[Feature] enable host memory for kv cache

[YZP17121579]

Optimal conditions for this feature:

1. The token count of your prompt significantly exceeds that of the generated output.
2. The commonly used prompt prefix length is substantial, leading to a high cache hit rate.
3. Your machine's GPU memory is insufficient to accommodate enough prefix key-value caches, resulting in a reduced cache hit rate.

Main idea

In the scenario described above, offloading key-value caches to host memory could be advantageous. The implementation is straightforward: we already have CPU blocks within the block manager, although they are currently designed exclusively for sequence preemption. Since CPU and GPU blocks share the same underlying block structure, we can store computed key-value cache blocks within CPU blocks.

With this approach, each time a GPU block is freed, we add it to the CPU allocator via swapping. When allocating immutable blocks, we first search the GPU allocator. If a GPU cache hit occurs, we proceed with the cached GPU block as usual. If no hit is found, we additionally check the CPU allocator, and in the event of a CPU cache hit, we swap the CPU block back to the GPU and eliminate the recomputation.

Originally, the block allocation logic without preemption:

Now we utilize the CPU blocks:

Note that the computed blocks in the CPU allocator are stored only in the evictor, and this change will not impact the original preemption logic.

Usage

Launch the vllm engine with

--enable-prefix-caching --enable-host-memory-caching --swap-space 48

in which --swap-space configures the host memory for both kv cache and preemption.

Shortages

If host memory for the KV cache is enabled, each GPU block added to the evictor will be swapped to host memory, potentially leading to a performance drop. Therefore, ensure that enabling the host memory KV cache will significantly improve your cache hit rate. In our case (4090*1), if the cache hit rate is below 20%, enabling this feature will result in a performance degradation.

I’ve marked this PR as a draft because I’m not sure whether it will be useful for the main users, and the modification of the code is somewhat intrusive. Please provide feedback if you think it is beneficial, and I look forward to hearing your thoughts.

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[github-actions]

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[comaniac]

Looks like the function you propose can be covered by LMCache or kv cache offloading? cc @KuntaiDu