[Core][Kernel][Misc] Support external swapper for vllm

[zeroorhero]

Hi,

In the previous version of vllm, when the GPU memory is insufficient,
the kv cache needs to be swapped to the CPU memory. Based on the CPU,
we abstracted an external swapper interface and implemented the local file
to store kv cache. Other distributed storage implementations may be added in the
future. By adding external swapper, the storage space of token is greatly expanded.

The specific design is as follows:
The kv cache is stored in a hierarchical structure, with the following levels:

GPU ---> CPU ----> External Swapper.

The storage space is getting bigger, but the latency is also getting longer.
When the generated kv cache space is small, it will be stored in the area
with smaller latency first. When the space in this area is insufficient, it will
be swapped to the outside area.
We continue to use the previous vllm scheduling and executor related strategies,
and simply abstract an external swapper interface under the cache engine to
connect to different swappers.

Currently, the external swapper of local file is implemented, and the external
swapper of valkey (RDMA version of redis) will be implemented next.

For the implemented local file, we also conducted some benchmark tests.
Our test environment is Nvidia A10 with 4 cards and NVME disk.

1. Kernel benchmark（benchmark/kernels/benchmark_swap_blocks）:
   The execution time of a single kernel increased by about 60%.
   Avg. GPU->CPU time taken for swapping blocks: 0.016023850440979003 seconds
Avg. GPU->File time taken for swapping blocks: 0.026778271198272707 seconds
Avg. File->GPU time taken for swapping blocks: 0.025919318199157715 seconds

2. Online server benchmark
   The throughput performance of tokens has dropped very little, about 2%.
   2.1 Swap to CPU(997 times swap):
   Server:
   python3 -m vllm.entrypoints.openai.api_server --model /root/Llama-2-7b-hf/ --tokenizer /root/Llama-2-7b-hf/ --num-gpu-blocks-override 270 --preemption-mode swap --tensor-parallel-size 4 --swap-space 40
   Client:
   python3 benchmark_serving.py --backend vllm --dataset-name random --model /root/Llama-2-7b-hf/ --tokenizer /root/Llama-2-7b-hf/ --random-input-len 1024 --random-output-len 1024 --request-rate 100 --num-prompts 1000
   

   2.2 Swap to File(996 times swap):
   Server:
   python3 -m vllm.entrypoints.openai.api_server --model /root/Llama-2-7b-hf/ --tokenizer /root/Llama-2-7b-hf/ --num-gpu-blocks-override 270 --preemption-mode swap --swap-space 0 --external-swapper file:///root/test --external-swapper-space 40 --tensor-parallel-size 4
   Client:
   python3 benchmark_serving.py --backend vllm --dataset-name random --model /root/Llama-2-7b-hf/ --tokenizer /root/Llama-2-7b-hf/ --random-input-len 1024 --random-output-len 1024 --request-rate 100 --num-prompts 1000

[github-actions]

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[noooop]

pin_memory has a great impact on swapping blocks.

more specifically

benchmarks/kernels/benchmark_swap_blocks.py

+ from light_vllm.utils import is_pin_memory_available
+ pin_memory = is_pin_memory_available()

- dst = torch.zeros_like(src).cpu()
+ dst = torch.zeros_like(src, pin_memory=pin_memory, device="cpu")

Avg. GPU->CPU time taken for swapping blocks: 0.016023850440979003 seconds
16ms is so weird

[zeroorhero]

pin_memory has a great impact on swapping blocks.

more specifically

benchmarks/kernels/benchmark_swap_blocks.py

+ from light_vllm.utils import is_pin_memory_available
+ pin_memory = is_pin_memory_available()

- dst = torch.zeros_like(src).cpu()
+ dst = torch.zeros_like(src, pin_memory=pin_memory, device="cpu")

Avg. GPU->CPU time taken for swapping blocks: 0.016023850440979003 seconds
16ms is so weird

@noooop Thank you very much for your suggestion. I will make some changes and conduct relevant tests. And could you help me find other reviewers? Help me see if this solution is feasible?

[noooop]

How should I say it tactfully?

in my scenario ddr4 3600 32G *4 only has ≈20G/s bandwidth （Compare with 4090 1T/s）, so cpu memory SWAP is almost useless to me.

[noooop]

Maybe external swapper can be used in future async schedulers, it is too slow for current synchronous schedulers.

[zeroorhero]

Add a benchmark result.
2. Online server benchmark
2.3 Recompute(997 times recompute):
The recompute result is basically the same as the result of swapping to the CPU.
Server:
python3 -m vllm.entrypoints.openai.api_server --model /root/Llama-2-7b-hf/ --tokenizer /root/Llama-2-7b-hf/ --num-gpu-blocks-override 270 --preemption-mode recompute --tensor-parallel-size 4
Client:
python3 benchmark_serving.py --backend vllm --dataset-name random --model /root/Llama-2-7b-hf/ --tokenizer /root/Llama-2-7b-hf/ --random-input-len 1024 --random-output-len 1024 --request-rate 100 --num-prompts 1000

[zeroorhero]

@DarkLight1337 @ywang96 @youkaichao hi, I have simply implemented an external storage. Please help review the codes.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @zeroorhero.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork