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To identify an AI network bottleneck, prove that communication is delaying the job before changing NCCL settings. Compare communication-heavy phases, verify the interfaces and peer paths NCCL selected, measure repeated point-to-point bandwidth and latency, then inspect fabric-specific counters. InfiniBand and RoCE need different evidence: InfiniBand troubleshooting centers on RDMA, subnet-manager and port-error counters, while RoCE requires congestion, pause/PFC, ECN, CNP and queue-drop telemetry. Correct the failing layer, rerun the same baseline, and only then consider NCCL tuning.
What a network-bound AI workload looks like
Distributed training and inference can spend substantial time exchanging gradients, activations, parameters or other tensors between GPUs on different hosts. A slow run does not prove that the fabric is at fault: GPU utilization, process launch, MPI or runtime setup, storage, synchronization and application scheduling can produce similar symptoms. NVIDIA’s NCCL troubleshooting documentation separates networking from GPU, runtime/MPI, logging, reliability, availability and serviceability (RAS), and performance-tuning investigations for this reason.
Start with an observable symptom rather than a theory. Capture the same job configuration across runs and compare phases that exchange data heavily with phases that are mostly local. A network problem is more credible when communication-heavy sections expand, collective operations become irregular across ranks, or step time changes with the number of participating hosts while local GPU work remains comparable.
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- Record the model, batch and sequence sizes, number of hosts and GPUs, process placement, software versions, and fabric type.
- Mark the time spent in communication-heavy collectives separately from forward, backward and optimizer work.
- Compare several runs using the same placement and input shape. Look for unstable tails, not just a slower average.
- Check that GPUs are ready, the runtime and MPI layer initialize cleanly, and there are no unrelated application or storage stalls.
If the communication phase is not the part that changed, move the investigation to the component that did. Do not “fix” a healthy network by adding unexplained environment variables.
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Use NCCL diagnostics before tuning
NCCL has passive readiness checks and active communication-path diagnostics. The diagnostics documentation identifies these built-in features as available as of NCCL v2.31. Use the diagnostics supported by the NCCL version installed in the cluster, and keep their output with the run record. A readiness failure can expose an interface, device or runtime problem without requiring a full training job; an active path test can show whether ranks can communicate over the route the job will use.
Validate the selected path and the physical baseline
An interface can be administratively up and still be unable to communicate with every peer. NCCL may select that interface unless the configuration or topology prevents it. Validate selection and connectivity before interpreting performance numbers.
Confirm which interfaces and devices are in use
- Record the host’s network interfaces, addresses, link layer and negotiated rate.
- Confirm that every participating host has a route to its peers over the intended fabric and that the selected device is attached to the expected GPU locality.
- Compare the interface NCCL reports or uses with the interface you intended. An “up” administrative state is not proof of peer reachability.
- Repeat the check on all hosts. A single host with a different interface, rate or route can make a collective appear randomly slow.
For InfiniBand, NVIDIA’s networking troubleshooting guidance points to ibstat and ibstatus for port state, link layer and rate checks. For Ethernet and RoCE NICs, inspect driver statistics with ethtool -S. Command output and available fields vary by NIC, driver and operating-system build, so use the syntax supported by the deployed version.
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ibstatus
ethtool -S <interface>Run point-to-point fabric tests
Use the perftest tools that are installed and permitted in your environment. ib_write_bw measures write bandwidth and ib_write_lat measures write latency. Run client and server roles on the intended hosts, use the GPU-memory or other options supported by your build, and document the exact command line. A test that uses host memory or a different path than the training job can hide the real problem.
# On the server host
ib_write_bw
# On the client host, use the server address and options supported by your build
ib_write_bw <server-address>
# Repeat the same pattern with latency
ib_write_lat
ib_write_lat <server-address>These tests are baselines, not guarantees of application performance. Run them between representative host pairs and, when possible, during the traffic pattern that resembles the workload. Preserve several samples so that tail behavior and run-to-run variation are visible.
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Measure latency as well as bandwidth
Peak throughput can look healthy while synchronization is delayed by queueing, retransmissions or intermittent link problems. Compare repeated bandwidth and latency observations rather than relying on a single maximum number.
Read the pattern, not one result
| Observation | What it suggests | Next check |
|---|---|---|
| Bandwidth is consistently low and latency is consistently high | A path, rate, topology or configuration limitation is plausible. | Verify selected interfaces, negotiated rate, peer reachability and physical link state. |
| Average bandwidth is near the expected link capability but latency has large or rising tails | Queueing, congestion, retransmissions or intermittent errors may be delaying synchronization. | Inspect fabric counters and switch telemetry while repeating the test. |
| Bandwidth fluctuates between repetitions and latency fluctuates with it | The path may be contended, dropping or retrying traffic. | Correlate counter changes with each test interval; check congestion and port health. |
| Point-to-point tests are stable but the training job is slow | The issue may be collective topology, rank placement, GPU readiness, runtime setup or application behavior rather than raw link capacity. | Compare rank timelines and NCCL diagnostic output before changing fabric settings. |
There is no universal “good” latency or bandwidth value that applies to every NIC, topology, message size and traffic pattern. The useful comparison is the same path and command before and after a change, under the same host and workload conditions.
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InfiniBand: inspect RDMA, subnet and port evidence
InfiniBand troubleshooting has transport and fabric signals that are not interchangeable with Ethernet counters. Collect them during a reproducible test and note whether they increase while latency tails or throughput instability appear.
Check RDMA error and retry counters
Inspect RDMA statistics for counters including rnr_nak_retry_err, packet_seq_err, implied_nak_seq_err and local_ack_timeout_err. These counters identify transport events worth correlating with the affected port, host pair and test interval. A single nonzero value without a time trend does not establish the cause; growing values during the slow operation are more actionable.
Verify Subnet Manager and port health
Use sminfo to check Subnet Manager status. A path that is operational at the host may still be affected by fabric-management or routing conditions. Use perfquery to inspect link and symbol errors, discards and related port statistics. Compare the local and remote ports and follow the path through the switches when the environment permits it.
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perfqueryWhen errors, discards or retries rise with the workload, investigate the port, optic, cable, switch path and rate negotiation before changing NCCL. Replace or reseat a suspect physical component only under your organization’s change-control process, then rerun the exact baseline.
RoCE: diagnose congestion and lossless-fabric behavior
RoCE uses Ethernet infrastructure, so its failure modes and telemetry differ from InfiniBand. Inspect NIC counters and any switch telemetry available to operators. Useful signals include packet drops, pause or Priority Flow Control (PFC), Explicit Congestion Notification (ECN), Congestion Notification Packets (CNP) and queue drops.
Correlate congestion indicators with workload behavior
- Rising PFC or pause events together with latency growth points toward queue pressure or lossless-fabric configuration.
- ECN marks and CNP traffic that rise while throughput becomes unstable indicate that congestion control is active or being triggered.
- Queue drops or packet drops during the same interval suggest that the path is not preserving the lossless behavior expected by the deployment.
- A clean counter sample taken after the workload has stopped cannot rule out a transient event; collect before-and-after values around the test.
NVIDIA’s NCCL Networking Troubleshooting documentation states: “RoCE performance problems are often tied to congestion control or lossless fabric settings rather than outright link failure.” Treat that as a direction for investigation, not a universal diagnosis. Confirm the behavior in your NIC and switch telemetry, then consult the documentation for the exact driver, firmware and switch configuration.
Check the complete Ethernet path
Validate MTU and priority handling consistently across hosts, NICs and switches according to your deployment standard. Verify that the traffic class used for RDMA is mapped consistently and that PFC and ECN policies match on every hop. The exact settings are environment-specific; avoid copying values from a different switch family or driver release.
Separate fabric, configuration and physical-link causes
Use the evidence to choose the smallest layer that explains the symptom. The following branches keep corrective action tied to an observation.
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| Evidence pattern | Likely layer to investigate | Action before NCCL tuning |
|---|---|---|
| NCCL selects an interface that cannot reach all peers | Interface selection or host connectivity | Correct addressing, routing, interface selection and peer reachability; rerun a path test. |
| Link state or negotiated rate differs from the deployment baseline | Physical link or port configuration | Check port, optic, cable, switch port and rate negotiation; involve the network operator if the path crosses managed switches. |
| InfiniBand retries, sequence errors, timeouts, link errors or discards grow during the slow run | RDMA transport or physical/fabric health | Trace the affected ports and path, correct the underlying health issue, then repeat bandwidth and latency tests. |
| RoCE PFC, ECN, CNP, pause or queue-drop indicators rise with latency tails | Congestion control or lossless-fabric configuration | Review queueing, priority, PFC, ECN and congestion-control settings with NIC and switch documentation. |
| Fabric tests are healthy but rank timelines show uneven work or initialization delays | GPU placement, runtime, MPI, application or collective topology | Investigate process mapping, GPU readiness, runtime logs and collective behavior rather than the physical network. |
Apply the correction at the diagnosed layer
Fix interface and connectivity problems first
Remove or repair unusable interfaces, correct routes and ensure every rank can reach its peers over the intended network. Confirm that all hosts use the same interface class and expected link rate. A configuration change is complete only when a path test succeeds from every relevant host pair.
Repair physical and port faults
For degraded links, inspect port state, cabling, optics, switch ports and negotiated speed. In InfiniBand, use RDMA and port statistics; in RoCE, use NIC and switch counters. Replace components or move a link only when the counter and path evidence identifies the affected segment.
Address congestion in RoCE fabrics
When congestion indicators rise, investigate congestion-control and lossless-fabric settings before touching NCCL. Check that PFC and ECN policy, queue allocation and traffic-class mapping are consistent end to end. Coordinate changes with the switch and NIC owners, because a host-only adjustment can make a multi-hop path less predictable.
Change NCCL tuning last
After system and fabric checks are healthy, test NCCL performance settings one at a time. Keep the original baseline, record the version and environment, and revert a change that improves one message size or topology while harming another. The NCCL performance and tuning guidance does not provide a universal setting that is correct for every network.
Rerun the same baseline after every material change
- Restore the original workload, host set, rank placement, message sizes and process configuration.
- Run the same passive or active NCCL diagnostic used during triage.
- Repeat the same
ib_write_bw,ib_write_lator Ethernet test, using the same hosts and supported options. - Capture before-and-after NIC, RDMA and switch counters over the same interval.
- Compare communication-phase duration, rank skew, bandwidth distribution and latency tails—not just the fastest sample.
- Keep the change only if the diagnosed symptom improves without introducing new errors, drops or instability.
If the baseline does not improve, revert the change and return to the evidence. A successful point-to-point test does not prove that the collective path or application is healthy, and a higher peak bandwidth does not prove that synchronization tails have improved.
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InfiniBand and RoCE troubleshooting compared
| Diagnostic axis | InfiniBand | RoCE |
|---|---|---|
| Link and transport health | Use ibstat, ibstatus, RDMA statistics and perfquery for state, rate, retries, errors and discards. |
Use ethtool -S and NIC/driver telemetry for link, packet, pause, error and drop counters. |
| Congestion and lossless behavior | Examine fabric and port behavior together with Subnet Manager status from sminfo. |
Inspect PFC or pause, ECN, CNP and queue-drop telemetry, plus end-to-end lossless-fabric policy. |
| Point-to-point validation | Use supported perftest bandwidth and latency tests on representative host pairs. | Use the same principle, while ensuring the test follows the intended Ethernet/RDMA path and traffic class. |
| Switch visibility | Port and fabric counters may require operator access to switches and Subnet Manager data. | Switch telemetry is essential when congestion, queueing or drops are suspected; host counters alone may not show every hop. |
The available guidance does not establish that either fabric is universally faster, cheaper or better. Suitability depends on topology, operations, congestion policy, telemetry and the workload’s traffic pattern.
Build an operating practice that catches regressions
Keep a versioned baseline
Record NCCL, driver, firmware, kernel, NIC, switch and perftest versions with each baseline. NCCL documentation cited here covers 2.31.2 networking and performance guidance, 2.32.3 troubleshooting, and 2.32.3 diagnostics; commands and counters can differ in other releases. Validate every command against the software actually deployed.
Collect counters around known workloads
For recurring jobs, sample host and switch counters before, during and after communication-heavy phases. Alert on changes from the established baseline rather than on a universal threshold. Correlate alerts with job placement and latency tails so operators can distinguish a busy but healthy fabric from a degraded path.
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Document which team owns GPU readiness, host networking, RDMA or RoCE configuration, switches, cabling and NCCL settings. A clear handoff prevents a fabric symptom from being passed between teams as an unexplained application slowdown.
Quick Recap
Field checklist
- Communication-heavy phases are slower or less stable than the local GPU work.
- GPU, runtime and MPI readiness checks do not explain the delay.
- NCCL’s selected interfaces and devices are verified on every host.
- Link state, link layer and negotiated rate match the expected baseline.
- Point-to-point bandwidth and latency are measured repeatedly on representative paths.
- Latency tails are reviewed alongside peak and average bandwidth.
- InfiniBand RDMA, Subnet Manager and port counters are checked when applicable.
- RoCE NIC and switch telemetry includes drops, PFC or pause, ECN, CNP and queue behavior.
- The suspected physical, configuration or congestion layer is corrected before NCCL tuning.
- The identical baseline is rerun, with counters and workload results compared before and after.
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