perform the steps of
gradient scaling conveniently, as shown in the following code snippets.</p>
<div class="section" id="typical-use">
<h3><a class="toc-backref" href="#id3">Typical Use</a><a class="headerlink" href="#typical-use" title="Permalink to this headline">¶</a></h3>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># Creates a GradScaler once at the beginning of training.</span>
<span class="n">scaler</span> <span class="o">=</span> <span class="n">GradScaler</span><span class="p">()</span>
<span class="k">for</span> <span class="n">epoch</span> <span class="ow">in</span> <span class="n">epochs</span><span class="p">:</span>
<span class="k">for</span> <span class="nb">input</span><span class="p">,</span> <span class="n">target</span> <span class="ow">in</span> <span class="n">data</span><span class="p">:</span>
<span class="n">optimizer</span><span class="o">.</span><span class="n">zero_grad</span><span class="p">()</span>
<span class="n">output</span> <span class="o">=</span> <span class="n">model</span><span class="p">(</span><span class="nb">input</span><span class="p">)</span>
<span class="n">loss</span> <span class="o">=</span> <span class="n">loss_fn</span><span class="p">(</span><span class="n">output</span><span class="p">,</span> <span class="n">target</span><span class="p">)</span>
<span class="c1"># Scales the loss, and calls backward() on the scaled loss to create scaled gradients.</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">scale</span><span class="p">(</span><span class="n">loss</span><span class="p">)</span><span class="o">.</span><span class="n">backward</span><span class="p">()</span>
<span class="c1"># scaler.step() first unscales the gradients of the optimizer's assigned params.</span>
<span class="c1"># If these gradients do not contain infs or NaNs, optimizer.step() is then called,</span>
<span class="c1"># otherwise, optimizer.step() is skipped.</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">step</span><span class="p">(</span><span class="n">optimizer</span><span class="p">)</span>
<span class="c1"># Updates the scale for next iteration.</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">update</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="working-with-unscaled-gradients">
<span id="id1"></span><h3><a class="toc-backref" href="#id4">Working with Unscaled Gradients</a><a class="headerlink" href="#working-with-unscaled-gradients" title="Permalink to this headline">¶</a></h3>
<p>All gradients produced by <code class="docutils literal notranslate"><span class="pre">scaler.scale(loss).backward()</span></code> are scaled. If you wish to modify or inspect
the parameters’ <code class="docutils literal notranslate"><span class="pre">.grad</span></code> attributes between <code class="docutils literal notranslate"><span class="pre">backward()</span></code> and <code class="docutils literal notranslate"><span class="pre">scaler.step(optimizer)</span></code>, you should
unscale them first. For example, gradient clipping manipulates a set of gradients such that their global norm
(see <a class="reference internal" href="../nn.html#torch.nn.utils.clip_grad_norm_" title="torch.nn.utils.clip_grad_norm_"><code class="xref py py-func docutils literal notranslate"><span class="pre">torch.nn.utils.clip_grad_norm_()</span></code></a>) or maximum magnitude (see <a class="reference internal" href="../nn.html#torch.nn.utils.clip_grad_value_" title="torch.nn.utils.clip_grad_value_"><code class="xref py py-func docutils literal notranslate"><span class="pre">torch.nn.utils.clip_grad_value_()</span></code></a>)
is <span class="math"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo><</mo><mo>=</mo></mrow><annotation encoding="application/x-tex"><=</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.5782em;vertical-align:-0.0391em;"></span><span class="mrel"><</span></span><span class="base"><span class="strut" style="height:0.36687em;vertical-align:0em;"></span><span class="mrel">=</span></span></span></span>
</span> some user-imposed threshold. If you attempted to clip <em>without</em> unscaling, the gradients’ norm/maximum
magnitude would also be scaled, so your requested threshold (which was meant to be the threshold for <em>unscaled</em>
gradients) would be invalid.</p>
<p><code class="docutils literal notranslate"><span class="pre">scaler.unscale_(optimizer)</span></code> unscales gradients held by <code class="docutils literal notranslate"><span class="pre">optimizer</span></code>’s assigned parameters.
If your model or models contain other parameters that were assigned to another optimizer
(say <code class="docutils literal notranslate"><span class="pre">optimizer2</span></code>), you may call <code class="docutils literal notranslate"><span class="pre">scaler.unscale_(optimizer2)</span></code> separately to unscale those
parameters’ gradients as well.</p>
<div class="section" id="gradient-clipping">
<h4><a class="toc-backref" href="#id5">Gradient clipping</a><a class="headerlink" href="#gradient-clipping" title="Permalink to this headline">¶</a></h4>
<p>Calling <code class="docutils literal notranslate"><span class="pre">scaler.unscale_(optimizer)</span></code> before clipping enables you to clip unscaled gradients as usual:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">scaler</span> <span class="o">=</span> <span class="n">GradScaler</span><span class="p">()</span>
<span class="k">for</span> <span class="n">epoch</span> <span class="ow">in</span> <span class="n">epochs</span><span class="p">:</span>
<span class="k">for</span> <span class="nb">input</span><span class="p">,</span> <span class="n">target</span> <span class="ow">in</span> <span class="n">data</span><span class="p">:</span>
<span class="n">optimizer</span><span class="o">.</span><span class="n">zero_grad</span><span class="p">()</span>
<span class="n">output</span> <span class="o">=</span> <span class="n">model</span><span class="p">(</span><span class="nb">input</span><span class="p">)</span>
<span class="n">loss</span> <span class="o">=</span> <span class="n">loss_fn</span><span class="p">(</span><span class="n">output</span><span class="p">,</span> <span class="n">target</span><span class="p">)</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">scale</span><span class="p">(</span><span class="n">loss</span><span class="p">)</span><span class="o">.</span><span class="n">backward</span><span class="p">()</span>
<span class="c1"># Unscales the gradients of optimizer's assigned params in-place</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">unscale_</span><span class="p">(</span><span class="n">optimizer</span><span class="p">)</span>
<span class="c1"># Since the gradients of optimizer's assigned params are unscaled, clips as usual:</span>
<span class="n">torch</span><span class="o">.</span><span class="n">nn</span><span class="o">.</span><span class="n">utils</span><span class="o">.</span><span class="n">clip_grad_norm_</span><span class="p">(</span><span class="n">model</span><span class="o">.</span><span class="n">parameters</span><span class="p">(),</span> <span class="n">max_norm</span><span class="p">)</span>
<span class="c1"># optimizer's gradients are already unscaled, so scaler.step does not unscale them,</span>
<span class="c1"># although it still skips optimizer.step() if the gradients contain infs or NaNs.</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">step</span><span class="p">(</span><span class="n">optimizer</span><span class="p">)</span>
<span class="c1"># Updates the scale for next iteration.</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">update</span><span class="p">()</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">scaler</span></code> records that <code class="docutils literal notranslate"><span class="pre">scaler.unscale_(optimizer)</span></code> was already called for this optimizer
this iteration, so <code class="docutils literal notranslate"><span class="pre">scaler.step(optimizer)</span></code> knows not to redundantly unscale gradients before
(internally) calling <code class="docutils literal notranslate"><span class="pre">optimizer.step()</span></code>.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p><code class="xref py py-meth docutils literal notranslate"><span class="pre">unscale_()</span></code> should only be called once per optimizer per <code class="xref py py-meth docutils literal notranslate"><span class="pre">step()</span></code> call,
and only after all gradients for that optimizer’s assigned parameters have been accumulated.
Calling <code class="xref py py-meth docutils literal notranslate"><span class="pre">unscale_()</span></code> twice for a given optimizer between each <code class="xref py py-meth docutils literal notranslate"><span class="pre">step()</span></code> triggers a RuntimeError.</p>
</div>
</div>
</div>
<div class="section" id="working-with-scaled-gradients">
<h3><a class="toc-backref" href="#id6">Working with Scaled Gradients</a><a class="headerlink" href="#working-with-scaled-gradients" title="Permalink to this headline">¶</a></h3>
<p>For some operations, you may need to work with scaled gradients in a setting where
<code class="docutils literal notranslate"><span class="pre">scaler.unscale_</span></code> is unsuitable.</p>
<div class="section" id="gradient-penalty">
<h4><a class="toc-backref" href="#id7">Gradient penalty</a><a class="headerlink" href="#gradient-penalty" title="Permalink to this headline">¶</a></h4>
<p>A gradient penalty implementation typically creates gradients out-of-place using
<a class="reference internal" href="../autograd.html#torch.autograd.grad" title="torch.autograd.grad"><code class="xref py py-func docutils literal notranslate"><span class="pre">torch.autograd.grad()</span></code></a>, combines them to create the penalty value,
and adds the penalty value to the loss.</p>
<p>Here’s an ordinary example of an L2 penalty without gradient scaling:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">for</span> <span class="n">epoch</span> <span class="ow">in</span> <span class="n">epochs</span><span class="p">:</span>
<span class="k">for</span> <span class="nb">input</span><span class="p">,</span> <span class="n">target</span> <span class="ow">in</span> <span class="n">data</span><span class="p">:</span>
<span class="n">optimizer</span><span class="o">.</span><span class="n">zero_grad</span><span class="p">()</span>
<span class="n">output</span> <span class="o">=</span> <span class="n">model</span><span class="p">(</span><span class="nb">input</span><span class="p">)</span>
<span class="n">loss</span> <span class="o">=</span> <span class="n">loss_fn</span><span class="p">(</span><span class="n">output</span><span class="p">,</span> <span class="n">target</span><span class="p">)</span>
<span class="c1"># Creates some gradients out-of-place</span>
<span class="n">grad_params</span> <span class="o">=</span> <span class="n">torch</span><span class="o">.</span><span class="n">autograd</span><span class="o">.</span><span class="n">grad</span><span class="p">(</span><span class="n">loss</span><span class="p">,</span> <span class="n">model</span><span class="o">.</span><span class="n">parameters</span><span class="p">(),</span> <span class="n">create_graph</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="c1"># Computes the penalty term and adds it to the loss</span>
<span class="n">grad_norm</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="n">grad</span> <span class="ow">in</span> <span class="n">grad_params</span><span class="p">:</span>
<span class="n">grad_norm</span> <span class="o">+=</span> <span class="n">grad</span><span class="o">.</span><span class="n">pow</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span><span class="o">.</span><span class="n">sum</span><span class="p">()</span>
<span class="n">grad_norm</span> <span class="o">=</span> <span class="n">grad_norm</span><span class="o">.</span><span class="n">sqrt</span><span class="p">()</span>
<span class="n">loss</span> <span class="o">=</span> <span class="n">loss</span> <span class="o">+</span> <span class="n">grad_norm</span>
<span class="n">loss</span><span class="o">.</span><span class="n">backward</span><span class="p">()</span>
<span class="n">optimizer</span><span class="o">.</span><span class="n">step</span><span class="p">()</span>
</pre></div>
</div>
<p>To implement a gradient penalty <em>with</em> gradient scaling, the loss passed to
<a class="reference internal" href="../autograd.html#torch.autograd.grad" title="torch.autograd.grad"><code class="xref py py-func docutils literal notranslate"><span class="pre">torch.autograd.grad()</span></code></a> should be scaled. The resulting out-of-place gradients
will therefore be scaled, and should be unscaled before being combined to create the
penalty value.</p>
<p>Here’s how that looks for the same L2 penalty:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">scaler</span> <span class="o">=</span> <span class="n">GradScaler</span><span class="p">()</span>
<span class="k">for</span> <span class="n">epoch</span> <span class="ow">in</span> <span class="n">epochs</span><span class="p">:</span>
<span class="k">for</span> <span class="nb">input</span><span class="p">,</span> <span class="n">target</span> <span class="ow">in</span> <span class="n">data</span><span class="p">:</span>
<span class="n">optimizer</span><span class="o">.</span><span class="n">zero_grad</span><span class="p">()</span>
<span class="n">output</span> <span class="o">=</span> <span class="n">model</span><span class="p">(</span><span class="nb">input</span><span class="p">)</span>
<span class="n">loss</span> <span class="o">=</span> <span class="n">loss_fn</span><span class="p">(</span><span class="n">output</span><span class="p">,</span> <span class="n">target</span><span class="p">)</span>
<span class="c1"># Scales the loss for the out-of-place backward pass, resulting in scaled grad_params</span>
<span class="n">scaled_grad_params</span> <span class="o">=</span> <span class="n">torch</span><span class="o">.</span><span class="n">autograd</span><span class="o">.</span><span class="n">grad</span><span class="p">(</span><span class="n">scaler</span><span class="o">.</span><span class="n">scale</span><span class="p">(</span><span class="n">loss</span><span class="p">),</span> <span class="n">model</span><span class="o">.</span><span class="n">parameters</span><span class="p">(),</span> <span class="n">create_graph</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="c1"># Unscales grad_params before computing the penalty. grad_params are not owned</span>
<span class="c1"># by any optimizer, so ordinary division is used instead of scaler.unscale_:</span>
<span class="n">inv_scale</span> <span class="o">=</span> <span class="mf">1.</span><span class="o">/</span><span class="n">scaler</span><span class="o">.</span><span class="n">get_scale</span><span class="p">()</span>
<span class="n">grad_params</span> <span class="o">=</span> <span class="p">[</span><span class="n">p</span><span class="o">*</span><span class="n">inv_scale</span> <span class="k">for</span> <span class="n">p</span> <span class="ow">in</span> <span class="n">scaled_grad_params</span><span class="p">]</span>
<span class="c1"># Computes the penalty term and adds it to the loss</span>
<span class="n">grad_norm</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="n">grad</span> <span class="ow">in</span> <span class="n">grad_params</span><span class="p">:</span>
<span class="n">grad_norm</span> <span class="o">+=</span> <span class="n">grad</span><span class="o">.</span><span class="n">pow</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span><span class="o">.</span><span class="n">sum</span><span class="p">()</span>
<span class="n">grad_norm</span> <span class="o">=</span> <span class="n">grad_norm</span><span class="o">.</span><span class="n">sqrt</span><span class="p">()</span>
<span class="n">loss</span> <span class="o">=</span> <span class="n">loss</span> <span class="o">+</span> <span class="n">grad_norm</span>
<span class="c1"># Applies scaling to the backward call as usual. Accumulates leaf gradients that are correctly scaled.</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">scale</span><span class="p">(</span><span class="n">loss</span><span class="p">)</span><span class="o">.</span><span class="n">backward</span><span class="p">()</span>
<span class="c1"># step() and update() proceed as usual.</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">step</span><span class="p">(</span><span class="n">optimizer</span><span class="p">)</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">update</span><span class="p">()</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="working-with-multiple-losses-and-optimizers">
<h3><a class="toc-backref" href="#id8">Working with Multiple Losses and Optimizers</a><a class="headerlink" href="#working-with-multiple-losses-and-optimizers" title="Permalink to this headline">¶</a></h3>
<p>If your network has multiple losses, you must call <code class="docutils literal notranslate"><span class="pre">scaler.scale</span></code> on each of them individually.
If your network has multiple optimizers, you may call <code class="docutils literal notranslate"><span class="pre">scaler.unscale_</span></code> on any of them individually,
and you must call <code class="docutils literal notranslate"><span class="pre">scaler.step</span></code> on each of them individually.</p>
<p>However, <code class="docutils literal notranslate"><span class="pre">scaler.update()</span></code> should only be called once,
after all optimizers used this iteration have been stepped:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">scaler</span> <span class="o">=</span> <span class="n">torch</span><span class="o">.</span><span class="n">cuda</span><span class="o">.</span><span class="n">amp</span><span class="o">.</span><span class="n">GradScaler</span><span class="p">()</span>
<span class="k">for</span> <span class="n">epoch</span> <span class="ow">in</span> <span class="n">epochs</span><span class="p">:</span>
<span class="k">for</span> <span class="nb">input</span><span class="p">,</span> <span class="n">target</span> <span class="ow">in</span> <span class="n">data</span><span class="p">:</span>
<span class="n">optimizer0</span><span class="o">.</span><span class="n">zero_grad</span><span class="p">()</span>
<span class="n">optimizer1</span><span class="o">.</span><span class="n">zero_grad</span><span class="p">()</span>
<span class="n">output0</span> <span class="o">=</span> <span class="n">model0</span><span class="p">(</span><span class="nb">input</span><span class="p">)</span>
<span class="n">output1</span> <span class="o">=</span> <span class="n">model1</span><span class="p">(</span><span class="nb">input</span><span class="p">)</span>
<span class="n">loss0</span> <span class="o">=</span> <span class="n">loss_fn</span><span class="p">(</span><span class="mi">2</span> <span class="o">*</span> <span class="n">output0</span> <span class="o">+</span> <span class="mi">3</span> <span class="o">*</span> <span class="n">output1</span><span class="p">,</span> <span class="n">target</span><span class="p">)</span>
<span class="n">loss1</span> <span class="o">=</span> <span class="n">loss_fn</span><span class="p">(</span><span class="mi">3</span> <span class="o">*</span> <span class="n">output0</span> <span class="o">-</span> <span class="mi">5</span> <span class="o">*</span> <span class="n">output1</span><span class="p">,</span> <span class="n">target</span><span class="p">)</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">scale</span><span class="p">(</span><span class="n">loss0</span><span class="p">)</span><span class="o">.</span><span class="n">backward</span><span class="p">(</span><span class="n">retain_graph</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">scale</span><span class="p">(</span><span class="n">loss1</span><span class="p">)</span><span class="o">.</span><span class="n">backward</span><span class="p">()</span>
<span class="c1"># You can choose which optimizers receive explicit unscaling, if you</span>
<span class="c1"># want to inspect or modify the gradients of the params they own.</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">unscale_</span><span class="p">(</span><span class="n">optimizer0</span><span class="p">)</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">step</span><span class="p">(</span><span class="n">optimizer0</span><span class="p">)</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">step</span><span class="p">(</span><span class="n">optimizer1</span><span class="p">)</span>
<span class="n">scaler</span><span class="o">.</span><span class="n">update</span><span class="p">()</span>
</pre></div>
</div>
<p>Each optimizer independently checks its gradients for infs/NaNs, and therefore makes an independent decision
whether or not to skip the step. This may result in one optimizer skipping the step
while the other one does not. Since step skipping occurs rarely (every several hundred iterations)
this should not impede convergence. If you observe poor convergence after adding gradient scaling
to a multiple-optimizer model, please file an issue.</p>
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