NVLS Multicast Slot Exhaustion (NVSwitch)
NCCL exhausts the NVSwitch's fixed pool of NVLink SHARP (NVLS) multicast slots, then treats the binding failure as fatal instead of falling back to a non-NVLS transport.
NCCL exhausts the NVSwitch's fixed pool of NVLink SHARP (NVLS) multicast slots, then treats the binding failure as fatal instead of falling back to a non-NVLS transport.
What this failure is
NVLS Multicast Slot Exhaustion (NVSwitch) is a Communication failure seen during ML training runs. NCCL exhausts the NVSwitch's fixed pool of NVLink SHARP (NVLS) multicast slots, then treats the binding failure as fatal instead of falling back to a non-NVLS transport. Common tags: Nccl, Nvls, Nvlink Sharp, Nvswitch.
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Why it happens (the mechanism)
NCCL aggressively allocates an NVLS multicast group per sub-communicator. The ~128 hardware multicast slots on the NVSwitch fill up. CuMulticastBindMem failure is treated as fatal with no graceful fallback. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- Training crashes when many sub-communicators are created (e.g. per-pair FSDP groups)
- Crash is fatal even though a non-NVLS path exists
- Started after upgrading NCCL
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| Failed to bind NVLink SHARP (NVLS) Multicast memory of size ... : CUDA error 2 'out of memory' | NCCL aggressively allocates an NVLS multicast group per sub-communicator |
| Fabric Manager: all the NVSwitch multicast resources/slot ids are used | The ~128 hardware multicast slots on the NVSwitch fill up |
| ncclUnhandledCudaError on communicator init | cuMulticastBindMem failure is treated as fatal with no graceful fallback |
Which systems are affected
- 8-GPU H100/H200 nodes with NVSwitch
- Workloads creating many small sub-communicators
- NCCL 2.29.x (regression from 2.28.x)
How to confirm this is the problem
Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: Failed to bind NVLink SHARP (NVLS) Multicast memory of size ... : CUDA error 2 'out of memory'
- ✓Verified signal present: Fabric Manager: all the NVSwitch multicast resources/slot ids are used
- ✓Verified signal present: ncclUnhandledCudaError on communicator init
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
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Install the free VS Code extensionNCCL errors in context
NCCL is where a distributed job reports failure, which is not the same as where it failed. The hub lists every common NCCL error next to what it actually indicates, and the environment variables that tell them apart.
Compare every nccl error side by sideRelated failures to investigate next
Root cause
- NCCL aggressively allocates an NVLS multicast group per sub-communicator
- The ~128 hardware multicast slots on the NVSwitch fill up
- cuMulticastBindMem failure is treated as fatal with no graceful fallback
The fix and how to prevent it
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