use flash attn fuse cross entropy loss to reduce metric memory usage #2987
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@cli99 consider doing it as in https://github.com/mosaicml/llm-foundry/pull/575/files, to avoid introducing flash attn as a composer dependency. |
j316chuck
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Feb 9, 2024
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| try: | ||
| from flash_attn.losses.cross_entropy import CrossEntropyLoss as FusedCrossEntropyLoss | ||
| self.loss_fn = FusedCrossEntropyLoss(ignore_index=ignore_index, reduction='sum') | ||
| except ImportError: | ||
| self.loss_fn = torch.nn.CrossEntropyLoss(ignore_index=ignore_index, reduction='sum') |
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Should this live in llm-foundry? CC: @dakinggg
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i have a very old pr that i never merged
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@cli99 should we close or is this still WIP |
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This PR uses fused cross entropy loss from flash attention in the metric LanguageCrossEntropy (also LanguagePerplexity).
The current torch.nn.CrossEntropyLoss call needs
6 * seq_len * vocab_sizeGPU memory, and can be the bottleneck memory usage when sequence length is long (where act ckpt is probably used). Using cross entropy loss from flash attn resolves this problem.Example test model with long sequence and full act ckpt:
with torch loss fn:
with flash_attn loss fn
