[Bugfix][Benchmark] Make sure the output length > 0 when testing prefill workload.

[KuntaiDu]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

When testing on random dataset using prefill workload (i.e. output_len == 1) with random_range_ratio > 0, the output_low will be 0, result in request execution failure. This PR fixes this.

Example benchmarking command that will trigger request execution failure:

python benchmark_serving.py --model meta-llama/Llama-3.1-8B-Instruct --dataset-name random --random-input-len 10000 --random-output-len 1 --random-range-ratio 0.9 --request-rate inf --num-prompts 100

Test Plan

Test Result

(Optional) Documentation Update

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[gemini-code-assist]

Summary of Changes

Hello @KuntaiDu, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request provides a critical bugfix for the benchmark suite, specifically targeting an issue where prefill workloads could fail due to an invalid output length calculation. The change ensures the robustness of benchmark tests by preventing the generation of requests with an effective output length of zero.

Highlights

• Bugfix: Benchmark Output Length: I've addressed a bug in the benchmark dataset sampling logic where output_low could incorrectly become 0 when output_len was 1 (prefill workload) and random_range_ratio was greater than 0. This previously led to request execution failures.
• Ensuring Minimum Output Length: The fix ensures that the calculated output_low value is always at least 1, preventing the generation of requests with zero output tokens in benchmark scenarios.

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[gemini-code-assist]

Code Review

This pull request correctly fixes a bug in the prefill workload benchmark where the calculated minimum output length could become zero, leading to request failures. The change ensures the minimum output length is always at least 1. My feedback includes a suggestion to make the explanatory comment more concise for improved readability.

[KuntaiDu]

Did a force push to resolve DCO issue.