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[3/N] Refactor scheduler for chunked prefill scheduling #3550

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rkooo567
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@rkooo567 rkooo567 commented Mar 21, 2024

Refactor the current scheduler to make it easy to understand with chunked prefill later.

This simply moves logic for prefill scheduling and decoding scheudling to a dedicated function. The purpose of doing this is we want the different scheduling policy for chunked prefill (by default, we do prefill -> decoding. But when chunked prefill is enabled, we want decoding -> prefill to reduce ITL impact).

The functionality must be exactly the same except that I made it use lora_enabled instead of directly checking if lora config is None.

Related: #3130

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@rkooo567
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rkooo567 commented Mar 29, 2024

before: Throughput: 2.01 requests/s, 972.94 tokens/s
after: Throughput: 1.99 requests/s, 961.40 tokens/s

Benchmark result. I'd say it is just the same

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rkooo567 commented Mar 29, 2024

@simon-mo Updated (plz take a look one more time);

  • swap is a separate API now
  • each API is more thoroughly tested. Also better unit testing for swapping.
  • general cleanup (e.g., use dataclass, use better naming).

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Comment on lines 537 to 539
self.running.extend([s.seq_group for s in prefills.seq_groups])
self.running.extend([s.seq_group for s in decodes.seq_groups])
self.running.extend([s.seq_group for s in swapped_in.seq_groups])
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can you help me understand what clears self.running each step?

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it is popped out when it is scheduled from each func!

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seems not

vllm/vllm/core/scheduler.py

Lines 281 to 287 in 810c56d

seq_group = self.running[0]
new_token_size = (
seq_group.num_seqs(status=SequenceStatus.RUNNING) *
self.num_decoding_tokens_per_seq)
if num_batched_tokens + new_token_size > token_budget:
break

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+1 The logic here is confusing. Some of the requests in self.running will be poped out in _schedule_decodes. But the lines here give people a feeling that self.running is a queue that keep extending to infinity.

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This is basically the same behavior as the master;

self.running = running

It is cleared up when the model output is processed

self.scheduler.free_finished_seq_groups()
.

I will comment it here

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Thanks for the changes! The code looks better than the last version. My main concern on this PR is that self.running seems to be a leaking abstraction that is used everywhere. Can we somehow make this interface a bit more clean?

Comment on lines 537 to 539
self.running.extend([s.seq_group for s in prefills.seq_groups])
self.running.extend([s.seq_group for s in decodes.seq_groups])
self.running.extend([s.seq_group for s in swapped_in.seq_groups])
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+1 The logic here is confusing. Some of the requests in self.running will be poped out in _schedule_decodes. But the lines here give people a feeling that self.running is a queue that keep extending to infinity.

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@rkooo567
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rkooo567 commented Apr 2, 2024

@zhuohan123 @simon-mo

As we discussed offline, I updated code based on the proposal I made.

  • Made all _schedule APIs as stateless as possible.
  • Each APIs can be used with any order
  • Increase test coverage A LOT compared to before including beam search case, block updates, swap & preemption, individual APIs, loras, max_seqs and max_batched_tokens.
  • Fix 2 bugs.
    • the first prefill doesn't include running batched tokens when it counts num_batched_tokens.
    • swapping doesn't break when it reaches to max_num_batched_tokens.

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Comment on lines 537 to 539
self.running.extend([s.seq_group for s in prefills.seq_groups])
self.running.extend([s.seq_group for s in decodes.seq_groups])
self.running.extend([s.seq_group for s in swapped_in.seq_groups])
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This is basically the same behavior as the master;

self.running = running

It is cleared up when the model output is processed

self.scheduler.free_finished_seq_groups()
.

I will comment it here

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@@ -573,17 +791,13 @@ def _preempt_by_recompute(
seq.status = SequenceStatus.WAITING
self.free_seq(seq)
seq.reset_state_for_recompute()
# NOTE: For FCFS, we insert the preempted sequence group to the front
# of the waiting queue.
self.waiting.appendleft(seq_group)
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updated within _schedule now

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rkooo567 commented Apr 3, 2024

sampler test failure seems unrelated

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rkooo567 commented Apr 3, 2024

lora test failure unrelated

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