[Benchmark] Add asyncio-based multi_turn benchmark v2 to fix deadlock

[Potterluo]

Purpose

Fixes #42226.

The original benchmark_serving_multi_turn.py deadlocks when mp.Queue.join_thread() waits for a feeder thread that cannot flush a full OS pipe (~1 MB on Linux). mp.Queue.empty() is documented as unreliable, so the queue drain loop exits prematurely, leaving data in the pipe that join_thread() then waits for indefinitely. The deadlock is deterministic when conversation data is large enough (30-40 turns, ~30-80 KB per conversation pickled) and intermittent with smaller conversations.

This PR adds benchmark_serving_multi_turn_v2.py (new file, original unchanged) that replaces the multiprocessing architecture with a single-process asyncio event loop, eliminating all mp.Queue usage.

Why not patch the existing file?

PR #42327 (CLOSED) attempted a conservative patch (get_nowait + cancel_join_thread). This only fixes the join_thread() deadlock but leaves other deadlock paths:

1. task_queue.get() — blocking call with no timeout; clients stuck here cannot check stop_event
2. conv_queue.get() — main process blocks indefinitely if a client crashes without sending TERM_SIGNAL
3. result_queue pipe buffer backpressure — clients block on put() when the pipe is full and the main process isn't draining fast enough

The asyncio rewrite eliminates all of these by design:

• asyncio.wait_for(task_queue.get(), timeout=1.0) + stop_event check
• asyncio.wait(client_tasks, timeout=5.0) monitoring loop detects task.exception()
• Shared in-process list/dict (no pipe, no backpressure)
• --benchmark-timeout-sec global safety net

Additional features

• --generate-only: generate dataset to JSON without running the benchmark (generate once, reuse for multiple runs)
• Shared aiohttp.ClientSession with TCPConnector (pattern from vllm/benchmarks/serve.py)
• Per-client RNG instances (avoids shared global state in asyncio)

Test Plan

Tested on 2× Ascend 910B3 NPU with Qwen3-0.6B (--data-parallel-size 2), VLLM_SERVER_DEV_MODE=1, --enable-prefix-caching. Cache cleared between tests via POST /reset_prefix_cache.

Linter commands:

python -m ruff check benchmarks/multi_turn/benchmark_serving_multi_turn_v2.py
python -m ruff format --check benchmarks/multi_turn/benchmark_serving_multi_turn_v2.py

Both pass.

Test phases:

1. Low concurrency performance comparison (v1 vs v2) — 30 conversations, 8-10 turns, --no-early-stop
2. Deadlock detection — 2000 conversations, 30-40 turns, --max-num-requests 200
3. v2 upper limit — up to 2000 concurrent clients + sustained load

Test Result

Phase 1: Low concurrency (small conversations — both pass)

Clients	Version	Samples	RPS	TTFT mean	TPOT mean	Latency mean
2	v1	120	1.76	160.6ms	9.6ms	1107ms
2	v2	120	2.09	58.6ms	8.8ms	931ms
4	v1	120	3.79	77.6ms	9.2ms	990ms
4	v2	120	3.63	89.2ms	9.2ms	1003ms

At 4 clients, performance is within normal variance. v1's higher mean TTFT at 2 clients is from multiprocessing startup overhead (p90 is comparable: 86.6 vs 78.2ms).

Phase 2: Deadlock detection (large conversations)

Clients	v1	v2
2	HANG (exit 124)	Pass
4	HANG (exit 124)	Pass
8	HANG (exit 124)	Pass
10	HANG (exit 124)	Pass
100	HANG (exit 124)	Pass

v1 deadlocks at all concurrency levels (2-100c) with 30-40 turn conversations. v2 completes at all levels.

Phase 3: v2 upper limit

Clients	Samples	RPS	TTFT mean	Status
200	200	18.10	1,313ms	Pass
500	970	18.77	2,816ms	Pass
1000	1,750	18.81	18,655ms	Pass
2000	3,558	18.85	54,558ms	Pass

v2 did not crash at 2000 concurrent clients. Peak throughput ~19 RPS. Server saturation is the bottleneck, not the client.

Sustained load (100c x 4000 requests, 50c x full 2000 conversations): ran 9+ minutes, graceful timeout exit, no memory leaks.

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This PR was prepared with AI assistance (GLM5.2 + ZCode). All code was reviewed and all tests were run by the human submitter.

• The purpose of the PR (link to issue [Bug]: benchmark_serving_multi_turn.py deadlocks after clients exit when --max-num-requests is used #42226)
• The test plan (test commands)
• The test results (before/after comparison and e2e results)

[claude]

Claude Code Review

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[Potterluo]

Design Rationale: Why a new file instead of patching the original?

I considered two approaches before submitting this PR:

Option A (this PR): New benchmark_serving_multi_turn_v2.py file

Why I chose this:

1. Zero risk to existing users — the original file is unchanged, so anyone currently using benchmark_serving_multi_turn.py is unaffected
2. Eliminates all deadlock paths, not just join_thread() — while join_thread() is the most commonly observed deadlock, the mp.Queue architecture has other failure modes (blocking get() with no timeout, empty() unreliability, pipe buffer backpressure on put()). The asyncio rewrite addresses all of them by design
3. Follows an existing pattern — vllm/benchmarks/serve.py already uses a pure-asyncio approach (shared aiohttp.ClientSession, asyncio.Semaphore, asyncio.create_task). v2 brings the multi-turn benchmark in line with that pattern
4. Additional features — --generate-only (separate dataset generation from benchmarking) and --benchmark-timeout-sec (global safety net) are easier to add in the asyncio model

Downside: Two files to maintain. I understand this is a concern.

Option B (alternative if maintainers prefer): Minimal patch to the existing file

If the maintainers prefer a direct patch to benchmark_serving_multi_turn.py, I can close this PR and submit a smaller one with these targeted fixes (~30-40 lines changed):

# 1. Replace empty()-based drain with get_nowait() (fixes unreliable empty())
import queue
while True:
    try:
        task_queue.get_nowait()
    except queue.Empty:
        break

# 2. Replace join_thread() with cancel_join_thread() (fixes feeder deadlock)
task_queue.cancel_join_thread()
result_queue.cancel_join_thread()
conv_queue.cancel_join_thread()
# (no need to call close() — cancel_join_thread() handles it)

# 3. Add timeout to task_queue.get() in client_main so stop_event is checked
conv_id, messages = task_queue.get(timeout=1)  # was: task_queue.get() (blocking)

This approach is similar to the closed PR #42327 but adds the task_queue.get(timeout=) fix which #42327 did not include.

My recommendation

I'm open to either approach. If Option B is preferred, I'll close this PR and submit the patch. Please let me know which direction the maintainers would like to go.

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This assessment was prepared with AI assistance (GLM5.2 + ZCode). Reviewed by the human submitter.