Is this a hardware or a software issue?

Hardware failures can look like either layer depending on the symptom. The discriminator is the resolution of the symptom to the recommended fix: if applying the steps under "How to fix it" eliminates the symptom, this is a software/configuration class issue regardless of the hardware it surfaced on. Repeat the fix on a fresh node to confirm.

How long does the fix typically take?

Most resolutions from this entry take 10 to 30 minutes of engineer time once the diagnosis is confirmed. Long fixes come from skipping the diagnosis step and chasing apparent symptoms (validation drift, memory growth, slow throughput) instead of the actual root cause listed above. Apply the fix in a small experiment first, then roll it forward.

Does this affect inference or only training?

Primarily this is a hardware failure observed during training, but the same misconfiguration can leak into downstream evaluation and serving. Treat the prevention checklist as a guardrail: any pipeline that touches a trained checkpoint should re-validate the failure mode before promoting the model.

What's the cheapest way to confirm this is the problem?

Cheapest signal: reproduce the symptom on a 1-GPU toy run with the same configuration. If the toy run exhibits the same behavior, the cause is the configuration rather than the cluster. If only the multi-GPU run is affected, the failure is a distributed interaction, so NCCL is the most likely layer to inspect.

Can the Denpex platform detect this automatically?

Yes. Denpex correlates the symptoms in this entry against your training logs in real time and surfaces the failure taxonomy, root cause, and exact remediation command on the console. Three full diagnoses are included free during signup, and the encyclopedia entry you are reading is generated from the same signal library that powers the platform.

What related failures should I check next?

Closely related failures cluster around #PCIe Configuration. After applying this fix, validate against the related entries below to confirm no overlapping cause is masking as this one. Engineers routinely discover that the dominant failure mode is actually a second, sibling failure compounded on top.

Will this recur after the fix?

Applied once with the prevention checklist, this failure does not recur under normal operation. The recurring variant most engineers encounter is the re-emergence of the same symptom on a different model or stack after PCIe Configuration is upgraded. Pin compatible versions and rerun the prevention smoke test on every dependency change.

Is there a fingerprint I can monitor?

Yes. Training does not crash, but multi-GPU scaling efficiency is severely degraded. is the highest-signal fingerprint. Wire a metric or alert on that signal (or its absence after the fix) so you can detect the recurrence automatically. The Denpex platform encodes this exact fingerprint and surfaces it directly in the dashboard.

What is the official documentation that covers this?

Reference documentation is listed in the References section at the bottom of this entry. The most authoritative URLs are the upstream framework docs and the issue trackers cited; the Denpex entry synthesizes both into the executive, engineer, and on-call perspectives so each audience can act without reading every linked source.

Does this entry include working code?

Yes. The "Code Examples" section contains copy-paste-ready snippets that target the most common stacks (PyTorch, Hugging Face Trainer, DeepSpeed, Megatron-LM, vLLM/Transformers). Each snippet is annotated with why it works so you can adapt the pattern to your own framework without losing fidelity.

PCIe ACS Blocking GPU Direct P2P

Q: What does this failure look like in production?

In production this failure typically appears during the first tens-to-hundreds of training steps: loss curves become non-monotonic, validation metrics plateau above the baseline, and downstream checkpoints either diverge or fail to improve. Engineers frequently misread it as data drift when the underlying mechanism is pcie access control services (acs) is enabled in the host motherboard's bios or the linux kernel. acs forces all pcie peer-to-peer traffic to be routed up to the root complex (cpu) for iommu translation/security checks. nccl detects this topology and disables direct gpu-to-gpu memory transfers (p2p) because routing through the root complex destroys performance and can cause hardware hangs.. Validating against the symptom list in this entry is the fastest way to confirm the diagnosis.

Q: How does this scale from 1 GPU to 1000 GPUs?

At single-GPU scale the symptom usually looks like slow convergence or a single anomalous loss value. At 64+ GPU scale, distributed interactions amplify pcie access control services (acs) is enabled in the host motherboard's bios or the linux kernel. acs forces all pcie peer-to-peer traffic to be routed up to the root complex (cpu) for iommu translation/security checks. nccl detects this topology and disables direct gpu-to-gpu memory transfers (p2p) because routing through the root complex destroys performance and can cause hardware hangs. through collectives, and the failure can manifest as a full job abort. Always reproduce the fix at the smallest cluster configuration that still exhibits the symptom before promoting to production scale.

PCIe Access Control Services (ACS) is enabled in the host motherboard's BIOS or the Linux kernel. ACS forces all PCIe peer-to-peer traffic to be routed up to the Root Complex (CPU) for IOMMU translation/security checks. NCCL detects this topology and disables direct GPU-to-GPU memory transfers (P2P) because routing through the root complex destroys performance and can cause hardware hangs.

Quick answer

PCIe Access Control Services (ACS) is enabled in the host motherboard's BIOS or the Linux kernel.

Symptom: NCCL INFO P2P Disable
Root cause: PCIe Access Control Services (ACS) is enabled in the host motherboard's BIOS or the Linux kernel. ACS forces all PCIe peer-to-peer traffic to be routed up to the Root Complex (CPU) for IOMMU translation/security checks. NCCL detects this topology and disables direct GPU-to-GPU memory transfers (P2P) because routing through the root complex destroys performance and can cause hardware hangs.
Recommended fix: pcie_acs_override=downstream,multifunction
How Denpex helps: Denpex matches PCIe ACS Blocking GPU Direct P2P across every rank in a distributed run and reports which rank failed first, so you act on the initiating node instead of the loudest one.

Hardware#PCIe Configuration

What this failure is

PCIe ACS Blocking GPU Direct P2P is a Hardware failure seen during ML training runs. PCIe Access Control Services (ACS) is enabled in the host motherboard's BIOS or the Linux kernel. ACS forces all PCIe peer-to-peer traffic to be routed up to the Root Complex (CPU) for IOMMU translation/security checks. NCCL detects this topology and disables direct GPU-to-GPU memory transfers (P2P) because routing through the root complex destroys performance and can cause hardware hangs. Common tags: PCIe Configuration.

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Why it happens (the mechanism)

There is no explicit crash; it just runs slowly. Engineers might spend days optimizing their dataloader or model architecture, completely unaware that the underlying hardware topology is actively blocking direct GPU communication.

What you'll observe

NCCL INFO P2P Disable
NCCL INFO Using SHM instead of P2P
Performance degradation during DDP training

Common symptoms and what they mean

Symptom	Why it happens
Training does not crash, but multi-GPU scaling efficiency is severely degraded.	PCIe Access Control Services (ACS) is enabled in the host motherboard's BIOS or the Linux kernel. ACS forces all PCIe peer-to-peer traffic to be routed up to the Root Complex (CPU) for IOMMU translation/security checks. NCCL detects this topology and disables direct GPU-to-GPU memory transfers (P2P) because routing through the root complex destroys performance and can cause hardware hangs.
NCCL logs show that Peer-to-Peer (P2P) transfers are disabled, and it is falling back to Shared Memory (SHM) routing through the host CPU.	PCIe Access Control Services (ACS) is enabled in the host motherboard's BIOS or the Linux kernel. ACS forces all PCIe peer-to-peer traffic to be routed up to the Root Complex (CPU) for IOMMU translation/security checks. NCCL detects this topology and disables direct GPU-to-GPU memory transfers (P2P) because routing through the root complex destroys performance and can cause hardware hangs.

Which systems are affected

NCCL
PCIe
Linux Kernel
Multi-GPU Nodes

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.

✓Check NCCL logs (`NCCL_DEBUG=INFO`) for `P2P Disable`.
✓Use `lspci -vvv | grep -i acs` to verify if Access Control Services are enabled on the PCIe bridges.

Searchable error signature

search key

NCCL INFO P2P Disable
NCCL INFO Using SHM instead of P2P

Use this text as a lookup key in logs and upstream issue trackers. It is not presented as a captured customer log. Confirm the cause from your own preceding events, versions, configuration and the cited references.

The fix and the prevention pattern

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Diagnose this failure in VS Code

Select the traceback or open the failed terminal, then run Denpex locally to see the initiating rank, collateral failures, exact fix, and verification command without uploading the log.

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Root cause

PCIe Access Control Services (ACS) is enabled in the host motherboard's BIOS or the Linux kernel. ACS forces all PCIe peer-to-peer traffic to be routed up to the Root Complex (CPU) for IOMMU translation/security checks. NCCL detects this topology and disables direct GPU-to-GPU memory transfers (P2P) because routing through the root complex destroys performance and can cause hardware hangs.

The fix and how to prevent it

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References

↗NVIDIA NCCL Documentation: Troubleshooting PCIe ACS

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