Redistribute test exercise extensively various sharding schemes and
redistribution between them. These tests uncovered more edge cases
that were not supported by the local tensor primarily different flavors
of uneven sharding. In order to handle these cases this change implements
missing functional collectives and adds support for uneven sharding
case where sharding group (ranks) is larger than the size of the dimension
being sharded. In the latter case the "missing" shards are represented
by zero sized tensors so that the rest of the local tensor machinery
can stay oblivious to this special case.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/166081
Approved by: https://github.com/ezyang
A LocalTensor is a tensor subclass which simulates a tensor that is
distributed across SPMD ranks. A LocalTensor might be size N, but in fact
there are world_size shards/replicas of it stored internally. When you do a
plain PyTorch operation on it, we apply the operation to each shard; when you
do a collective, we do the mathematically equivalent operation on the local
shards. A LocalTensor is associated with a list of ranks which specify
which ranks it holds local tensors for.
NB, this is NOT a DataParallel like abstraction where you can run operations
on multiple different GPUs. It is intended purely for *debugging* purposes,
the overhead is almost certainly too high to keep eight GPUs (even the C++
autograd needs multithreading to keep up!) (It might potentially be possible
to trace through this with torch.compile and then compile it with CUDA graphs
but this is currently a non-goal.)
In order to handle MPMD, we provide a helper decorator that allows you to
run a function with no side effects for each LocalTensor shard and combine
results back into LocalTensor or LocalIntNode.
Note: This PR convert all DTensor ops and some DTensor tests to illustrate
intended usage and ensure conrrectness. In subsequent PR more tests will be
converted. DUring test conversion we aim to share as much as possible of
test logic between multi-process / multi-threaded and local tensor tests.
We would like to developers to be able to run both flavors of the tests.
Note: This work is based on the original proposal
by @ezyang (WIP PR https://github.com/pytorch/pytorch/pull/162753).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/164537
Approved by: https://github.com/ezyang
