def alltoall(self, x, mesh_axis, split_axis, concat_axis):
"""Grouped alltoall (like MPI alltoall with splitting and concatenation).
Args:
x: a LaidOutTensor
mesh_axis: an integer the mesh axis along which to group
split_axis: an integer (the Tensor axis along which to split)
concat_axis: an integer (the Tensor axis along which to concatenate)
Returns:
a LaidOutTensor
"""
x = x.to_laid_out_tensor()
t = x.one_slice
group_assignment = self._create_group_assignment([mesh_axis])
dtype = t.dtype
if dtype == tf.float32:
# There seems to be a bug with float32 alltoall.
# Do it in bfloat16 until the bug is fixed.
# TODO(noam): file a bug
t = tf.to_bfloat16(t)
t = tpu_ops.all_to_all(t,
concat_dimension=concat_axis,
split_dimension=split_axis,
split_count=len(group_assignment[0]),
group_assignment=group_assignment)
t = tf.cast(t, dtype)
x = self.LaidOutTensor([t])
return x
def _to_bfloat16_unbiased(x, noise):
"""Convert a float32 to a bfloat16 using randomized roundoff.
Args:
x: A float32 Tensor.
noise: a float32 Tensor with values in [0, 1), broadcastable to tf.shape(x)
Returns:
A float32 Tensor.
"""
x_sign = tf.sign(x)
# Make sure x is positive. If it is zero, the two candidates are identical.
x = x * x_sign + 1e-30
cand1 = tf.to_bfloat16(x)
cand1_f = tf.to_float(cand1)
# This relies on the fact that for a positive bfloat16 b,
# b * 1.005 gives you the next higher bfloat16 and b*0.995 gives you the
# next lower one. Both 1.005 and 0.995 are ballpark estimation.
cand2 = tf.to_bfloat16(
tf.where(tf.greater(x, cand1_f), cand1_f * 1.005, cand1_f * 0.995))
ret = _randomized_roundoff_to_bfloat16(x, noise, cand1, cand2)
return ret * tf.to_bfloat16(x_sign)
