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jax.lax.all_gather

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jax.lax.all_gather#

jax.lax.all_gather(x, axis_name, *, axis_index_groups=None, axis=0, tiled=False)[source]#

Gather values of x across all replicas.

If x is a pytree then the result is equivalent to mapping this function to each leaf in the tree.

This is equivalent to, but faster than, all_to_all(broadcast(x)).

Parameters:

  • x – array(s) with a mapped axis named axis_name.

  • axis_name – hashable Python object used to name a pmapped axis (see the jax.pmap() documentation for more details).

  • axis_index_groups – optional list of lists containing axis indices (e.g. for an axis of size 4, [[0, 1], [2, 3]] would run all gather over the first two and last two replicas). Groups must cover all axis indices exactly once, and all groups must be the same size.

  • axis – a positional axis into which the chunks along axis_name will be concatenated.

  • tiled – when False, the chunks will be stacked into a fresh positional axis at index axis in the output. When True, axis has to refer to an existing positional dimension and the chunks will be concatenated into that dimension.

Returns:

Array(s) representing the result of an all-gather along the axis axis_name. Shapes are the same as x.shape, but:

  • when tiled is False, there is a new dimension equal to the size of axis axis_name in position axis,

  • when tiled is True, the size of dimension in position axis is multiplied by the size of axis axis_name.

For example, with 4 XLA devices available:

>>> x = np.arange(4)
>>> y = jax.pmap(lambda x: jax.lax.all_gather(x, 'i'), axis_name='i')(x)
>>> print(y)
[[0 1 2 3]
 [0 1 2 3]
 [0 1 2 3]
 [0 1 2 3]]

An example of using axis_index_groups, groups split by even & odd device ids:

>>> x = np.arange(16).reshape(4, 4)
>>> print(x)
  [[ 0  1  2  3]
   [ 4  5  6  7]
   [ 8  9 10 11]
   [12 13 14 15]]
>>> def f(x):
...   return jax.lax.all_gather(
...       x, 'i', axis_index_groups=[[0, 2], [3, 1]])
>>> y = jax.pmap(f, axis_name='i')(x)
>>> print(y)
[[[ 0  1  2  3]
  [ 8  9 10 11]]
 [[12 13 14 15]
  [ 4  5  6  7]]
 [[ 0  1  2  3]
  [ 8  9 10 11]]
 [[12 13 14 15]
  [ 4  5  6  7]]]
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