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fix(rccl): default cheap post-send fence off for gfx950, add force-on override - #10339
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wenkaidu merged 4 commits intoAug 19, 2026
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… override The arch-tuned auto path enabled the cheap post-send fence (skipping the full __threadfence_system()) on gfx942, gfx950 and gfx1250. Drop gfx950 from the auto-enable set so it defaults to the full fence, while gfx942/gfx1250 are unchanged. Extend RCCL_CHEAP_POST_SEND_FENCE_OFF with a new value 2 that force-enables the cheap fence regardless of arch (overriding auto), so gfx950 can be re-enabled when desired. Values remain: 0 = arch-tuned auto, 1 = force off, 2 = force on. Co-authored-by: Cursor <cursoragent@cursor.com>
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Address review feedback on the cheap post-send fence change: - Extract the decision into rcclComputeCheapPostSendFenceOff(cudaArch, param, uncachedMemSupported) declared in rccl_common.h, so the call site in initTransportsRank collapses to a single assignment. - Switch arch detection from IsArchMatch() string matching to the existing numeric comm->cudaArch (gfx942 = 940, gfx950 = 950, gfx1250 = 1250). - Add CheapPostSendFenceTests unit test covering the full matrix (uncached-mem gating, arch-tuned auto, force-off, force-on override, other params, unknown arch). Co-authored-by: Cursor <cursoragent@cursor.com>
alex-breslow-amd
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Aug 18, 2026
corey-derochie-amd
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Aug 18, 2026
amd-jiali
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August 18, 2026 19:37
amd-jiali
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Aug 18, 2026
initTransportsRank() uses goto fail for error handling; a local const bool uncachedMemSupported sat after those labels and failed to compile. Pass the HIP_UNCACHED_MEMORY flag inline instead. Co-authored-by: Cursor <cursoragent@cursor.com>
Define rcclComputeCheapPostSendFenceOff inline in rccl_common.h so rccl-UnitTestsFixtures can exercise it without importing a hidden librccl symbol, and move CheapPostSendFenceTests there. Co-authored-by: Cursor <cursoragent@cursor.com>
cmcknigh
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Aug 25, 2026
… override (#10339) ## Motivation default cheap post-send fence off for gfx950, add force-on override ## Technical Details The arch-tuned auto path enabled the cheap post-send fence (skipping the full __threadfence_system()) on gfx942, gfx950 and gfx1250. Drop gfx950 from the auto-enable set so it defaults to the full fence, while gfx942/gfx1250 are unchanged. Extend RCCL_CHEAP_POST_SEND_FENCE_OFF with a new value 2 that force-enables the cheap fence regardless of arch (overriding auto), so gfx950 can be re-enabled when desired. Values remain: 0 = arch-tuned auto, 1 = force off, 2 = force on. ## Issue Tracking JIRA ID AICOMRCCL-1873) ## Test Plan Confirm cheap fence can be turned on/off through env var ## Test Result Passed ## Submission Checklist - [ ] Look over the contributing guidelines at https://github.com/ROCm/rocm-systems/blob/develop/CONTRIBUTING.md. --------- Co-authored-by: Cursor <cursoragent@cursor.com>
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alexander-sannikov
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Oct 30, 2026
Backported PRs: - #10670 fix(rccl): LL protocol explicit 128-bit vector loads/stores - #10339 fix(rccl): default cheap post-send fence off for gfx950 - #9336 LL: cache-bypass user buffers, plain/non-temporal comm FIFO Partially backported PRs: - #9347 [RCCL] [JIRA ID : ROCM-25479] LL128: generate separate kernels for UBR Aligned with develop: - RCCL_GFX9_CHEAP_FENCE_OFF default Co-authored-by: Cursor <cursoragent@cursor.com>
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Motivation
default cheap post-send fence off for gfx950, add force-on override
Technical Details
The arch-tuned auto path enabled the cheap post-send fence (skipping the full __threadfence_system()) on gfx942, gfx950 and gfx1250. Drop gfx950 from the auto-enable set so it defaults to the full fence, while gfx942/gfx1250 are unchanged.
Extend RCCL_CHEAP_POST_SEND_FENCE_OFF with a new value 2 that force-enables the cheap fence regardless of arch (overriding auto), so gfx950 can be re-enabled when desired. Values remain: 0 = arch-tuned auto, 1 = force off, 2 = force on.
Issue Tracking
JIRA ID AICOMRCCL-1873)
Test Plan
Confirm cheap fence can be turned on/off through env var
Test Result
Passed
Submission Checklist