[Misc] Add --max-duration-sec to benchmark_serving_multi_turn.py

[nikonyrh-siloai]

Purpose

Currently it is difficult to estimate how long a benchmark run will take. In addition, we'd like to take all benchmark measurements in a "steady state", but currently the request rate drops towards the end (if --no-early-stop is used). If early stopping isn't used, the benchmark duration is very random if the number of turns / session has high variance.

This PR adds an explicit, simple and optional --max-duration-sec. Stopping is handled by pre-existing stop_event.set().

Test Plan

Run the benchmark without max duration, and see that the run time is random. With the patch the run time is deterministic.

time python benchmark_serving_multi_turn.py \
  --model deepseek-ai/DeepSeek-V3 \
  --input-file config.json \
  --num-clients 20 \
  --request-rate 0.1 \
  --max-active-conversations 200 \
  --max-num-requests 360

Test Result

Adding --max-duration-sec 30 stops the benchmark after 30 seconds.

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[gemini-code-assist]

Code Review

This pull request adds a --max-duration-sec option to the multi-turn serving benchmark to limit its total execution time. The changes include adding the CLI argument, updating the BenchmarkArgs configuration, and implementing a runtime check that sets a stop event when the duration is exceeded. Feedback indicates that the duration check should not depend on the task queue being non-empty, as multi-turn processing continues after the queue is exhausted, and a code suggestion was provided to fix this logic.

[gemini-code-assist]

The condition not task_queue.empty() incorrectly limits the duration check to only the period when new conversations are being dispatched. In multi-turn benchmarks, the initial dispatch often finishes quickly, leaving the queue empty while clients continue to process multiple turns. This would cause the --max-duration-sec limit to be ignored for the remainder of the benchmark once all conversations have started. Removing this condition ensures the duration limit is respected throughout the entire run, including the multi-turn processing phase.

            if (
                not stop_event.is_set() and
                bench_args.max_duration_sec > 0 and
                runtime_sec > bench_args.max_duration_sec
            ):