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[https://nvbugs/6379406][fix] Use coherent managed memory for EPLB when GDRCopy is unavailable - #20156
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info
WalkthroughThe allocator can use CUDA managed memory for host access when the active GPU supports direct access and GDRCopy is not forced. Allocation and free accept this mode. A unit test checks data visibility, and the CDMM guidance describes the fallback. ChangesManaged memory host access
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Bug fix Sequence Diagram(s)sequenceDiagram
participant HostAccessibleDeviceAllocator
participant GDRCopy
participant CUDARuntime
HostAccessibleDeviceAllocator->>GDRCopy: Attempt initialization
GDRCopy-->>HostAccessibleDeviceAllocator: Initialization fails
HostAccessibleDeviceAllocator->>CUDARuntime: Query direct managed-memory host access
CUDARuntime-->>HostAccessibleDeviceAllocator: Return GPU capability
HostAccessibleDeviceAllocator->>CUDARuntime: Allocate managed memory and advise host access
Suggested reviewers: Merge Risk: 🔵 Low · up to
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| Check name | Status | Explanation | Resolution |
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| Docstring Coverage |  |
Docstring coverage is 30.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 3 files. | Write docstrings for the functions missing them to satisfy the coverage threshold. |
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Check name Status Explanation Linked Issues check Check skipped because no linked issues were found for this pull request. Out of Scope Changes check Check skipped because no linked issues were found for this pull request. Description check The description clearly explains the issue, solution, affected platforms, fallback behavior, test coverage, validation results, documentation update, and checklist status. It is complete and aligned w… Title check The title follows the required ticket and type format and clearly summarizes the main change: using coherent managed memory for EPLB when GDRCopy is unavailable.
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Inline comments:
Review comments at
@cpp/tensorrt_llm/runtime/moeLoadBalancer/hostAccessibleDeviceAllocator.cpp:
- Line 199: Update isSupported() to return the cached support decision when the
allocator is initialized, using its recorded NUMA, GDRCopy, managed-fallback,
and host-direct managed-access state instead of querying the current CUDA
device; preserve the existing device-query path before initialization.
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cpp/tensorrt_llm/runtime/moeLoadBalancer/hostAccessibleDeviceAllocator.cppcpp/tensorrt_llm/runtime/moeLoadBalancer/hostAccessibleDeviceAllocator.hcpp/tests/unit_tests/runtime/hostAccessibleDeviceAllocatorTest.cuexamples/wide_ep/README.md
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Barry-Delaney
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Reviewed 991ea71: NUMA and GDRCopy retain their existing priority, the automatic managed-memory fallback is gated on direct host access support, and forced GDRCopy does not silently enter that fallback. Allocation/free use the recorded fallback mode; allocation retains its initialized-device check. The accessed-by host hint matches the CUDA unified-memory guidance (https://docs.nvidia.com/cuda/cuda-programming-guide/04-special-topics/unified-memory.html). No blocking code finding. Hardware validation was not independently rerun locally.
brnguyen2
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Approving — the comments below are optional touch-ups, not blockers.
The new branch only runs where the allocator used to assert, and forced-GDRCopy and non-coherent behavior are unchanged, so the regression surface is small. My main concern is validation, not code:
- The fix's premise hasn't been tested on the target configuration. The change assumes two things on a GB200 CDMM node without
gdrdrv: thatcudaDevAttrDirectManagedMemAccessFromHostreads 1, and thatcudaMemAdviseSetAccessedBy(host)keeps host writes from migrating pages. Neither has been checked. Before merging, or at least before closing nvbugs/6379406, please run the failing GB200 disagg perf-sanity stage (or just the existing allocator tests) on a node where/dev/gdrdrvis absent, and confirm the new"using managed memory with direct host access"log line appears. If the attribute reads 0 under CDMM, this PR is a no-op for the bug. - Fallback visibility: the switch away from GDRCopy is logged at INFO, so it's easy to miss. Consider logging it at WARNING, since it means a node is missing
gdrdrv, which operators may want to know about. - Nit: the header comment just above
mGdrHandle(// if Not, we should use GDRCopy) is now incomplete. The new member's comment covers the third option, but the comment block above it still describes two options.
mikeiovine
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Stamp on behalf of runtime devs, delegating proper review to @NVIDIA/trt-llm-moe-devs; please ping me if you think this is not accurate
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Automatically added "ci: full pre-merge approved" because this PR has satisfied the required GitHub review approvals. Unresolved review conversations and other required checks remain independent merge requirements. |
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@brnguyen2 thanks for the review. Updates in f3ab25b:
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/bot run --disable-fail-fast |
…en GDRCopy is unavailable
Online EPLB needs device memory that the host copy threads can write. When
GPU memory is not exposed as a NUMA node (e.g. GB200/GB300 with CDMM
enabled), HostAccessibleDeviceAllocator could only use GDRCopy. On nodes
where gdr_open() fails because the gdrdrv module is missing, the
MoeLoadBalancer constructor asserted ("HostAccessibleDeviceAllocator is not
supported on current platform, please install gdrcopy(gdrdrv)") and the GEN
workers aborted during executor initialization.
On hardware-coherent platforms (cudaDevAttrDirectManagedMemAccessFromHost=1),
the host can access device-resident managed memory directly, without page
faults or migration. This also holds in CDMM mode. In that case, fall back
to CUDA managed memory with the preferred location on the device and an
accessed-by hint for the host.
The allocator makes the decision once in init() and logs the fallback at
WARNING. The NUMA and GDRCopy paths, TRTLLM_EPLB_FORCE_GDRCOPY, and the
behavior on non-coherent platforms are unchanged.
Add ManagedMemoryFallbackWithoutGdrCopy. It exercises the fallback where it
applies: allocate, memcpyToDevice, then check the data on the device and on
the host.
Signed-off-by: qgai <qgai@nvidia.com>
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Barry-Delaney
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Reviewed d096b9a against the previously approved implementation and the release/1.3 base. The allocator retains the cached coherent-managed-memory decision, forced-GDRCopy behavior, device check, and matching allocation/free path. The meaningful additions strengthen the fallback test with host-to-device and device-to-host verification; other changes are logging/documentation. No blocking code findings. Hardware tests were not rerun locally.
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/bot run --disable-fail-fast |
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PR_Github #77726 [ run ] triggered by Bot. Commit: |
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PR_Github #77726 [ run ] completed with state |
….3: Use coherent managed memory for EPLB when GDRCopy is unavailable Forward-port of release/1.3 PR NVIDIA#20156 (release/1.3 commit 248825e) to main. Signed-off-by: qgai <qgai@nvidia.com> Signed-off-by: Simeng Liu <simengl@nvidia.com>
….3: Use coherent managed memory for EPLB when GDRCopy is unavailable Forward-port of release/1.3 PR NVIDIA#20156 (release/1.3 commit 248825e) to main. Signed-off-by: qgai <qgai@nvidia.com> Signed-off-by: Simeng Liu <simengl@nvidia.com>
….3: Use coherent managed memory for EPLB when GDRCopy is unavailable Forward-port of release/1.3 PR NVIDIA#20156 (release/1.3 commit 248825e) to main. Signed-off-by: qgai <qgai@nvidia.com> Signed-off-by: Simeng Liu <simengl@nvidia.com>
….3: Use coherent managed memory for EPLB when GDRCopy is unavailable Forward-port of release/1.3 PR NVIDIA#20156 (release/1.3 commit 248825e) to main. Signed-off-by: qgai <qgai@nvidia.com> Signed-off-by: Simeng Liu <simengl@nvidia.com>
….3: Use coherent managed memory for EPLB when GDRCopy is unavailable Forward-port of release/1.3 PR NVIDIA#20156 (release/1.3 commit 248825e) to main. Signed-off-by: qgai <qgai@nvidia.com> Signed-off-by: Simeng Liu <simengl@nvidia.com>
….3: Use coherent managed memory for EPLB when GDRCopy is unavailable Forward-port of release/1.3 PR NVIDIA#20156 (release/1.3 commit 248825e) to main. Signed-off-by: qgai <qgai@nvidia.com> Signed-off-by: Simeng Liu <simengl@nvidia.com>
Dev Engineer Review
The allocator adds managed-memory fallback when GPU memory has no NUMA node, GDRCopy is unavailable, and
cudaDevAttrDirectManagedMemAccessFromHostis enabled. NUMA and GDRCopy remain preferred, and forced GDRCopy still prevents fallback. The fallback logs a warning and advises CUDA about host access. The README documents the coherent-platform requirement. GB200 under R580 CDMM remains unverified; the reported Vera Rubin and x86 B200 results are author-reported.QA Engineer Review
hostAccessibleDeviceAllocatorTest.cureplaces the support-only fallback test withManagedMemoryFallbackWithoutGdrCopy. The test skips when direct access is unsupported, GDRCopy is forced or available, or GPU memory has a NUMA node. Otherwise, it verifies host-to-device and device-to-host data access through the fallback. CMake registers the test; no integration test-list change is included. Coverage verdict: needs follow-up for GB200 CDMM validation. The reported allocator and online-update test results are author-reported.Per-File QA Perspective
cpp/tensorrt_llm/runtime/moeLoadBalancer/hostAccessibleDeviceAllocator.cpp: Verify fallback selection, managed allocation access advice, and allocation/free behavior on direct-access coherent hardware. Verify that forced GDRCopy retains its error when GDRCopy is unavailable.cpp/tensorrt_llm/runtime/moeLoadBalancer/hostAccessibleDeviceAllocator.h: Adds state for direct managed-memory access and documents the NUMA, GDRCopy, and managed-memory preference order. Verify the state matches the selected allocation path.cpp/tests/unit_tests/runtime/hostAccessibleDeviceAllocatorTest.cu: AddsManagedMemoryFallbackWithoutGdrCopyto verify host-to-device and device-to-host access through managed fallback. It is registered incpp/tests/unit_tests/runtime/CMakeLists.txt; it is not listed in integrationtest-db/orqa/lists.examples/wide_ep/README.md: Documents GDRCopy use and the managed-memory fallback in CDMM mode. Verify the guidance correctly limits fallback to hardware-coherent platforms.Description
Target branch. This fix targets
release/1.3, because the bug is committed for v1.3.0. The four touched files and the allocator's dependencies (gdrwrap,topologyDetector,moeLoadBalancer,moeLoadBalanceOp) are identical onmainandrelease/1.3.Issue. Online EPLB (
moe_config.load_balancer.layer_updates_per_iter > 0) needs device memory that the host copy threads can write.HostAccessibleDeviceAllocatorsupports two mechanisms:With CDMM enabled (the R580+ driver option for GB200/GB300), GPU memory is not a NUMA node, so GDRCopy is the only path. Passing GB200 CI logs confirm this: they print
GDRCopy enabled library 2.5 driver 2.5during online EPLB setup. On a node where thegdrdrvkernel module is not loaded,gdr_open()fails, and theMoeLoadBalancerconstructor asserts:The GEN workers then abort during executor initialization, and the disaggregated perf-sanity case (
..._gen1_dep32_eplb416_mtp3_ccb-NIXLon GB200) runs until the stage timeout.Fix. On hardware-coherent platforms (
cudaDevAttrDirectManagedMemAccessFromHost == 1, e.g. Grace-based GB200/GB300), the host can access device-resident managed memory directly, without page faults or migration. This also holds in CDMM mode. When GPU memory is not a NUMA node and GDRCopy is unavailable, the allocator now falls back to CUDA managed memory on such platforms:cudaMemAdviseSetAccessedByset for the host.The allocator makes this decision once in
init(), for the device it was initialized on. It logs the fallback at WARNING, because it means the node is missinggdrdrv.Allocation order is unchanged:
TRTLLM_EPLB_FORCE_GDRCOPY=1keeps its meaning: if GDRCopy is forced but unavailable, the existing error is still raised instead of silently using managed memory.TLLM_HOST_ACCESSIBLE_ALLOW_MANAGED_FALLBACKtest override keeps working.examples/wide_ep/README.mdis updated accordingly.Test Coverage
New C++ unit test
HostAccessibleDeviceAllocatorTest.ManagedMemoryFallbackWithoutGdrCopy(cpp/tests/unit_tests/runtime/hostAccessibleDeviceAllocatorTest.cu). It runs only where the fallback is used:It copies a pattern with
memcpyToDeviceand checks it on the device, then writes from the device and checks the result on the host. It is skipped elsewhere, including the A30runtime-80stage and nodes where GDRCopy works. To validate on a GB200 CDMM node, run it where/dev/gdrdrvis absent.The existing allocator tests (
HostDeviceDataAccess, etc.) also exercise the new path whenever GDRCopy is missing on a coherent platform.Validation on GPUs. GB200 nodes were not available to me, so I reproduced the failure mechanism deterministically. On a Vera Rubin system with CDMM enabled (
CoherentGPUMemoryMode: "driver",cudaDevAttrNumaConfig=0,cudaDevAttrDirectManagedMemAccessFromHost=1), I simulated a node withoutgdrdrvwith a stublibgdrapi.sowhosegdr_open()returns NULL. I ran the matrix twice, once withmainand once withrelease/1.3as the baseline, and got identical results. In each run both variants were built from the same sources.gdr_openfailsgdr_openfailsgdr_openfails +TRTLLM_EPLB_FORCE_GDRCOPY=1*
MoeLoadBalancerTestvariant with these changes, used locally only:setUseGpuMemcpy(false);HostAccessibleDeviceAllocator;It checks that slot contents match the expected experts after host-side weight and placement updates.
MoeLoadBalancer(0, 1, layer_updates_per_iter=1)) reproduce the same assertion on the CDMM system whengdr_openfails.layer_updates_per_iter=0still works.gdr_copy_to_mapping.cudaMallocHBM bandwidth after host writes, so the pages stay device-resident.cudaDevAttrDirectManagedMemAccessFromHost=0):mainand this PR behave identically.PR Checklist
Please review the following before submitting your PR:
PR description clearly explains what and why. If using CodeRabbit's summary, please make sure it makes sense.
PR Follows TRT-LLM CODING GUIDELINES to the best of your knowledge.
Test cases are provided for new code paths (see test instructions)
If PR introduces API changes, an appropriate PR label is added - either
api-compatibleorapi-breaking. Forapi-breaking, includeBREAKINGin the PR title.Any new dependencies have been scanned for license and vulnerabilities
CODEOWNERS updated if ownership changes
Documentation updated as needed
Update tava architecture diagram if there is a significant design change in PR.
The reviewers assigned automatically/manually are appropriate for the PR.
Please check this after reviewing the above items as appropriate for this PR.
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