# Multi-GPU models¶

Training deep learning models often requires large amounts of data and computation resources, rendering the process quite time-expensive. Models are trained for hours and/or days, which makes experimenting with different configurations cumbersome and slow.

Performance gain with multiple GPUs in emloop-tensorflow framework.

One way to speed up the process is utilizing multiple GPUs for model training. This may significantly lower the required time, but the implementation is not always straightforward. Luckily enough, we implemented the necessary routines to make arbitrary TensorFlow models eligible for multi-GPU training.

## Compatible models¶

Chances are, your TensorFlow model is already ready for multi-GPU training in emloop-tensorflow. In fact, the only requirement is to use tf.get_variable instead of tf.Variable, so that variables can be reused and shared over multiple GPUs.

## Multi-GPU training¶

To train on (multiple) GPUs, define the n_gpus parameter in the configuration file. E.g.:

config.yaml
model:
class: nets.MyMultiGPUNet
n_gpus: 4
...


That’s it! Now run the emloop training and check if all your GPUs are in fact utilized.

Tip

Full example may be found in our emloop examples repository @GitHub.

Tip

Increase your dataset batch size together with the n_gpus parameter. Each batch is distributed evenly between the GPUs alowing for much greater batch sizes!

## Implementation details¶

Tower architecture for multi GPU training.

In order to leverage the true potential of multiple GPUs, emloop-tensorflow creates a bit more complicated computational graph than usual. In fact, the whole model is multiplicated so that each GPU has its own tower. The variables to be trained are, naturally, shared between the towers. Each GPU computes its own feed-forward pass and the respective variable gradients. Consequently, the gradients are averaged and used for update.

During the training emloop-tensorflow distributes each batch equally among the towers. The fetched outputs are concatenated preserving their orders which ensures perfect abstraction for the remaining emloop components. From the perspective of hooks and datasets, nothing has changed. Additionally, emloop-tensorflow handles incomplete batches automatically.

A model trained on multiple GPUs may be restored and used for inference with equal or less amount of GPUs or even on CPU without any trouble.