graph/tuning: disable LL128 on PATH_P2C inter-node paths (GH200 corruption fix)#2053
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ryanhankins wants to merge 1 commit intoNVIDIA:masterfrom
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graph/tuning: disable LL128 on PATH_P2C inter-node paths (GH200 corruption fix)#2053ryanhankins wants to merge 1 commit intoNVIDIA:masterfrom
ryanhankins wants to merge 1 commit intoNVIDIA:masterfrom
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… corruption On GH200 (Grace Hopper), the inter-node NIC->GPU receive path is classified as PATH_P2C (PCIe->NVLink-C2C). LL128 requires that when the receiver observes the flag at the end of a 128-byte chunk, all preceding bytes in that chunk are already visible in GPU memory. That ordering guarantee is not met on GH200: the NIC writes directly into GPU HBM via PCIe->NVLink-C2C, and the two 64-byte halves of the 128-byte chunk can become visible to the GPU independently. If the flag half (last 8 bytes) becomes visible before the data half, the consumer reads stale or incomplete data, resulting in silent corruption. Previously the condition to enable LL128 on Hopper/Blackwell was: typeInter <= PATH_PXN This included PATH_P2C because numerically P2C < PXN. The fix changes the condition to: typeInter <= PATH_PXB || typeInter == PATH_PXN which explicitly excludes PATH_P2C. LL128 remains enabled for NVLink-switch fabrics (PATH_PXN) and closer intra-node paths where the ordering guarantees hold. LL and Simple are unaffected and were verified clean on GH200. Verified: NCCL_PROTO=Simple - clean NCCL_PROTO=LL - clean NCCL_PROTO=LL128 - corrupt (fixed by this change) NCCL_LL128_C2C=0 - clean (env-var workaround, superseded by this fix) Signed-off-by: Ryan Hankins <ryan.hankins@hpe.com>
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On GH200 (Grace Hopper), the inter-node NIC->GPU receive path is classified as PATH_P2C (PCIe->NVLink-C2C). LL128 requires that when the receiver observes the flag at the end of a 128-byte chunk, all preceding bytes in that chunk are already visible in GPU memory. That ordering guarantee is not met on GH200: the NIC writes directly into GPU HBM via PCIe->NVLink-C2C, and the two 64-byte halves of the 128-byte chunk can become visible to the GPU independently. If the flag half (last 8 bytes) becomes visible before the data half, the consumer reads stale or incomplete data, resulting in silent corruption.
Previously the condition to enable LL128 on Hopper/Blackwell was:
typeInter <= PATH_PXN
This included PATH_P2C because numerically P2C < PXN. The fix changes the condition to:
typeInter <= PATH_PXB || typeInter == PATH_PXN
which explicitly excludes PATH_P2C. LL128 remains enabled for NVLink-switch fabrics (PATH_PXN) and closer intra-node paths where the ordering guarantees hold. LL and Simple are unaffected and were verified clean on GH200.
Verified:
NCCL_PROTO=Simple - clean
NCCL_PROTO=LL - clean
NCCL_PROTO=LL128 - corrupt (fixed by this change)
NCCL_LL128_C2C=0 - clean (env-var workaround, superseded by this fix)
Description
graph/tuning: disable LL128 on PATH_P2C inter-node paths (GH200 corruption fix)
On GH200 (Grace Hopper), the NIC->GPU receive path is classified as PATH_P2C (PCIe->NVLink->C2C). NCCL's LL128 protocol requires that when the receiver sees the flag byte at
the end of a 128-byte chunk, all preceding data in that chunk is already visible in GPU memory. That guarantee does not hold on GH200: the two 64-byte halves of the chunk
can become visible independently across the PCIe->C2C boundary, causing silent data corruption.
Root cause: the previous condition typeInter <= PATH_PXN inadvertently included PATH_P2C (numerically P2C < PXN).
Fix: change the condition to typeInter <= PATH_PXB || typeInter == PATH_PXN, explicitly excluding PATH_P2C. LL128 stays enabled for NVLink-switch fabrics and tighter intra-node paths. LL and Simple are unaffected.
Verified clean on GH200: with SIMPLE,LL,LL128 protocols.
Reference any related issues or PRs:
#2001
Performance Impact
N/A