[Bugfix] Fix Gemma4 MTP block_table batch_size mismatch under concurrent load

[Dymasik]

Purpose

Fix RuntimeError: batch_size must be equal to batch_size_k that occurs with Gemma4 + MTP + FlashAttention under concurrent load when the batch is partially occupied.

Gemma4Proposer.set_per_group_block_table() captures block tables with shape (num_reqs_padded, max_blocks) during _prepare_inputs. Later, spec_decode_common_attn_metadata is unpadded to num_reqs via .unpadded(), but the per-group block tables stored on the proposer instance are never sliced. When build_per_group_and_layer_attn_metadata swaps in the stored block table for secondary KV cache groups, FlashAttention sees page_table.size(0) == num_reqs_padded while cu_seqlens_q implies batch_size == num_reqs, triggering the assertion.

Fix: slice _per_group_block_tables[gid] to [:batch_size] in build_per_group_and_layer_attn_metadata.

Stack Trace

File "/usr/local/lib/python3.12/dist-packages/vllm/v1/worker/gpu_model_runner.py", line 4825, in propose_draft_token_ids
  self._draft_token_ids = self.propose_draft_token_ids(
File "/usr/local/lib/python3.12/dist-packages/vllm/model_executor/models/gemma4_mtp.py", line 550, in forward
  return self.optimized_call(*args, **kwargs)
File "/usr/local/lib/python3.12/dist-packages/vllm/compilation/decorators.py", line 520, in __call__
  return self._op(*args, **kwargs)
File "/usr/local/lib/python3.12/dist-packages/vllm/model_executor/layers/attention/attention.py", line 723, in unified_attention_with_output
  self.impl.forward(
RuntimeError: batch_size must be equal to batch_size_k

Test Plan

Server start command

vllm serve ./gemma-4-31B-it --host 0.0.0.0 --port 8000 --served-model-name gemma --tensor-parallel-size 8 --uvicorn_log_level error --max_num_seqs 8 --trust-remote-code --gpu-memory-utilization 0.95 --max-model-len 65536 --enable-chunked-prefill --max-num-batched-tokens 8192 --speculative-config.num_speculative_tokens 5 --speculative-config.model ./gemma-4-31B-it-assistant --max-cudagraph-capture-size 1024 --enable-prefix-caching --stream-interval 20 --async-scheduling --attention-backend FLASH_ATTN

Test script

test_gemma4_mtp_batch.py

Run test

python test_gemma4_mtp_batch.py --url http://localhost:8000 --concurrency 8 --num-requests 32

Test Result

Before fix (FlashAttention backend)

Crashes with RuntimeError: batch_size must be equal to batch_size_k at >4 RPS when num_reqs < num_reqs_padded (partial batch with CUDA graph padding).

Result

=== Phase 1: Warmup === [req 0] OK - 128 tokens in 0.67s

=== Phase 2: Concurrent load (concurrency=8) ===

--- Wave 1 ---
[req 1] OK - 80 tokens in 1.89s
[req 4] OK - 128 tokens in 1.81s
[req 2] OK - 128 tokens in 1.91s
[req 3] ERROR 500: {"error":{"message":"EngineCore encountered an issue
. See stack trace (above) for the root cause.","type":"InternalServerEr
ror","param":null,"code":500}}
[req 7] ERROR 500: {"error":{"message":"EngineCore encountered an issue
. See stack trace (above) for the root cause.","type":"InternalServerEr
ror","param":null,"code":500}}
[req 5] ERROR 500: {"error":{"message":"EngineCore encountered an issue
. See stack trace (above) for the root cause.","type":"InternalServerEr
ror","param":null,"code":500}}
[req 6] ERROR 500: {"error":{"message":"EngineCore encountered an issue
. See stack trace (above) for the root cause.","type":"InternalServerEr
ror","param":null,"code":500}}
[req 8] ERROR 500: {"error":{"message":"EngineCore encountered an issue
. See stack trace (above) for the root cause.","type":"InternalServerEr
ror","param":null,"code":500}}
FAILURES in wave 1: 5/8

--- Wave 2 ---
[req 9] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [M
ultiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0)
, [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 10] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
[req 11] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 12] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
[req 13] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 14] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
[req 15] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 16] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
FAILURES in wave 2: 8/8

--- Wave 3 ---
[req 17] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 18] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
[req 19] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 20] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
[req 21] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 22] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
[req 23] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 24] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
FAILURES in wave 3: 8/8

--- Wave 4 ---
[req 25] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 26] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
[req 27] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 28] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
[req 29] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 30] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
[req 31] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('::1', 8000, 0, 0
), [Errno 111] Connect call failed ('127.0.0.1', 8000)]
[req 32] EXCEPTION: Cannot connect to host localhost:8000 ssl:default [
Multiple exceptions: [Errno 111] Connect call failed ('127.0.0.1', 8000
), [Errno 111] Connect call failed ('::1', 8000, 0, 0)]
FAILURES in wave 4: 8/8

=== Results: 3 OK, 29 FAILED ===
BUG REPRODUCED: batch_size mismatch likely triggered

After fix (FlashAttention backend)

All 32 requests succeed across 4 waves of 8 concurrent requests with staggered timing:

Result

=== Phase 1: Warmup ===
[req 0] OK - 128 tokens in 0.35s

=== Phase 2: Concurrent load (concurrency=8) ===

--- Wave 1 ---
[req 1] OK - 72 tokens in 0.39s
[req 2] OK - 128 tokens in 0.36s
[req 3] OK - 128 tokens in 0.34s
[req 4] OK - 128 tokens in 0.36s
[req 5] OK - 128 tokens in 0.38s
[req 6] OK - 128 tokens in 0.37s
[req 7] OK - 128 tokens in 0.35s
[req 8] OK - 128 tokens in 0.34s

--- Wave 2 ---
[req 9] OK - 128 tokens in 0.36s
[req 10] OK - 128 tokens in 0.39s
[req 11] OK - 128 tokens in 0.41s
[req 12] OK - 128 tokens in 0.37s
[req 13] OK - 128 tokens in 0.37s
[req 14] OK - 128 tokens in 0.35s
[req 15] OK - 128 tokens in 0.33s
[req 16] OK - 128 tokens in 0.37s

--- Wave 3 ---
[req 18] OK - 128 tokens in 0.36s
[req 17] OK - 74 tokens in 0.41s
[req 19] OK - 128 tokens in 0.36s
[req 20] OK - 128 tokens in 0.33s
[req 21] OK - 128 tokens in 0.37s
[req 22] OK - 128 tokens in 0.35s
[req 23] OK - 128 tokens in 0.34s
[req 24] OK - 128 tokens in 0.34s

--- Wave 4 ---
[req 25] OK - 128 tokens in 0.36s
[req 26] OK - 128 tokens in 0.38s
[req 27] OK - 128 tokens in 0.41s
[req 28] OK - 128 tokens in 0.37s
[req 29] OK - 128 tokens in 0.37s
[req 30] OK - 128 tokens in 0.35s
[req 31] OK - 128 tokens in 0.33s
[req 32] OK - 128 tokens in 0.37s

=== Results: 32 OK, 0 FAILED ===
All requests succeeded - fix appears to work

TritonAttention backend

Also tested with --attention-backend TRITON_ATTN. No impact — TritonAttention was already working correctly before and after the fix since it does not perform the same batch_size == batch_size_k assertion on the page table dimension.

---

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[benchislett]

LGTM, seems reasonable.

@LucasWilkinson @TheEpicDolphin have we seen this before? Any idea why this took so long to pop up?