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<a href="https://github.com/williamFalcon/pytorch-lightning/edit/master/docs/Trainer/Distributed training.md" title="Edit this page" class="md-icon md-content__icon">&#xE3C9;</a>
<h1>Distributed training</h1>
<p>Lightning makes multi-gpu training and 16 bit training trivial.</p>
<p><em>Note:</em> <br />
None of the flags below require changing anything about your lightningModel definition. </p>
<hr />
<h4 id="choosing-a-backend">Choosing a backend</h4>
<p>Lightning supports two backends. DataParallel and DistributedDataParallel. Both can be used for single-node multi-GPU training.
For multi-node training you must use DistributedDataParallel. </p>
<p>You can toggle between each mode by setting this flag.</p>
<pre><code class="python"># DEFAULT uses DataParallel
trainer = Trainer(distributed_backend='dp')
# change to distributed data parallel
trainer = Trainer(distributed_backend='ddp')
</code></pre>
<p>If you request multiple nodes, the back-end will auto-switch to ddp.
We recommend you use DistributedDataparallel even for single-node multi-GPU training. It is MUCH faster than DP but <em>may</em>
have configuration issues depending on your cluster.</p>
<p>For a deeper understanding of what lightning is doing, feel free to read <a href="https://medium.com/@_willfalcon/9-tips-for-training-lightning-fast-neural-networks-in-pytorch-8e63a502f565">this guide</a>. </p>
<hr />
<h4 id="distributed-and-16-bit-precision">Distributed and 16-bit precision.</h4>
<p>Due to an issue with apex and DistributedDataParallel (PyTorch and NVIDIA issue), Lightning does
not allow 16-bit and DP training. We tried to get this to work, but it's an issue on their end. </p>
<p>Below are the possible configurations we support. </p>
<table>
<thead>
<tr>
<th>1 GPU</th>
<th>1+ GPUs</th>
<th>DP</th>
<th>DDP</th>
<th>16-bit</th>
<th>command</th>
</tr>
</thead>
<tbody>
<tr>
<td>Y</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td><code>Trainer(gpus=[0])</code></td>
</tr>
<tr>
<td>Y</td>
<td></td>
<td></td>
<td></td>
<td>Y</td>
<td><code>Trainer(gpus=[0], use_amp=True)</code></td>
</tr>
<tr>
<td></td>
<td>Y</td>
<td>Y</td>
<td></td>
<td></td>
<td><code>Trainer(gpus=[0, ...])</code></td>
</tr>
<tr>
<td></td>
<td>Y</td>
<td></td>
<td>Y</td>
<td></td>
<td><code>Trainer(gpus=[0, ...], distributed_backend='ddp')</code></td>
</tr>
<tr>
<td></td>
<td>Y</td>
<td></td>
<td>Y</td>
<td>Y</td>
<td><code>Trainer(gpus=[0, ...], distributed_backend='ddp', use_amp=True)</code></td>
</tr>
</tbody>
</table>
<hr />
<h4 id="cuda-flags">CUDA flags</h4>
<p>CUDA flags make certain GPUs visible to your script.
Lightning sets these for you automatically, there's NO NEED to do this yourself.</p>
<pre><code class="python"># lightning will set according to what you give the trainer
# os.environ[&quot;CUDA_DEVICE_ORDER&quot;] = &quot;PCI_BUS_ID&quot;
# os.environ[&quot;CUDA_VISIBLE_DEVICES&quot;] = &quot;0&quot;
</code></pre>
<hr />
<h4 id="16-bit-mixed-precision">16-bit mixed precision</h4>
<p>16 bit precision can cut your memory footprint by half. If using volta architecture GPUs it can give a dramatic training speed-up as well. <br />
First, install apex (if install fails, look <a href="https://github.com/NVIDIA/apex">here</a>):</p>
<pre><code class="bash">$ git clone https://github.com/NVIDIA/apex
$ cd apex
$ pip install -v --no-cache-dir --global-option=&quot;--cpp_ext&quot; --global-option=&quot;--cuda_ext&quot; ./
</code></pre>
<p>then set this use_amp to True.</p>
<pre><code class="python"># DEFAULT
trainer = Trainer(amp_level='O2', use_amp=False)
</code></pre>
<hr />
<h4 id="single-gpu">Single-gpu</h4>
<p>Make sure you're on a GPU machine. </p>
<pre><code class="python"># DEFAULT
trainer = Trainer(gpus=[0])
</code></pre>
<hr />
<h4 id="multi-gpu">multi-gpu</h4>
<p>Make sure you're on a GPU machine. You can set as many GPUs as you want.
In this setting, the model will run on all 8 GPUs at once using DataParallel under the hood.</p>
<pre><code class="python"># to use DataParallel (default)
trainer = Trainer(gpus=[0,1,2,3,4,5,6,7], distributed_backend='dp')
# RECOMMENDED use DistributedDataParallel
trainer = Trainer(gpus=[0,1,2,3,4,5,6,7], distributed_backend='ddp')
</code></pre>
<hr />
<h4 id="multi-node">Multi-node</h4>
<p>Multi-node training is easily done by specifying these flags.</p>
<pre><code class="python"># train on 12*8 GPUs
trainer = Trainer(gpus=[0,1,2,3,4,5,6,7], nb_gpu_nodes=12)
</code></pre>
<p>In addition, make sure to set up your SLURM job correctly via the <a href="https://williamfalcon.github.io/test-tube/hpc/SlurmCluster/">SlurmClusterObject</a>. In particular, specify the number of tasks per node correctly.</p>
<pre><code class="python">cluster = SlurmCluster(
hyperparam_optimizer=test_tube.HyperOptArgumentParser(),
log_path='/some/path/to/save',
)
# OPTIONAL FLAGS WHICH MAY BE CLUSTER DEPENDENT
# which interface your nodes use for communication
cluster.add_command('export NCCL_SOCKET_IFNAME=^docker0,lo')
# see output of the NCCL connection process
# NCCL is how the nodes talk to each other
cluster.add_command('export NCCL_DEBUG=INFO')
# setting a master port here is a good idea.
cluster.add_command('export MASTER_PORT=%r' % PORT)
# good to load the latest NCCL version
cluster.load_modules(['NCCL/2.4.7-1-cuda.10.0'])
# configure cluster
cluster.per_experiment_nb_nodes = 12
cluster.per_experiment_nb_gpus = 8
cluster.add_slurm_cmd(cmd='ntasks-per-node', value=8, comment='1 task per gpu')
</code></pre>
<p>Finally, make sure to add a distributed sampler to your dataset. The distributed sampler copies a
portion of your dataset onto each GPU. (World_size = gpus_per_node * nb_nodes). </p>
<pre><code class="python"># ie: this:
dataset = myDataset()
dataloader = Dataloader(dataset)
# becomes:
dataset = myDataset()
dist_sampler = torch.utils.data.distributed.DistributedSampler(dataset)
dataloader = Dataloader(dataset, sampler=dist_sampler)
</code></pre>
<hr />
<h4 id="self-balancing-architecture">Self-balancing architecture</h4>
<p>Here lightning distributes parts of your module across available GPUs to optimize for speed and memory. </p>
<p>COMING SOON.</p>
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