[Core] Block Allocator to support KV cache CPU offloading #11532
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… scheduler Signed-off-by: ApostaC <[email protected]> Co-authored-by: KuntaiDu <[email protected]>
ApostaC
requested review from
zhuohan123,
youkaichao,
alexm-neuralmagic,
comaniac and
njhill
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Signed-off-by: ApostaC <[email protected]>
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Note: This PR is part of the big CPU offloading PR #10874 -- this PR contains the CPU-offloading block allocator implementation as well as the changes in the scheduler.
TL; DR: CPU offloading is better than prefix caching in our benchmark, we also found that the evictor can be optimized to save 10-30% of the runtime.
End-to-end benchmarking results:
A long document QA workload (see
benchmarks/benchmark_long_document_qa.py) running on A100-40G-SXM GPU. The GPU can cache 8 documents and the CPU can cache 30 documents.(Following are the original data for the above figure)
Implementation
This PR has much less features compared to #8694, but it is really minimum and creates very little core change. So I guess we can use this PR to enable CPU KV cache offloading first, and then focus on disk.
The key idea of this implementation is to maintain those allocated blocks that didn't hit the cache, and constantly copy them into CPU after each scheduler step.
Here is the flow diagram
This idea is borrowed from ConServe (paper link: https://arxiv.org/abs/2410.01228), based on the assumption that the CPU-GPU bandwidth is much higher than GPU KV cache generation throughput. Thanks Yifan for this idea.