Processing the entire Dataset#

For efficiency reasons, we form batches that contain multiple examples and process them in parallel. Especially when evaluating a model, it is important that we process all examples and avoid losing the remainder of examples that does not form a complete batch at the end.

The problem#

When evaluating on a single device, one can either drop the last incomplete batch, or one can form a last batch with a shape different from the preceding batches. Doing the latter has the disadvantage that this will trigger a recompilation of the eval_step() because XLA is not shape polymorphic.

    for batch in tfds.load('mnist', split='test').batch(per_device_batch_size)
# output:
# Counter({(272, 28, 28, 1): 1, (512, 28, 28, 1): 19})

The problem is accentuated when using multiple devices for data parallelism. If the batch size is not divisible by the number devices, then that last step must be executed on a single device (or a subset of devices). Usually one would drop the last batch, but this will lead to incorrect results.

    for batch in tfds.load('mnist', split='test')
        .batch(per_device_batch_size, drop_remainder=True)
# output:
# 9728

Using multiple hosts further complicates the situation because JAX uses the SPMD paradigm and every host must execute the same program. We would usually form non-overlapping splits for different hosts with tfds.split_for_jax_process(), but this can lead to different numbers for different hosts, resulting in different JAX programs when all examples are to be processed.

process_count = 6
    len(tfds.load(dataset_name, split=tfds.split_for_jax_process(
        'test', process_index=process_index, process_count=process_count)))
    for process_index in range(process_count)
# output:
# [1667, 1667, 1667, 1667, 1666, 1666]

The solution: padding#

Even though it’s possible to solve this problem by cleverly adjusting the number of batches executed by different devices on different hosts, such a solution quickly becomes complicated and makes the main eval loop hard to read with a lot of cumbersome logic.

The more straightforward solution to this problem is to use padding at the end of the dataset to make sure that the last batch has the same size as the preceding batches.

Manual implementation#

The last batch is manually padded to contain the same number of examples as in the preceding batches. The predictions for the padded examples are discarded from the computation.

shard = lambda x: einops.rearrange(
    x, '(d b) ... -> d b ...', d=jax.local_device_count())
unshard = lambda x: einops.rearrange(x, 'd b ... -> (d b) ...')

correct = total = 0
for batch in ds.as_numpy_iterator():
  images = batch['image']
  n = len(images)
  padding = np.zeros([per_host_batch_size - n, *images.shape[1:]], images.dtype)
  padded_images = np.concatenate([images, padding])
  preds = unshard(get_preds(variables, shard(padded_images)))[:n]
  total += n
  correct += (batch['label'] == preds.argmax(axis=-1)).sum()

Using pad_shard_unpad()#

The above pattern, namely the pad→shard→predict→unshard→unpad sequence, can be extracted into a utility wrapper pad_shard_unpad(), which greatly simplifies above evaluation loop.

correct = total = 0
for batch in ds.as_numpy_iterator():
  preds = flax.jax_utils.pad_shard_unpad(get_preds)(
      vs, batch['image'], min_device_batch=per_device_batch_size)
  total += len(batch['image'])
  correct += (batch['label'] == preds.argmax(axis=-1)).sum()

Computing metrics in eval_step()#

Instead of returning the predictions and computing the metrics in the main evaluation loop, we would often want to make the metric computation part of the evaluation step, especially when using libraries like clu.metrics, or clu.metrics.

In that case we would want to pass the metrics as a static_argnums (i.e. do not shard/pad it), and treat the return value as static_return too (i.e. no un-sharding or un-padding):

def eval_step(metrics, variables, batch):
  print('retrigger compilation', {k: v.shape for k, v in batch.items()})
  preds = model.apply(variables, batch['image'])
  correct = (batch['mask'] & (batch['label'] == preds.argmax(axis=-1))).sum()
  total = batch['mask'].sum()
  return dict(
      correct=metrics['correct'] + jax.lax.psum(correct, axis_name='batch'),
      total=metrics['total'] + jax.lax.psum(total, axis_name='batch'),

eval_step = jax.pmap(eval_step, axis_name='batch')
eval_step = flax.jax_utils.pad_shard_unpad(
    eval_step, static_argnums=(0, 1), static_return=True)

Adding “infinite padding”#

The above solution works in most cases, but it has some limitations:

  1. In the rare case where even splitting of the dataset on multiple hosts leads to a different number of batches. Imagine having a dataset of n=4097 examples, and evaluating this on h=8, each having d=8 local devices, and forming on-device batch sizes of b=128. With even dataset splitting, the first host would get 4096/8+1==513 examples, and all other hosts would get 4096/8==512 examples. Forming per-host batches of d*b==512 this would lead to two batches on the first host, and a single batch on all other hosts, violating SPMD principles and hanging the multi-host setup in the last psum() directive (which would only be executed by the first host, but not the others).

  2. When dropping examples dynamically by using ds.filter().

In these more complicated cases we could add “infinite padding” to the dataset, on each of the hosts independently, and continuing processing examples until all hosts run out of unpadded examples.

correct = total = 0
for batch in ds.as_numpy_iterator():
  n = count_p(batch['mask'])[0].item()  # adds sync barrier
  if not n: break

  preds = get_preds(vs, batch['image']).argmax(axis=-1)
  total += n
  correct += count_correct_p(batch['label'], preds, batch['mask'])[0]