Training on a TPU Pod

Overview

One of the benefits of Cloud TPU is that you can scale training between different sized TPU types. All TPU types use the same data-parallel architecture. The only change is that the parallelism increase from 8 cores to 2048 cores.

However, you must tune several parameters when you train on larger TPU types. This document explains some common issues and the necessary changes to your models.

Scaling batch size and train steps

To achieve linear scaling on larger TPU types, keep the per core batch size the same.

For example if you use a batch size of 1024 on a v2-8, use a batch size of 4096 (4 * 1024) on a v2-32. This is to fully utilize the TPU hardware. You can use smaller batch sizes, however your training will not scale linearly if you do so.

Many models include a train_steps flag where one step corresponds to a single batch of data. If you increase the batch size, scale down the number of training steps, so the total number of training examples remains the same. For example if the original configuration was a batch size of 1024 for 1000 steps, after increasing the batch size to 4096 you should reduce the steps to 250 (1000 / 4). If the model uses an epochs flag, you do not need to scale the number of steps.

Larger batch sizes can change convergence behavior of the model, so you might also tune some hyperparameters.

Using large batch optimizers

If you scale up to use larger batch sizes, you might also need to use an optimizer that supports large batch sizes. For example, the reference Resnet model uses the LARSOptimizer for larger TPU types.

Using regional Google Cloud Storage buckets in the same region as the TPU Pod

In general, the best practice for TPU training is to always use resources in the same region. Resource region is particularly important when using TPU Pods because the rate of data transfer from Google Cloud Storage tends to be higher. Ensure you are using a regional Google Cloud Storage bucket in the same region as the TPU for training datasets and checkpoints.

Providing the TPU name and region to TPUClusterResolver

For single device training, you can specify either the TPU name or an IP address, for example: grpc://1.2.3.4:8470.

For TPU Pods you must use the TPU name so that TensorFlow can discover the IP addresses of all the hosts available for training distribution.

Specifying the IP address for a TPU Pod generates the following error:

ValueError: TPUConfig.num_shards is not set correctly ....

If you need to run your model and TPU in different zones or regions, you must provide TPUClusterResolver with the TPU zone.

You can pass the tpu and tpu_zone flags to reference models or directly set the values for TPUClusterResolver. For example:

cluster = tf.contrib.cluster_resolver.TPUClusterResolver(
  tpu='tpuname',
  zone='europe-west4-a',
  project='myproject')

A related issue that can occur when using TPUEstimator is not passing in the cluster resolver to tf.contrib.tpu.RunConfig. This could happen if RunConfig is initialized with the master parameter. Instead, initialize it with the cluster parameter.

config = tf.contrib.tpu.RunConfig(cluster=cluster)

If you are using a TPU Pod and tf.Session without using TPUEstimator, you must configure the session config, as follows:

with tf.Session(cluster.get_master(), config=config.session_config) as sess:

Evaluation on TPU Pods

Evaluation is neither supported nor cost-effective on TPU Pods. Google recommends running evaluation on a single-device TPU using the mode flag, which is supported by most models.

For training on a TPU Pod or single-device TPU, set the flag to train. For evaluation, set eval for single-device TPUs only. You can also use the mode train_and_eval on single-device TPUS, but not on TPU Pods.

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