/asyncopenai-concurrency-httpx-pool

name: asyncopenai-concurrency-httpx-pool
description: |
  Raise real concurrency in asyncio LLM batch scorers built on the OpenAI SDK
  (AsyncOpenAI, including OpenAI-compatible providers like DeepSeek). Use when:
  (1) raising an asyncio.Semaphore above ~100 produces no throughput gain,
  (2) a batch pipeline saturates near 100 in-flight requests despite a larger
  semaphore, (3) planning a high-concurrency campaign against a provider with
  no hard rate limit (DeepSeek v4-flash tolerates 2000+ in flight). Root cause:
  AsyncOpenAI's default httpx pool caps max_connections at 100, silently
  bottlenecking any larger semaphore — you must pass a custom http_client with
  httpx.Limits sized to the semaphore.
author: Claude Code
version: 1.0.0
date: 2026-07-17

AsyncOpenAI Concurrency: the Hidden httpx Pool Cap

Problem

Async batch scorers typically gate concurrency with `asyncio.Semaphore(N)`. Raising N above ~100 silently does nothing: the OpenAI SDK's default httpx transport caps the connection pool at `max_connections=100`, so excess tasks queue inside httpx instead of reaching the provider. The semaphore looks like the throttle but is not the binding constraint — there is no error, just a throughput ceiling.

Context / Trigger Conditions

• `asyncio.Semaphore(N)` with N > 100 around `client.chat.completions.create`

shows the same throughput as N = 100

• Client constructed as `AsyncOpenAI(api_key=..., base_url=...)` with no

`http_client` argument (the default transport)

• Provider is known to allow high concurrency (DeepSeek v4-flash: ~2500)
• Symptom check: requests-in-flight measured at the server never exceeds ~100

Solution

Size the httpx pool to the semaphore when constructing the client:

import httpx
from openai import AsyncOpenAI

CONCURRENCY = 2000
client = AsyncOpenAI(
    api_key=..., base_url="https://api.deepseek.com",
    http_client=httpx.AsyncClient(limits=httpx.Limits(
        max_connections=CONCURRENCY,
        max_keepalive_connections=CONCURRENCY)))
sem = asyncio.Semaphore(CONCURRENCY)

Both edits are required; either alone caps the other. Keep the per-request retry loop — at high concurrency transient failures are more likely, and the retry envelope is what turns them into non-events.

Verification

Throughput scales with N. Verified 2026-07-17 on DeepSeek v4-flash (deepseek-chat, JSON-mode unit scoring, ~1.5k-token prompts): at semaphore 50 a cold 13.8k-request chunk took ~70 min; at semaphore 2000 + matched pool, a 14.4k-request chunk took ~11 min (~40 req/s sustained, ~250/s burst on a 1.7k-request tail chunk), 0 failed requests, 0 schema-invalid responses. Effective speedup ~7x rather than 40x — server-side queuing absorbs the rest — but with zero reliability cost.

Example

Specialist Directors US T1 campaign: final 8 chunks (100,243 calls) completed in ~55 minutes for $8.08 after the fix, versus a projected ~9 hours at the old setting. The edit is two lines in the scorer; the semaphore constant alone would have been a silent no-op.

Notes

• DeepSeek publishes no hard rate limit and handled 2000 in-flight cleanly;

the practical ceiling reported is ~2500. Other providers enforce RPM/TPM caps — check before sizing.

• Windows: default asyncio proactor loop handled 2000 sockets without

tuning; no ulimit-style adjustment needed.

• Companion ops lesson from the same campaign: when a run's *plan* changes

scale (two-night legs -> one-shot), re-audit the launch glue's hardcoded limits — a wrapper `MAX-HOURS=10` safety cap sized for the old plan hard-killed a healthy runner at 97/108 chunks. Caps and budgets in supervisor scripts must be revisited whenever expected duration changes.

• Concurrency is a pure throughput knob: per-request outputs are unchanged

(temperature 0, independent requests), so raising it mid-campaign does not create a scoring seam — unlike model/prompt changes, which do (see [llm-campaign-drift-gate]).