[Core] Adding Priority Scheduling

[apatke]

FILL IN THE PR DESCRIPTION HERE

There are three major changes implemented:


1. Addition of a new priority scheduling policy to the scheduler config. Also adds a user-defined priority variable to sequence.

2. All requests in the running queue and the waiting queue are sorted first based on this priority. If there is a tie, it falls back to the FCFS policy.

3. Force preemption of request from the running queue back into the waiting queue.
   If there are requests in the running queue whose priority is lower than the requests in the waiting queue, they are forcefully preempted out back into the waiting queue to allow immediate execution of the higher priority request.

@njhill @saurabhjha1 @youkaichao @simon-mo

FIX #6077 (link existing issues this PR will resolve)

BEFORE SUBMITTING, PLEASE READ THE CHECKLIST BELOW AND FILL IN THE DESCRIPTION ABOVE

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[njhill]

@apatke you need to run ./format.sh on the code to fix the linter errors.

[njhill]

Something we had been discussing is whether it would make sense for the API to take some kind of scheduing_params dataclass containing the priority, to allow for fields related to future scheduling policy additions without having to add them all as separate top-level parameters.

[apatke]

Performance slowdown from _schedule_priority_preemption is <4% with the priority policy for Llama 8B. No performance degradation when policy is not enabled.

priority: Throughput: 14.56 requests/s, 6052.59 tokens/s
fcfs: Throughput: 15.15 requests/s, 6299.22 tokens/s

[apatke]

@youkaichao Would you be able to take a look at the PR?

[njhill]

Thanks @apatke! I did a first pass.

As discussed, it's great that that this has no performance impact when the priority scheduling is disabled which is the most important thing.

Would be good to get @youkaichao's thoughts too.

[youkaichao]

will take a look when i have time :)

[njhill]

Thanks @apatke, could you resolve the new conflicts?

[youkaichao]

is this necessary to add in vllm's codebase? looks like the code duplication is large.

[njhill]

There is already a lot of duplication in our benchmarking and examples scripts ... could be a good separate task for someone to look at to streamline that. I will see if there's someone on my side who can look at it.

[youkaichao]

I give approval and hand it over to @njhill . I think it is fine since it does not affect default operation

[ZJUFangzh]

hi， why this priority scheduling not support AsyncLLMEngine？

[schoennenbeck]

@youkaichao @njhill Do you know if somebody is already working on supporting this in AsyncLLMEngine? If not I could go ahead open a PR.

[schoennenbeck]

Port can be found here: #8850

[tonyaw]

Thanks for your effort!
How can I use it with openai client?
The vllm version I'm using is vllm-0.6.3.dev152+gde895f16.d20241010.
Currently, I'm putting priority into 'extra_body' of client.chat.completions.create:

{'model': 'hugging-quants/Meta-Llama-3.1-70B-Instruct-AWQ-INT4', 'stream': False, 'max_tokens': 20480, 'temperature': 0, 'n': 1, 'seed': 42, 'extra_body': {'top_k': 1, 'priority': 10}}

Is it right?

[schoennenbeck]

@tonyaw Yes, that should be correct. The support was added in another PR. Remember that a lower value for priority means earlier handling (this is in line with how python's queue.PriorityQueue works).

[tonyaw]

@schoennenbeck Thanks for your prompt response!
Next question is: When the preemption can handle by priority?

        if len(prefills.seq_groups
               ) == 0 and self.scheduler_config.policy == "priority":
            self._schedule_priority_preemption(budget)

Does it mean as long as there is one request in prefill stage, the priority_preemption will never be triggered?

[tonyaw]

Also, I realized if "--enable-chunked-prefill" is set, priority scheduling won't be triggered.
To get better performance, I need to enable chunked-prefill, but in this case, I can't use priority scheduling any more.
May I ask the reason?

[tonyaw]

@apatke @schoennenbeck ,
I met some issue, and think current code has some issue:
#9272
Could you please help to check? Thanks in advance! :-)

[tonyaw]

@apatke and @schoennenbeck , as I mentioned in #9272, even if the priority is propagated successfully, vllm always crashes as long as preemption happens.
I just tested with vllm-0.6.3.dev173+g36ea7907.d20241011. The only change I made is following fix and some logs:
#9277

Could you please help to guide me how to WR it?
Also, I realized --enable_chunked_prefill is default be True for Llama3.1 as it is a long context length model.
Why enable_chunked_prefill can't work with priority scheduling together? It will reduce the vllm performance a lot.

Reproduce procedure:

1. Start vllm:

python3 -m vllm.entrypoints.openai.api_server --model hugging-quants/Meta-Llama-3.1-70B-Instruct-AWQ-INT4 \
        --host 0.0.0.0 --port 8080  --seed 42 --trust-remote-code --scheduling-policy priority \
        --tensor-parallel-size 2 --max-num-seqs 10 --enable_chunked_prefill False

2. Use openai client to make a 15 concurrent load.
3. Use another openai client to send some requests with priority -100.
4. As long as preemption is triggered, vllm crashes:

INFO 10-11 06:51:31 engine.py:292] Added request chat-29ccd21f37fb4a8ea96c1c8c189a6a49.
INFO 10-11 06:51:31 engine.py:294] tonyaw:Added request -100.
INFO 10-11 06:51:31 scheduler.py:1025] tonyaw: len(prefills.seq_groups) = 0
INFO 10-11 06:51:31 scheduler.py:1025] tonyaw: len(prefills.seq_groups) = 0
INFO 10-11 06:51:31 scheduler.py:807] tonyaw: _schedule_priority_preemption: waiting_queue is not None.
INFO 10-11 06:51:31 scheduler.py:808] tonyaw: seq_group chat-29ccd21f37fb4a8ea96c1c8c189a6a49 priority:-100
INFO 10-11 06:51:31 scheduler.py:837] tonyaw: _schedule_priority_preemption: vseq_group chat-55b8bdce4ae14eb8869839561fac50f9 is pop up, and will preempt.
WARNING 10-11 06:51:31 scheduler.py:1493] Sequence group chat-55b8bdce4ae14eb8869839561fac50f9 is preempted by PreemptionMode.RECOMPUTE mode because there is not enough KV cache space. This can affect the end-to-end performance. Increase gpu_memory_utilization or tensor_parallel_size to provide more KV cache memory. total_num_cumulative_preemption=1
INFO 10-11 06:51:31 model_runner_base.py:120] Writing input of failed execution to /tmp/err_execute_model_input_20241011-065131.pkl...
WARNING 10-11 06:51:31 model_runner_base.py:143] Failed to pickle inputs of failed execution: Can't get local object 'weak_bind.<locals>.weak_bound'
INFO:     10.254.17.246:54142 - "POST /v1/chat/completions HTTP/1.1" 400 Bad Request
INFO:     10.254.17.246:54086 - "POST /v1/chat/completions HTTP/1.1" 400 Bad Request
ERROR 10-11 06:51:31 engine.py:160] ValueError('Error in model execution: seq_group.get_last_latency() should not be called if the seq_group is in prefill phase.')
ERROR 10-11 06:51:31 engine.py:160] Traceback (most recent call last):
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/worker/model_runner_base.py", line 116, in _wrapper
ERROR 10-11 06:51:31 engine.py:160]     return func(*args, **kwargs)
ERROR 10-11 06:51:31 engine.py:160]            ^^^^^^^^^^^^^^^^^^^^^
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/worker/model_runner.py", line 1698, in execute_model
ERROR 10-11 06:51:31 engine.py:160]     model_input.async_callback()
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/utils.py", line 1122, in weak_bound
ERROR 10-11 06:51:31 engine.py:160]     unbound(inst, *args, **kwargs)
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/engine/llm_engine.py", line 1210, in _process_model_outputs
ERROR 10-11 06:51:31 engine.py:160]     self.do_log_stats(scheduler_outputs, outputs, finished_before,
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/engine/llm_engine.py", line 1543, in do_log_stats
ERROR 10-11 06:51:31 engine.py:160]     stats = self._get_stats(scheduler_outputs, model_output,
ERROR 10-11 06:51:31 engine.py:160]             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/engine/llm_engine.py", line 1664, in _get_stats
ERROR 10-11 06:51:31 engine.py:160]     latency = seq_group.get_last_latency(now)
ERROR 10-11 06:51:31 engine.py:160]               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/sequence.py", line 772, in get_last_latency
ERROR 10-11 06:51:31 engine.py:160]     raise ValueError(
ERROR 10-11 06:51:31 engine.py:160] ValueError: seq_group.get_last_latency() should not be called if the seq_group is in prefill phase.
ERROR 10-11 06:51:31 engine.py:160] 
ERROR 10-11 06:51:31 engine.py:160] The above exception was the direct cause of the following exception:
ERROR 10-11 06:51:31 engine.py:160] 
ERROR 10-11 06:51:31 engine.py:160] Traceback (most recent call last):
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/engine/multiprocessing/engine.py", line 158, in start
ERROR 10-11 06:51:31 engine.py:160]     self.run_engine_loop()
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/engine/multiprocessing/engine.py", line 221, in run_engine_loop
ERROR 10-11 06:51:31 engine.py:160]     request_outputs = self.engine_step()
ERROR 10-11 06:51:31 engine.py:160]                       ^^^^^^^^^^^^^^^^^^
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/engine/multiprocessing/engine.py", line 239, in engine_step
ERROR 10-11 06:51:31 engine.py:160]     raise e
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/engine/multiprocessing/engine.py", line 230, in engine_step
ERROR 10-11 06:51:31 engine.py:160]     return self.engine.step()
ERROR 10-11 06:51:31 engine.py:160]            ^^^^^^^^^^^^^^^^^^
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/engine/llm_engine.py", line 1386, in step
ERROR 10-11 06:51:31 engine.py:160]     outputs = self.model_executor.execute_model(
ERROR 10-11 06:51:31 engine.py:160]               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/executor/distributed_gpu_executor.py", line 82, in execute_model
ERROR 10-11 06:51:31 engine.py:160]     driver_outputs = self._driver_execute_model(execute_model_req)
ERROR 10-11 06:51:31 engine.py:160]                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/executor/multiproc_gpu_executor.py", line 155, in _driver_execute_model
ERROR 10-11 06:51:31 engine.py:160]     return self.driver_worker.execute_model(execute_model_req)
ERROR 10-11 06:51:31 engine.py:160]            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/worker/worker_base.py", line 327, in execute_model
ERROR 10-11 06:51:31 engine.py:160]     output = self.model_runner.execute_model(
ERROR 10-11 06:51:31 engine.py:160]              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/torch/utils/_contextlib.py", line 116, in decorate_context
ERROR 10-11 06:51:31 engine.py:160]     return func(*args, **kwargs)
ERROR 10-11 06:51:31 engine.py:160]            ^^^^^^^^^^^^^^^^^^^^^
ERROR 10-11 06:51:31 engine.py:160]   File "/usr/local/lib/python3.12/dist-packages/vllm/worker/model_runner_base.py", line 146, in _wrapper
ERROR 10-11 06:51:31 engine.py:160]     raise type(err)(f"Error in model execution: "
ERROR 10-11 06:51:31 engine.py:160] ValueError: Error in model execution: seq_group.get_last_latency() should not be called if the seq_group is in prefill phase.

[justadogistaken]

@tonyaw Hi, I'm curious that does "Why enable_chunked_prefill can't work with priority scheduling together" fixed?

[hxt365]

Is this usable now? I don't find the usage guide anywhere in the doc