[Core] Reduce TTFT with concurrent partial prefills (vllm-project#10235)

Signed-off-by: Joe Runde <[email protected]>
Signed-off-by: Prashant Gupta <[email protected]>
Co-authored-by: Prashant Gupta <[email protected]>
Co-authored-by: Cody Yu <[email protected]>

Author: 3 people

tests/basic_correctness/test_chunked_prefill.py

@@ -8,7 +8,6 @@
 Run `pytest tests/models/test_chunked_prefill.py`.
 """
 import os
-from contextlib import nullcontext
 
 import pytest
 
@@ -233,7 +232,6 @@ def test_with_prefix_caching(
 
     max_num_batched_tokens = max_num_seqs = chunk_size
     outputs = {}  # type: ignore
-    check_result = True
     for enable in (True, False):
         with vllm_runner(
                 model,
@@ -245,25 +243,17 @@ def test_with_prefix_caching(
                 enforce_eager=enforce_eager,
                 max_num_seqs=max_num_seqs,
         ) as vllm_model:
-            # It should fail when prefix caching is enable and chunk
-            # size is not a multiple of block size (16).
-            should_fail = chunk_size % 16 != 0 and enable
-            check_result &= not should_fail
             outputs[enable] = []
-            # Send the request one-by-one to ensure the cache is populated.
-            with pytest.raises(ValueError) if should_fail else nullcontext():
-                for prompt in full_prompts:
-                    outputs[enable] += vllm_model.generate_greedy([prompt],
-                                                                  max_tokens)
-
-    # Check results only if we did not expect a failure.
-    if check_result:
-        check_outputs_equal(
-            outputs_0_lst=outputs[False],
-            outputs_1_lst=outputs[True],
-            name_0="w/o prefix caching",
-            name_1="with prefix caching",
-        )
+            for prompt in full_prompts:
+                outputs[enable] += vllm_model.generate_greedy([prompt],
+                                                              max_tokens)
+
+    check_outputs_equal(
+        outputs_0_lst=outputs[False],
+        outputs_1_lst=outputs[True],
+        name_0="w/o prefix caching",
+        name_1="with prefix caching",
+    )
 
 
 @pytest.mark.parametrize("model", ["facebook/opt-125m"])

tests/core/test_chunked_prefill_scheduler.py

@@ -7,6 +7,9 @@
 
 from vllm.config import CacheConfig, SchedulerConfig
 from vllm.core.scheduler import Scheduler
+from vllm.engine.arg_utils import EngineArgs
+from vllm.engine.llm_engine import LLMEngine
+from vllm.sampling_params import SamplingParams
 from vllm.sequence import Logprob, SequenceGroup
 
 from .utils import create_dummy_prompt
@@ -16,7 +19,7 @@ def get_sequence_groups(scheduler_output):
     return [s.seq_group for s in scheduler_output.scheduled_seq_groups]
 
 
-def append_new_token(seq_group, token_id: int):
+def append_new_token(seq_group: SequenceGroup, token_id: int):
     for seq in seq_group.get_seqs():
         seq.append_token_id(token_id, {token_id: Logprob(token_id)})
 
@@ -123,6 +126,232 @@ def test_chunk():
     assert out.num_batched_tokens == 57
 
 
+def test_concurrent_chunking():
+    """Verify prefills are chunked properly when 
+    --max-num-partial-prefills is > 1"""
+    block_size = 4
+    max_seqs = 60
+    max_model_len = 2000
+    max_num_batched_tokens = 64
+    scheduler_config = SchedulerConfig(
+        "generate",
+        max_num_batched_tokens,
+        max_seqs,
+        max_model_len,
+        enable_chunked_prefill=True,
+        max_num_partial_prefills=2,  # Up to 2 partial prefills at a time
+    )
+    cache_config = CacheConfig(block_size, 1.0, 1, "auto")
+    cache_config.num_cpu_blocks = 32
+    cache_config.num_gpu_blocks = 32
+    scheduler = Scheduler(scheduler_config, cache_config, None)
+    running: List[SequenceGroup] = []
+
+    # Add seq groups to scheduler.
+    for i in range(2):
+        _, seq_group = create_dummy_prompt(str(i),
+                                           prompt_length=60,
+                                           block_size=block_size)
+        scheduler.add_seq_group(seq_group)
+        running.append(seq_group)
+
+    # Verify both requests are chunked with half of max_num_batched_tokens each
+    seq_group_meta, out = schedule_and_update_computed_tokens(scheduler)
+    assert set(get_sequence_groups(out)) == set(running)
+    assert seq_group_meta[0].token_chunk_size == 32
+    assert seq_group_meta[1].token_chunk_size == 32
+    assert out.num_prefill_groups == 2
+    assert out.num_batched_tokens == 64
+
+    # After one iteration, both should have 60 - 32 = 28 tokens left to prefill
+    seq_group_meta, out = schedule_and_update_computed_tokens(scheduler)
+    assert set(get_sequence_groups(out)) == set(running)
+    assert seq_group_meta[0].token_chunk_size == 28
+    assert seq_group_meta[1].token_chunk_size == 28
+    assert out.num_prefill_groups == 2
+    assert out.num_batched_tokens == 56
+
+
+def test_concurrent_chunking_large_requests():
+    """Verify large prefill requests are run one at a time"""
+    block_size = 4
+    max_seqs = 60
+    max_model_len = 2000
+    max_num_batched_tokens = 64
+    scheduler_config = SchedulerConfig(
+        "generate",
+        max_num_batched_tokens,
+        max_seqs,
+        max_model_len,
+        enable_chunked_prefill=True,
+        max_num_partial_prefills=2,  # Up to 2 partial prefills at a time
+    )
+    cache_config = CacheConfig(block_size, 1.0, 1, "auto")
+    cache_config.num_cpu_blocks = 3200  # large KV cache size for large requests
+    cache_config.num_gpu_blocks = 3200
+    scheduler = Scheduler(scheduler_config, cache_config, None)
+
+    # Add seq groups to scheduler.
+    for i in range(2):
+        _, seq_group = create_dummy_prompt(
+            str(i),
+            prompt_length=1200,  # Very large prompt
+            block_size=block_size)
+        scheduler.add_seq_group(seq_group)
+
+    # Verify only a single request is chunked, and it gets all 64 tokens
+    seq_group_meta, out = schedule_and_update_computed_tokens(scheduler)
+    assert len(get_sequence_groups(out)) == 1
+    assert seq_group_meta[0].token_chunk_size == 64
+    assert out.num_prefill_groups == 1
+    assert out.num_batched_tokens == 64
+
+
+def test_short_prompts_jump_long_prompts_in_queue():
+    """Verify large prefill requests are punted behind smaller ones if 
+    another large prefill request is already running"""
+    block_size = 4
+    max_seqs = 60
+    max_model_len = 2000
+    max_num_batched_tokens = 64
+    scheduler_config = SchedulerConfig(
+        "generate",
+        max_num_batched_tokens,
+        max_seqs,
+        max_model_len,
+        enable_chunked_prefill=True,
+        max_num_partial_prefills=2,  # Up to 2 partial prefills at a time
+    )
+    cache_config = CacheConfig(block_size, 1.0, 1, "auto")
+    cache_config.num_cpu_blocks = 3200  # large KV cache size for large requests
+    cache_config.num_gpu_blocks = 3200
+    scheduler = Scheduler(scheduler_config, cache_config, None)
+    long_seqs: List[SequenceGroup] = []
+    short_seqs: List[SequenceGroup] = []
+
+    # Add 2 large seq groups to scheduler.
+    for i in range(2):
+        _, seq_group = create_dummy_prompt(
+            str(i),
+            prompt_length=1200,  # Very large prompt
+            block_size=block_size)
+        scheduler.add_seq_group(seq_group)
+        long_seqs.append(seq_group)
+        assert seq_group.is_prefill()
+
+    # Add 2 small seq groups behind them
+    for i in range(2):
+        _, seq_group = create_dummy_prompt(
+            str(i + 2),
+            prompt_length=40,  # Very small prompt
+            block_size=block_size)
+        scheduler.add_seq_group(seq_group)
+        short_seqs.append(seq_group)
+        assert seq_group.is_prefill()
+
+    # Verify one large req and 1 small req chunked
+    seq_group_meta, out = schedule_and_update_computed_tokens(scheduler)
+    assert seq_group_meta[0].token_chunk_size == 32  # large req gets 32 tokens
+    assert seq_group_meta[1].token_chunk_size == 32  # small req gets 32 tokens
+
+    # all 4 are prefilling
+    assert long_seqs[0].is_prefill()
+    assert long_seqs[1].is_prefill()
+    assert short_seqs[0].is_prefill()
+    assert short_seqs[1].is_prefill()
+    # First short and first long sequences have been scheduled
+    assert long_seqs[0].first_seq.get_num_computed_tokens() == 32
+    assert long_seqs[1].first_seq.get_num_computed_tokens() == 0
+    assert short_seqs[0].first_seq.get_num_computed_tokens() == 32
+    assert short_seqs[1].first_seq.get_num_computed_tokens() == 0
+
+    assert out.num_prefill_groups == 2
+    assert out.num_batched_tokens == 64
+
+    # in the second iteration,
+    # the first small request had only 8 tokens left
+    # so it went to decode
+    # The other small req is scheduled
+    seq_group_meta, out = schedule_and_update_computed_tokens(scheduler)
+    # the new small req got 64 - (32+8) tokens
+    assert seq_group_meta[0].token_chunk_size == 24
+    assert seq_group_meta[1].token_chunk_size == 32  # large req still got 32
+    # the other small request had only 8 tokens left
+    assert seq_group_meta[2].token_chunk_size == 8  # 40-32
+
+    # The first small request got to decode now
+    assert long_seqs[0].is_prefill()
+    assert long_seqs[1].is_prefill()
+    assert not short_seqs[0].is_prefill()
+    assert short_seqs[1].is_prefill()
+    # Both small requests have started in front of the second long request
+    assert long_seqs[0].first_seq.get_num_computed_tokens() == 64
+    assert long_seqs[1].first_seq.get_num_computed_tokens() == 0
+    assert short_seqs[0].first_seq.get_num_computed_tokens() == 40
+    assert short_seqs[1].first_seq.get_num_computed_tokens() == 24
+
+    assert out.num_prefill_groups == 3
+    assert out.num_batched_tokens == 64
+    # the first small seq group has a new token appended.
+    append_new_token(short_seqs[0], 1)
+
+    # in the third iteration,
+    # the first small request is already decoding
+    # the second small request only has 16 tokens left and will enter decoding
+    seq_group_meta, out = schedule_and_update_computed_tokens(scheduler)
+    assert seq_group_meta[0].token_chunk_size == 32  # large still got 32
+    # small req finished prefilling 40-24=16 tokens
+    assert seq_group_meta[1].token_chunk_size == 16
+    assert seq_group_meta[2].token_chunk_size == 1  # decode
+    assert out.num_prefill_groups == 2
+    assert out.num_batched_tokens == 49  # (32+16+1 decode)
+
+    # both small requests have now reached decode
+    assert long_seqs[0].is_prefill()
+    assert long_seqs[1].is_prefill()
+    assert not short_seqs[0].is_prefill()
+    assert not short_seqs[1].is_prefill()
+    assert long_seqs[0].first_seq.get_num_computed_tokens() == 96
+    assert long_seqs[1].first_seq.get_num_computed_tokens() == 0
+    assert short_seqs[0].first_seq.get_num_computed_tokens() == 41
+    assert short_seqs[1].first_seq.get_num_computed_tokens() == 40
+
+    # both the small seq groups have a new token appended
+    append_new_token(short_seqs[0], 1)
+    append_new_token(short_seqs[1], 1)
+
+    # in the fourth iteration, both small requests are decoding
+    # so large request gets all the budget
+    seq_group_meta, out = schedule_and_update_computed_tokens(scheduler)
+
+    # large req gets 62 tokens (minus 2 for decode)
+    assert seq_group_meta[0].token_chunk_size == 62
+    assert seq_group_meta[1].token_chunk_size == 1  # decode
+    assert seq_group_meta[2].token_chunk_size == 1  # decode
+    assert out.num_prefill_groups == 1
+    assert out.num_batched_tokens == 64
+
+    assert long_seqs[0].first_seq.get_num_computed_tokens() == 158
+
+    # assert long_seqs[0].is_prefill()
+    # assert long_seqs[1].is_prefill()
+    # assert not short_seqs[0].is_prefill()
+    # assert not short_seqs[1].is_prefill()
+
+    # # both the small seq groups have a new token appended
+    # append_new_token(short_seqs[0], 1)
+    # append_new_token(short_seqs[1], 1)
+
+    # # in the fifth iteration, large request gets all the budget
+    # # while both small requests are decoding
+    # seq_group_meta, out = schedule_and_update_computed_tokens(scheduler)
+    # assert seq_group_meta[0].token_chunk_size == 62
+    # assert seq_group_meta[1].token_chunk_size == 1  # decode
+    # assert seq_group_meta[2].token_chunk_size == 1  # decode
+    # assert out.num_prefill_groups == 1
+    # assert out.num_batched_tokens == 64
+
+
 def test_complex():
     block_size = 4
     max_seqs = 60
@@ -508,7 +737,7 @@ def test_chunked_prefill_max_seqs():
     assert not running[1].is_prefill()
 
 
-def test_perfix_caching():
+def test_prefix_caching():
     """Verify allocating full blocks when prefix caching is enabled."""
     block_size = 4
     max_seqs = 10
@@ -548,3 +777,86 @@ def test_perfix_caching():
     assert seq_group_meta[1].token_chunk_size == 12
     assert out.num_prefill_groups == 2
     assert out.num_batched_tokens == 62
+
+
+def test_prefix_caching_with_concurrent_partial_prefills():
+    """Verify allocating full blocks when prefix caching is enabled with 
+    --max-num-partial-prefills > 1."""
+    block_size = 4
+    max_seqs = 10
+    max_model_len = 8000
+    max_num_batched_tokens = 60  # With two slots, each slot will get 30 tokens
+    scheduler_config = SchedulerConfig("generate",
+                                       max_num_batched_tokens,
+                                       max_seqs,
+                                       max_model_len,
+                                       enable_chunked_prefill=True,
+                                       max_num_partial_prefills=2)
+    cache_config = CacheConfig(block_size,
+                               1.0,
+                               1,
+                               "auto",
+                               enable_prefix_caching=True)
+    cache_config.num_cpu_blocks = 0
+    cache_config.num_gpu_blocks = 32
+    scheduler = Scheduler(scheduler_config, cache_config, None)
+    running: List[SequenceGroup] = []
+
+    # Add seq groups to scheduler.
+    for i in range(2):
+        _, seq_group = create_dummy_prompt(str(i),
+                                           block_size=block_size,
+                                           prompt_length=50)
+        scheduler.add_seq_group(seq_group)
+        running.append(seq_group)
+
+    seq_group_meta, out = schedule_and_update_computed_tokens(scheduler)
+    assert set(get_sequence_groups(out)) == set(running)
+    # To partially prefill both sequences, both can chunk up to 30 tokens
+    # But the next lowest multiple of the block size (4) is 28
+    assert seq_group_meta[0].token_chunk_size == 28
+    assert seq_group_meta[1].token_chunk_size == 28
+    assert out.num_prefill_groups == 2
+    assert out.num_batched_tokens == 56
+
+    # On the next iteration, both sequences should finish prefill
+    seq_group_meta, out = schedule_and_update_computed_tokens(scheduler)
+    assert set(get_sequence_groups(out)) == set(running)
+    # Both sequences have 50 - 28 = 22 tokens left to prefill.
+    # This is not a multiple of the block size, but we don't care since we don't
+    # cache the final partial block of prefix sequences
+    assert seq_group_meta[0].token_chunk_size == 22
+    assert seq_group_meta[1].token_chunk_size == 22
+    assert out.num_prefill_groups == 2
+    assert out.num_batched_tokens == 44
+
+
+@pytest.mark.parametrize("model", ["facebook/opt-125m"])
+@pytest.mark.parametrize("max_num_partial_prefills", [2, 4, 8])
+def test_chunked_prefill_with_actual_engine(model: str,
+                                            max_num_partial_prefills: int):
+    """Make sure the model can actually sample with concurrent 
+    partial prefills
+    """
+
+    prompt = "hello" * 40
+
+    engine_args = EngineArgs(
+        model=model,
+        max_num_partial_prefills=max_num_partial_prefills,
+        max_num_batched_tokens=40,
+        max_num_seqs=8,
+        enable_chunked_prefill=True,
+        gpu_memory_utilization=0.8,
+    )
+
+    engine = LLMEngine.from_engine_args(engine_args)
+    sampling_params = SamplingParams(temperature=0)
+
+    for req_num in range(max_num_partial_prefills):
+        engine.add_request(f"{req_num}", prompt, sampling_params)
+    # first step
+    request_outputs = engine.step()
+    # means all are prefilling
+    assert len(request_outputs) == 0
+    assert len(engine.scheduler[0].running) == max_num_partial_prefills