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Use allgatherv for sparse allreduce #22226

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pietern opened this issue Jun 25, 2019 · 1 comment
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Use allgatherv for sparse allreduce #22226

pietern opened this issue Jun 25, 2019 · 1 comment
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@zhaojuanmao
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enhancement Not as big of a feature, but technically not a bug. Should be easy to fix oncall: distributed Add this issue/PR to distributed oncall triage queue triaged This issue has been looked at a team member, and triaged and prioritized into an appropriate module

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@pietern
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pietern commented Jun 25, 2019

The current sparse allreduce in ProcessGroupGloo pads the indices and values tensors to the maximum length across all processes and then performs a regular allgather (because they'll have equal size across processes). Instead, we can use allgatherv, and avoid the padding trick, once #22225 is merged. This is mostly a win for memory usage is there is severe size imbalance between processes. The runtime likely won't change much, do to the nature of the underlying allgather implementation (it takes N steps where each step takes an amount of time proportional to the size of the largest contribution).
@pietern pietern added oncall: distributed Add this issue/PR to distributed oncall triage queue triaged This issue has been looked at a team member, and triaged and prioritized into an appropriate module enhancement Not as big of a feature, but technically not a bug. Should be easy to fix labels Jun 25, 2019
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pietern commented Jun 27, 2019

cc @zhaojuanmao

@zhaojuanmao zhaojuanmao self-assigned this Jul 1, 2019
@zhaojuanmao zhaojuanmao mentioned this issue Aug 7, 2019
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zhaojuanmao added a commit to zhaojuanmao/pytorch that referenced this issue Sep 18, 2019
@zhaojuanmao @facebook-github-bot
use allgatherv for sparse all reduce (pytorch#23917)
e7800ca 
Summary:
per pytorch#22226, The current sparse allreduce in ProcessGroupGloo pads the indices and values tensors to the maximum length across all processes and then performs a regular allgather (because they'll have equal size across processes). Instead, we can use allgatherv. This is mostly a win for memory usage if there is severe size imbalance between processes.

close pytorch#22226
Pull Request resolved: pytorch#23917

Test Plan:
buck run mode/dev-nosan caffe2/test:c10d -- test_c10d.ProcessGroupGlooTest.test_sparse_allreduce_basics

buck run mode/dev-nosan caffe2/test:c10d -- test_c10d.ProcessGroupGlooTest.test_sparse_allreduce_basics_cuda

buck run mode/dev-nosan caffe2/test:c10d -- test_c10d.ProcessGroupGlooTest.test_sparse_allreduce_checks

Differential Revision: D16664985

Pulled By: zhaojuanmao

fbshipit-source-id: a9a139da2b64617d2bb7f0b12f272e920140e5d1
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enhancement Not as big of a feature, but technically not a bug. Should be easy to fix oncall: distributed Add this issue/PR to distributed oncall triage queue triaged This issue has been looked at a team member, and triaged and prioritized into an appropriate module
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use allgatherv for sparse all reduce zhaojuanmao/pytorch
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