[Bugfix][NIXL] Release a dead peer's NIXL state without waiting for TTL

[YannikHinteregger]

Purpose

When a pull-mode prefiller restarts with a new engine ID at the same host and port, the decoder can keep the previous peer's NIXL resources and cached metadata until TTL cleanup.

This PR detects the replacement after a successful handshake and marks the old peer for cleanup. It waits for requests using the old peer and pending handshakes to finish before releasing the old state. This avoids removing resources that are still in use and allows cleanup without restarting the decoder or waiting for TTL eviction.

Test Plan

.venv/bin/python -m pytest tests/v1/kv_connector/unit/test_nixl_connector.py 
143 passed

Also ran a GPU test on two RTX PRO 4500 GPUs using Qwen3-0.6B, with one GPU for P and one for D. P was restarted twice normally and once after a forced shutdown while D stayed running. Each time, P returned with a new engine ID at the same address and D cleared the old peer state.

All new requests after the restarts succeeded and matched the outputs from the original P/D setup. Requests interrupted by the forced shutdown failed and were not resumed.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @YannikHinteregger.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[NickLucche]

Hey @YannikHinteregger thanks for the PR!
Reporting some of my thoughts here: I am not a huge fan of the indirect way in which D should stretch to figure out whether a P crashed.
As D has no way to figure that out (a xfer could fail once and wouldnt be a good proxy to tell P is a goner) ~by design, as the full knowledge of the system is assumed to be residing at the router level, I am not sure we should try to infer P state to allow users to set static engine_ids (which is by definition tricky to ensure as engine_id is effectively being used as id in many parts).

What I would propose instead is to expose a proper endpoint in which the router can communicate to D that P is dead.
This way we keep the full knowledge with the router level, and expose a clear interface that users wanting to use fixed engine_ids should have to abide to: that is when they scale down before routing to the P<>D pair they should hit D new endpoint.
We would also be able to piggyback on that "P is dead" event with some other logic for a couple of other features @hasB4K.

The change would be non-breaking for the rest: routers that do not abide to this interface won't get the logic attached to this new event.

[chaunceyjiang]

I agree with @NickLucche’s point. It might be better to handle this at a higher level of the system.

[YannikHinteregger]

Thanks both! Yeah, that makes sense to me. An explicit "peer is gone" event would also enable fixes for a few rows on #48633.

Just so I understand you correctly: You want an endpoint that a router can call so that it can tell an instance "peer X is gone, drop everything you have for it". So the knowledge stays at the router level and D never has to infer anything.

Something like this?

POST /fault_tolerance/event
{
  "event_type": "peer_gone",
  "nodes": {"engine_ids": ["..."]}
}

Happy to build it. Would you rather I do that in this PR or close this one and open a separate PR for the endpoint?

A few questions if you have a moment:

• In llm-d every D is connected to every P: So I'd expect the router to broadcast to the whole pool and not track pairs. That means the endpoint must be idempotent and defaults to no-ops for cleared/unknown nodes. Does that sound right? Sadly I don't know how other routers behave.
• Depending on the router it might not know vLLM engine ids, right? Should the endpoint accept a host:port too or should the orchestrator learn engine ids? I'm leaning towards including the address in the post.
• Should this also fail the in-flight requests waiting on the dead peer, or only drop the cached state? Failing them is what actually fixes a hang, but on a graceful scale-down it could kill requests that would still have completed. Maybe that's two event types.
• This is a type of control endpoint so it probably shouldn't sit behind authentication, matching /reset_prefix_cache and /sleep (they're outside the api-key guarded prefixes). What do you think?
• Should it only exist when --enable-fault-tolerance is set, like /fault_tolerance/apply?

Anything I'm missing?

[tylerjiang]

Hey @YannikHinteregger thanks for the PR! Reporting some of my thoughts here: I am not a huge fan of the indirect way in which D should stretch to figure out whether a P crashed. As D has no way to figure that out (a xfer could fail once and wouldnt be a good proxy to tell P is a goner) ~by design, as the full knowledge of the system is assumed to be residing at the router level, I am not sure we should try to infer P state to allow users to set static engine_ids (which is by definition tricky to ensure as engine_id is effectively being used as id in many parts).

What I would propose instead is to expose a proper endpoint in which the router can communicate to D that P is dead. This way we keep the full knowledge with the router level, and expose a clear interface that users wanting to use fixed engine_ids should have to abide to: that is when they scale down before routing to the P<>D pair they should hit D new endpoint. We would also be able to piggyback on that "P is dead" event with some other logic for a couple of other features @hasB4K.

The change would be non-breaking for the rest: routers that do not abide to this interface won't get the logic attached to this new event.

I'd like to add a perspective on the responsibility split.

I agree that monitoring the health status of P/D and other peer nodes is the router's job. That's a separate concern. But the issue here is different: a P or D node is holding a stale NIXL communication handle. From a responsibility standpoint, when P and D are able to clean up their own stale handles, they should do so locally rather than depending entirely on an external component to tell them to.

There's a key point worth emphasizing: in this scenario the IP:port remains unchanged, and only the engine ID changes after a restart. That is a very strong, deterministic signal that the P node has restarted — not a flaky heuristic like "a transfer failed once." Given that, cleaning up the old handle immediately is clearly correct and reasonable.

What do you think?

[tylerjiang]

Hey @YannikHinteregger thanks for the PR! Reporting some of my thoughts here: I am not a huge fan of the indirect way in which D should stretch to figure out whether a P crashed. As D has no way to figure that out (a xfer could fail once and wouldnt be a good proxy to tell P is a goner) ~by design, as the full knowledge of the system is assumed to be residing at the router level, I am not sure we should try to infer P state to allow users to set static engine_ids (which is by definition tricky to ensure as engine_id is effectively being used as id in many parts).

What I would propose instead is to expose a proper endpoint in which the router can communicate to D that P is dead. This way we keep the full knowledge with the router level, and expose a clear interface that users wanting to use fixed engine_ids should have to abide to: that is when they scale down before routing to the P<>D pair they should hit D new endpoint. We would also be able to piggyback on that "P is dead" event with some other logic for a couple of other features @hasB4K.

The change would be non-breaking for the rest: routers that do not abide to this interface won't get the logic attached to this new event.

And there's another important consideration: if the router doesn't currently support this, the changes required on the router side would be enormous. It would also require P and D to implement a new interface for the cleanup logic.

The cost here is worth calling out clearly:

Router-side changes are substantial. To notify D to clean up P's handles, the router would need health-check/heartbeat capabilities to detect P crashes or scale-downs — something many routers (e.g. simple load balancers) simply don't have. It would also need to maintain a P→D topology mapping to know which D instances are connected to which P, plus a reliable delivery mechanism (retries, acknowledgements) and strict ordering guarantees (the notification must arrive before D sends its next request to P).

P and D still need to implement the cleanup interface. And here's the key point: the cleanup logic D runs when it receives a peer_gone event is essentially the same code D would run if it detected the stale handle itself. So the router-based approach doesn't save any D-side work — it only moves the trigger from "D detects it internally" to "D waits for the router to tell it."

Compared to that, the D-side approach (detecting the stale handle locally) requires changes only in base_worker.py, no router changes, no new interfaces, and no external dependency. It solves the same problem at a fraction of the cost.

Given that, I'd suggest the D-side detection as the immediate fix, with the router notification as a future enhancement (to cover scale-down/migration scenarios) rather than a replacement.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @YannikHinteregger.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[YannikHinteregger]

@NickLucche Update before you look at this next week.

While discussing #55471 with @xijiaat we found that NIXL itself drops its copy of a peer's metadata when a transfer returns REMOTE_DISCONNECT but vLLM keeps the Python cache. From then on every request to that peer fails, even when the peer is still alive. xijiaat saw this in production after a single P-D link failed.

Two conclusions here:

• The "peer is gone" endpoint wouldn't fix this, because a router can't see from the outside that this one link is unhealthy.
• With xijiaat's trigger D wouldn't infer whether P is dead. It would just drop its own cache once NIXL has dropped the peer.

Given that, I'd now suggest folding xijiaat's trigger into this PR and implementing the endpoint as a separate effort. But that's your call.

[liuzijing2014]

@YannikHinteregger is this PR ready to review? We would like to accelerate the process to have this merged.

cc @NickLucche @njhill

[YannikHinteregger]

Thanks @liuzijing2014! Yes this PR is ready for another round of review.
I’ve addressed the comments as discussed and NickLucche’s prerequisite PR #56104 has been merged. I’ve also retested it on a GPU cluster. Appreciate you helping move this forward!

[NickLucche]

Thanks for the work @YannikHinteregger !

Only two things:

• I think we can drop _replaced_remote_engines if we flip the map to (host, port) -> engine_id. The replaced engines can then be derived at cleanup time: self._remote_agents.keys() - self._engine_by_address.values(), returning early when not in pull mode. We have one fewer data structure to keep track of.
• the whole _cleanup_remote_engine is called while holding _handshake_lock, but the only ds that needs it is _remote_agents. We can collect and clean in 2 steps

[NickLucche]

can you leave a TODO to address as a follow up for push?
They have handshakes both ways. Otherwise I would've moved this method into pull_worker.py if it was pull only

[YannikHinteregger]

done 👍

[NickLucche]

Suggested change

	if len(self._replaced_remote_engines) == 0:
	return
	with self._handshake_lock:
	if len(self._handshake_futures) > 0:
	return
	busy = {
	meta.remote.engine_id
	for meta in self._recving_metadata.values()
	if meta.remote is not None
	}
	for engine_id in self._replaced_remote_engines - busy:
	if engine_id in self._remote_agents:
	self._cleanup_remote_engine(engine_id, log_eviction=False)
	logger.info(
	"Released NIXL state for replaced remote engine %s.", engine_id
	)
	self._replaced_remote_engines.discard(engine_id)
	if len(self._replaced_remote_engines) == 0:
	return
	busy = {
	meta.remote.engine_id
	for meta in self._recving_metadata.values()
	if meta.remote is not None
	}
	with self._handshake_lock:
	if len(self._handshake_futures) > 0:
	return
	idle = self._replaced_remote_engines - busy
	self._replaced_remote_engines -= idle
	to_clean = [eid for eid in idle if eid in self._remote_agents]
	for engine_id in to_clean:
	self._cleanup_remote_engine(engine_id, log_eviction=False)
	logger.info(
	"Released NIXL state for replaced remote engine %s.", engine_id
	)

[YannikHinteregger]

Thanks, applied a similar version

[YannikHinteregger]

Thanks for the review @NickLucche!
I addressed your comments in the latest commit. Deriving the replaced engines makes a lot of sense

[NickLucche]

LGTM , unblocking for Nick to take a look at

[njhill]

Thanks @YannikHinteregger. I have a few comments and then LGTM

[njhill]

Thanks @YannikHinteregger

[njhill]

/ci run

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[github-actions]

✅ @YannikHinteregger, CI is now available for this PR.

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[YannikHinteregger]

/ci run

[github-actions]

✅ Triggered Buildkite CI #91436 for commit dfb7dd92a406.