[Bugfix][Mooncake] Fix DP worker bootstrap register ReadTimeout race

[qingyuan18]

The Mooncake bootstrap server (uvicorn) runs inside the DP0 worker process. During startup, DP0 holds the GIL for several seconds while registering large KV-cache memory regions with the transport backend (e.g. EFA MR registration across many NICs, ~2-3s per chunk), which stalls the uvicorn event loop. Non-DP0 workers calling /register concurrently then hit httpx.ReadTimeout under the default 5s client timeout, even though the server eventually responds 200 OK. The old code only retried httpx.ConnectError and re-raised all other exceptions, so ready_event was never set and register_kv_caches blocked forever, leading to a NCCL collective timeout across the DP group.

Fix:

• Raise the httpx client timeout to 30s to tolerate event-loop starvation caused by GIL-bound backend registration on DP0.
• Retry httpx.TimeoutException and httpx.RemoteProtocolError the same way we already retry httpx.ConnectError, since both are transient symptoms of a momentarily busy bootstrap server. Non- transient failures (e.g. HTTPStatusError) still propagate.

Purpose

Fix a startup-time deadlock in the Mooncake KV connector where non-DP0
workers fail to register with the bootstrap server under a GIL-contended
DP0, causing the whole DP group to hang until a NCCL collective timeout.

File touched: vllm/distributed/kv_transfer/kv_connector/v1/mooncake/mooncake_connector.py
(15 insertions, 3 deletions — most of that is a comment explaining the timing constraint.)

Root cause

The Mooncake bootstrap server is a uvicorn app that runs inside the
DP0 worker process. All DP workers (DP0..DP_N-1) POST /register to
it concurrently during startup, right after register_kv_caches.

On DP0, register_kv_caches calls into the transport backend to
register the KV-cache memory regions. On EFA in particular, each chunk
takes ~2–3s to register, and large KV caches may be spread across many
NICs (we observe 16 NICs on p5en), so this call can hold the GIL for a
while.

While DP0 is in that path, the uvicorn event loop inside the same
process is starved and cannot respond to in-flight POSTs. The httpx
client on the caller side — httpx.AsyncClient() with the default
5s total timeout — then raises httpx.ReadTimeout even though the
server eventually does respond 200 OK a few seconds later.

Old behavior in register_worker_with_bootstrap:

try:
    async with httpx.AsyncClient() as client:         # timeout=5s (default)
        response = await client.post(url, json=...)
        response.raise_for_status()
    break
except httpx.ConnectError:
    await asyncio.sleep(1)                            # only retries ConnectError
except Exception as e:
    err_msg = (... if isinstance(e, HTTPStatusError) else str(e))
    logger.error("Error registering %s with bootstrap server: %s", payload, err_msg)
    raise e                                           # <-- fatal for ReadTimeout

Because httpx.ReadTimeout is neither ConnectError nor
HTTPStatusError, it fell through to raise e and killed that
worker's registration coroutine. ready_event is never set, the
_mooncake_sender_listener background task never completes, and
register_kv_caches blocks until the NCCL watchdog across the DP group
fires and tears the whole startup down.

One additional cosmetic symptom: str(httpx.ReadTimeout("", ...)) is
"", so the log line reads
Error registering engine_id='..._dp2' ... with bootstrap server:
with empty err_msg, which makes the failure extra hard to diagnose.

On our reproducer (2× p5en.48xlarge, DP=8), the failing rank is
random (DP1/DP2 most often, but any non-DP0 rank can lose the race),
which further argues for a timing problem rather than a configuration
one.

Fix

• httpx.AsyncClient() → httpx.AsyncClient(timeout=30.0). 30s
  comfortably covers the observed GIL-contended window (a few seconds
  per chunk × a handful of chunks) while still catching genuinely
  broken deployments.
• Add httpx.TimeoutException and httpx.RemoteProtocolError to the
  retry except clause. Both are transient symptoms of a momentarily
  busy bootstrap server; the existing retry loop around
  httpx.ConnectError is the right handler for them.
• except Exception as e: ... raise e path is unchanged. Real
  errors (4xx/5xx from the server, auth issues, etc.) still propagate
  as before.

---

Test Plan

Verified on the same environment where the deadlock was originally hit:

• Hardware: 2× p5en.48xlarge (H200, 16× EFA per node)
• Model: DeepSeek-V4-Pro
• vLLM layout: 1P1D (prefill + decode pods), DP=8 + EP
• KV transfer backend: Mooncake (EFA transport)

Steps:

1. Deploy prefill + decode pods against vLLM main without the fix —
   observe the deadlock (logs pasted below).
2. Apply this patch (mooncake_connector.py only) and redeploy the
   exact same config.
3. Confirm both prefill and decode reach
   Application startup complete, bind :8001 / :8002, and that the
   8 DP workers complete register_kv_caches without ReadTimeout.
4. Run vllm bench serve end-to-end against the service.

---

Test Result

Before the fix (reproducer)

Representative lines from a failing startup on vLLM HEAD + Mooncake +
EFA, DeepSeek-V4-Pro on 2× p5en.48xlarge, DP=8:

ERROR mooncake_connector.py:912 Error registering engine_id='<engine>_dp2' ... with bootstrap server: ⏎
... (other DP ranks stuck in register_kv_caches) ...
[rank0] NCCL WARN ... collective operation timeout ...

(The trailing empty line after bootstrap server: is str(e) == ""
from httpx.ReadTimeout.)

Failing rank is random across runs (observed DP1 and DP2 most often,
but any non-DP0 rank can hit it). Server-side the 8 POSTs eventually
all return 200 OK — the clients are already gone by then, so the
registration is effectively lost.

After the fix

• All 8 DP workers complete register_kv_caches without ReadTimeout.
• Prefill + decode both reach Application startup complete and
  listen on :8001 / :8002 as expected.
• End-to-end: vllm bench serve → 100 successful / 100 requests.
  End-to-end request path OK.
• Re-ran the same startup multiple times; deadlock no longer
  reproducible.

[claude]

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[gemini-code-assist]

Code Review

This pull request increases the HTTP client timeout to 30 seconds and expands exception handling to include timeouts and protocol errors during worker registration, addressing potential event loop starvation caused by GIL contention. The reviewer suggests catching the broader httpx.TransportError instead of only httpx.ConnectError to more robustly handle transient network issues like read or write errors that could otherwise lead to startup deadlocks.

[gemini-code-assist]

While adding httpx.TimeoutException and httpx.RemoteProtocolError addresses the specific ReadTimeout issue, it is recommended to catch the broader httpx.TransportError instead of just httpx.ConnectError. Transient network issues or a heavily loaded bootstrap server can manifest as httpx.ReadError or httpx.WriteError (both subclasses of TransportError), which would currently fall through to the except Exception block, potentially logging an empty error message and causing the same startup deadlock the PR aims to fix.

Suggested change

	except (
	httpx.ConnectError,
	httpx.TimeoutException,
	httpx.RemoteProtocolError,
	):
	except (
	httpx.TransportError,
	httpx.TimeoutException,
	httpx.RemoteProtocolError,
	):