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<h1>Checkpointing</h1>
<p>Lightning can automate saving and loading checkpoints.</p>
<hr />
<h3 id="model-saving">Model saving</h3>
<p>Checkpointing is enabled by default to the current working directory.
To change the checkpoint path pass in :</p>
<table class="codehilitetable"><tr><td class="linenos"><div class="linenodiv"><pre>1</pre></div></td><td class="code"><div class="codehilite"><pre><span></span><span class="n">Trainer</span><span class="p">(</span><span class="n">default_save_path</span><span class="o">=</span><span class="s1">&#39;/your/path/to/save/checkpoints&#39;</span><span class="p">)</span>
</pre></div>
</td></tr></table>
<p>To modify the behavior of checkpointing pass in your own callback.</p>
<table class="codehilitetable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
2
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13</pre></div></td><td class="code"><div class="codehilite"><pre><span></span><span class="kn">from</span> <span class="nn">pytorch_lightning.callbacks</span> <span class="kn">import</span> <span class="n">ModelCheckpoint</span>
<span class="c1"># DEFAULTS used by the Trainer</span>
<span class="n">checkpoint_callback</span> <span class="o">=</span> <span class="n">ModelCheckpoint</span><span class="p">(</span>
<span class="n">filepath</span><span class="o">=</span><span class="n">os</span><span class="o">.</span><span class="n">getcwd</span><span class="p">(),</span>
<span class="n">save_best_only</span><span class="o">=</span><span class="bp">True</span><span class="p">,</span>
<span class="n">verbose</span><span class="o">=</span><span class="bp">True</span><span class="p">,</span>
<span class="n">monitor</span><span class="o">=</span><span class="s1">&#39;val_loss&#39;</span><span class="p">,</span>
<span class="n">mode</span><span class="o">=</span><span class="s1">&#39;min&#39;</span><span class="p">,</span>
<span class="n">prefix</span><span class="o">=</span><span class="s1">&#39;&#39;</span>
<span class="p">)</span>
<span class="n">trainer</span> <span class="o">=</span> <span class="n">Trainer</span><span class="p">(</span><span class="n">checkpoint_callback</span><span class="o">=</span><span class="n">checkpoint_callback</span><span class="p">)</span>
</pre></div>
</td></tr></table>
<hr />
<h3 id="restoring-training-session">Restoring training session</h3>
<p>You might want to not only load a model but also continue training it. Use this method to
restore the trainer state as well. This will continue from the epoch and global step you last left off.<br />
However, the dataloaders will start from the first batch again (if you shuffled it shouldn't matter). </p>
<p>Lightning will restore the session if you pass a logger with the same version and there's a saved checkpoint. </p>
<table class="codehilitetable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
2
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16</pre></div></td><td class="code"><div class="codehilite"><pre><span></span><span class="kn">from</span> <span class="nn">pytorch_lightning</span> <span class="kn">import</span> <span class="n">Trainer</span>
<span class="kn">from</span> <span class="nn">pytorch_lightning.logging</span> <span class="kn">import</span> <span class="n">TestTubeLogger</span>
<span class="n">logger</span> <span class="o">=</span> <span class="n">TestTubeLogger</span><span class="p">(</span>
<span class="n">save_dir</span><span class="o">=</span><span class="s1">&#39;./savepath&#39;</span><span class="p">,</span>
<span class="n">version</span><span class="o">=</span><span class="mi">1</span> <span class="c1"># An existing version with a saved checkpoint</span>
<span class="p">)</span>
<span class="n">trainer</span> <span class="o">=</span> <span class="n">Trainer</span><span class="p">(</span>
<span class="n">logger</span><span class="o">=</span><span class="n">logger</span><span class="p">,</span>
<span class="n">default_save_path</span><span class="o">=</span><span class="s1">&#39;./savepath&#39;</span>
<span class="p">)</span>
<span class="c1"># this fit call loads model weights and trainer state</span>
<span class="c1"># the trainer continues seamlessly from where you left off</span>
<span class="c1"># without having to do anything else.</span>
<span class="n">trainer</span><span class="o">.</span><span class="n">fit</span><span class="p">(</span><span class="n">model</span><span class="p">)</span>
</pre></div>
</td></tr></table>
<p>The trainer restores: </p>
<ul>
<li>global_step </li>
<li>current_epoch </li>
<li>All optimizers </li>
<li>All lr_schedulers </li>
<li>Model weights</li>
</ul>
<p>You can even change the logic of your model as long as the weights and "architecture" of
the system isn't different. If you add a layer, for instance, it might not work. </p>
<p>At a rough level, here's <a href="https://github.com/williamFalcon/pytorch-lightning/blob/master/pytorch_lightning/root_module/model_saving.py#L63">what happens inside Trainer</a>: </p>
<table class="codehilitetable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
2
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15</pre></div></td><td class="code"><div class="codehilite"><pre><span></span><span class="bp">self</span><span class="o">.</span><span class="n">global_step</span> <span class="o">=</span> <span class="n">checkpoint</span><span class="p">[</span><span class="s1">&#39;global_step&#39;</span><span class="p">]</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_epoch</span> <span class="o">=</span> <span class="n">checkpoint</span><span class="p">[</span><span class="s1">&#39;epoch&#39;</span><span class="p">]</span>
<span class="c1"># restore the optimizers</span>
<span class="n">optimizer_states</span> <span class="o">=</span> <span class="n">checkpoint</span><span class="p">[</span><span class="s1">&#39;optimizer_states&#39;</span><span class="p">]</span>
<span class="k">for</span> <span class="n">optimizer</span><span class="p">,</span> <span class="n">opt_state</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">optimizers</span><span class="p">,</span> <span class="n">optimizer_states</span><span class="p">):</span>
<span class="n">optimizer</span><span class="o">.</span><span class="n">load_state_dict</span><span class="p">(</span><span class="n">opt_state</span><span class="p">)</span>
<span class="c1"># restore the lr schedulers</span>
<span class="n">lr_schedulers</span> <span class="o">=</span> <span class="n">checkpoint</span><span class="p">[</span><span class="s1">&#39;lr_schedulers&#39;</span><span class="p">]</span>
<span class="k">for</span> <span class="n">scheduler</span><span class="p">,</span> <span class="n">lrs_state</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">lr_schedulers</span><span class="p">,</span> <span class="n">lr_schedulers</span><span class="p">):</span>
<span class="n">scheduler</span><span class="o">.</span><span class="n">load_state_dict</span><span class="p">(</span><span class="n">lrs_state</span><span class="p">)</span>
<span class="c1"># uses the model you passed into trainer </span>
<span class="n">model</span><span class="o">.</span><span class="n">load_state_dict</span><span class="p">(</span><span class="n">checkpoint</span><span class="p">[</span><span class="s1">&#39;state_dict&#39;</span><span class="p">])</span>
</pre></div>
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