[Cherry-Pick][Optimization]Change default workers and max-concurrency when launch api-server(#7457)

[EmmonsCurse]

Cherry-pick of #7457 (authored by @K11OntheBoat) to release/2.6.

devPR:#7457

---

Motivation

API-Server 启动服务时，将workers默认从1改为4，服务最大concurrency同步改为2048. 避免了GPU 运行快，但是请求不足的性能瓶颈. Blackwell架构，长输入短输出数据，512并发场景，GLM4.5-Air模型TPS提升1.74倍.
multi_api_server中这两个参数保持不变.

Modifications

API-Server 启动服务时，将workers默认从1改为4，服务最大concurrency同步改为2048.

Checklist

• Add at least a tag in the PR title.
• Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
• You can add new tags based on the PR content, but the semantics must be clear.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[paddle-bot]

Thanks for your contribution!

[PaddlePaddle-bot]

🤖 AI Code Review | 2026-04-20 22:40:39

📋 Review 摘要

PR 概述：将 API-Server 启动时 workers 默认值从 1 改为 4，max-concurrency 从 512 改为 2048，提升 GPU 利用率；multi_api_server 模式保持原值不变。
变更范围：entrypoints/api_server.py、entrypoints/openai/utils.py、CI 配置
影响面 Tag：[APIServer] [CI]

问题

未发现阻塞性问题。

总体评价

变更逻辑清晰，通过 FD_ENABLE_MULTI_API_SERVER 环境变量合理区分单服务与多服务模式的默认参数，CI 测试也通过显式传参 --workers 1 进行了适配，避免默认值变更影响已有测试用例。Cherry-pick 内容与原 PR #7457 一致。

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release/2.6@f4f7760). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
fastdeploy/entrypoints/api_server.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##             release/2.6    #7516   +/-   ##
==============================================
  Coverage               ?   74.18%           
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  Files                  ?      376           
  Lines                  ?    52999           
  Branches               ?     8279           
==============================================
  Hits                   ?    39317           
  Misses                 ?    10919           
  Partials               ?     2763           

Flag	Coverage Δ
GPU	74.18% <75.00%> (?)

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