[Model] Phi-3 4k sliding window temp. fix #4380
Conversation
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@caiom Should we apply this padding in |
For a window size of 2047 we have:
This is true for all Models that uses a sliding window. Regardless of the PR, for Mistral, the block_manager_v1 will think the window size is smaller than it actually is (with the flash backend). The window size of mistralai/Mistral-7B-v0.1 is 4096 and hence the flash backend will have a window with 4097 tokens, while block_manager_v1 is assuming 4096 tokens. For Phi-3, with this PR, block_manager_v1 may assume a larger number of tokens than the actual number. For instance, with the window size of 2047, the PR will add one and hence the block_manager_v1 will see it as 2048 tokens. However, with sdpa, that number is 2047. So, there's mismatch between block_manager_v1 and the actual number of tokens in the window. This is true for all Models, for more or for less and depending on the backend. Now, I'm not sure if any of these cases of mismatch are a significant problem as I don't really know how block_manager_v1 actually works. Hopefully my explanation is good enough :) |
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@caiom Thanks for your detail explanation. Look good to me. @cadedaniel WDYT? |
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What if instead we round up the sliding window in the block manager to nearest block size? |
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@cadedaniel I have same thought before, but it seems that there's a few references to |
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I'm guessing that for PagedAttention, what matters is what is in the blocks_table. The max context length for mistralai/Mistral-7B-v0.1 is 4097 but the PagedAttention will consider only 4096 tokens. This is fine since the model can handle 4096 tokens. For Phi-3, using this PR, PagedAttention have the correct/maximum number of tokens in context. So, if we are going to round it, we should always round down so that we never feed more tokens than the model was trained for. Unless we are just rounding by 1, so the +1 in my PR should be safe.
I may be totally wrong, though. |
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So the block manager's concern here (and only concern) should be allocating enough space for sliding window. As for how the model would like to use that space is up to the model. So from a north-star perspective the block manager should not be involved in the minutia of context len for sliding window. It's OK if the model_runner uses a subset of the blocks for context. While this PR is a working hotfix, in design terms it couples the exact sliding window length used by attention closely with the block manager, which isn't necessary. Ideally we avoid this! Can we test out my suggestion? There may be some other coupling between block manager and exact context length that I don't know about. |
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Thanks for the review @cadedaniel, I have implemented your suggestion and tested microsoft/Phi-3-mini-4k-instruct using several long prompts, everything looks good. |
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Done! I tested the mistralai/Mistral-7B-Instruct-v0.1 model by inputting Wikipedia articles, asking it to create summaries, and then eyeballing the results. Everything looks good. Additionally, the rounding should not affect Mistral models. |
hi, I find Phi-3 medium is released ,when i try it with vllm, framework logged it was using xformers because of sliding window, my vllm is building from latest source, do you have any idea how to use flash-attn instead of xformers?
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@ShadowTeamCN Same problem here for phi3-mini. Cannot use FlashAttention-2 backend due to sliding window. |
The model microsoft/Phi-3-mini-4k-instruct uses a sliding window of size 2047 (which should result in 2048 tokens of context) but due to the issue #3385 the model loading will break:
File "/usr/local/lib/python3.10/dist-packages/vllm/core/block_manager_v1.py", line 223, in __init__ assert sliding_window % block_size == 0, (sliding_window, AssertionError: (2047, 16)My idea is to have this quick fix so that people can use the model, I can also create a draft PR for a definitive solution. However, a definitive solution to the issue can be involved, the issue affects xformers and sdpa but not flash backend. It also affects block_manager_v1 which is the bit I would need some help.
Let me know your thoughts.
RELATED: #3385
Update:
After some discussion, we decided to change only
block_manager_v1by removing the assert and rounding upblock_sliding_window. This change has no effect anywhere else (model_runneror actual model code).Models tested by feeding long prompts:
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