Race conditions in metadata deserialization cause intermittent failures

[dmvevents]

Description

Multiple race conditions in metadata serialization/deserialization cause intermittent failures during KV cache transfer setup, especially in multi-rail configurations.

Environment

• NIXL 0.8.0
• Multi-rail libfabric backend
• Disaggregated inference (separate prefill/decode workers)

Issues Found

1. Empty Descriptor Crash (v81)

File: src/infra/nixl_memory_section.cpp

When remote agent publishes metadata before KV cache is allocated, empty descriptor list causes error:

if (s_desc.descCount()==0)
    return NIXL_ERR_NOT_FOUND;  // Should continue, not fail

2. Missing MemSection Marker (v82)

File: src/core/nixl_agent.cpp

Missing marker causes permanent failure instead of retry:

if (sd.getStr("") != "MemSection") {
    return NIXL_ERR_MISMATCH;  // Should warn and retry
}

3. Premature Backend Removal (v85)

File: src/core/nixl_listener.cpp

On metadata load failure, backend connection is removed, requiring full restart:

remoteBackends.erase(remote_name);  // Should keep connection

4. Segment Count Race (v86)

File: src/infra/nixl_memory_section.cpp

seg_count can indicate more backends than actually serialized:

if (nixl_backend.size()==0)
    return NIXL_ERR_INVALID_PARAM;  // Should break loop

5. Stale Cache Entries (v88/v89)

File: src/core/nixl_agent.cpp

Empty or failed metadata loads are cached permanently, preventing retry.

Impact

These issues cause intermittent connection failures that require pod restarts to resolve, especially in multi-worker disaggregated inference setups.

Proposed Fixes

See individual patches in our repository for detailed fixes. General approach:

• Replace hard errors with warnings + retry
• Don't cache failed/empty metadata
• Keep connections alive on transient failures

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[fengjica]

While libfabric is in the setup, the behavior discussed here is defined in nixl's core or infra code. From the code quoted in this issue, I cannot easily tell whether it will crash or just return an error code to their caller site.

I think these issues will need more discussion with nixl core maintainers.

[ColinNV]

See individual patches in our repository for detailed fixes.

Could you please provide the links to these patches?

[ColinNV]

@dmvevents If you could please list the relevant patches in the repo that you mentioned above.

[dmvevents]

Summary

@fengjica @ColinNV - Responding to the request for more details.

The Key Evidence

Your own TODO at nixl_listener.cpp:780:

// src/core/nixl_listener.cpp:779-786
const nixl_status_t ret = remoteSections[remote_name]->loadRemoteData(&sd, backendEngines);
// TODO: can be more graceful, if just the new MD blob was improper  <-- THIS TODO
if (ret != NIXL_SUCCESS) {
    delete remoteSections[remote_name];    // Deletes ALL cached metadata
    remoteSections.erase(remote_name);
    remoteBackends.erase(remote_name);
    return ret;
}

The Problem

ANY loadRemoteData() error permanently deletes ALL cached metadata for that agent. This is too aggressive for transient errors during concurrent metadata updates.

Race scenario:

1. Prefill worker serializes metadata to etcd
2. Decode worker reads before write completes
3. Partial data causes deserialization failure
4. Current: Returns permanent error, deletes all cached metadata
5. System never recovers without restart

Reproduction (P5.48xlarge required)

# Two workers - disaggregated inference
# Prefill publishes metadata, decode reads it
resources:
  limits:
    nvidia.com/gpu: "8"
    vpc.amazonaws.com/efa: "32"
env:
  NIXL_BACKEND: "LIBFABRIC"
  FI_EFA_USE_DEVICE_RDMA: "1"

Watch for:

kubectl logs -f -l component=decode-worker 2>&1 | grep -i "deserialization\|NOT_FOUND"

Expected error:

E1224 20:12:21 serdes.cpp:52] Deserialization of tag nixlDList failed

What PR #1220 Does

Implements the TODO by making error handling graceful:

1. Empty descriptors → skip instead of error
2. Partial metadata → allow retry instead of permanent failure
3. Preserve previously cached valid metadata

Verification (No Hardware Needed)

1. Look at nixl_listener.cpp:780 - the TODO exists
2. Trace the error path - any failure deletes all cached data
3. Confirm this is the intended behavior for transient errors

Would a unit test demonstrating the error path help move this forward?