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[https://nvbugs/6900756][perf] KVCMv2: fill the initial host tier in parallel, populate in 4 MiB calls - #20790
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WalkthroughInitial host-memory filling now uses seven helper threads to populate backing-memory chunks. ChangesHost-Memory Fill Flow
Priority: ⬇️ Low Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Other Sequence Diagram(s)sequenceDiagram
participant HostMem
participant FillHelpers
participant HostMemBacking
HostMem->>HostMemBacking: Open the fill range
HostMem->>FillHelpers: Launch helpers to populate chunks
FillHelpers->>HostMemBacking: Populate claimed chunks
HostMem->>HostMemBacking: Commit populated chunks in order
HostMem->>HostMemBacking: Decommit the uncommitted tail on fill failure
Suggested reviewers: Merge Risk: ⚪ Minimal · up to
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Docstring coverage is 45.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 20 functions across 6 files. | Write docstrings for the functions missing them to satisfy the coverage threshold. |
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Check name Status Explanation Title check The title clearly summarizes the main changes: parallel initial host-tier filling and 4 MiB prefault calls. It includes a valid NVBugs reference and a concise change type. 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 pull request description is complete and relevant. It explains the problem, implementation, behavior change, performance results, test coverage, and checklist status. The supplied context does not… Full details: Description check
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@cpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMem.cpp:
- Around line 245-278: In the fill setup around `stepStates` and
`helpers.reserve`, perform all potentially throwing allocations before calling
`mBacking.open`; open the host range only after those allocations succeed,
before population begins. Preserve the existing helper-thread creation handling.
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cpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMem.cppcpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMem.hcpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMemBacking.cppcpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMemBacking.hcpp/tests/unit_tests/batch_manager/kvCacheManagerV2HostMemTest.cpp
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@cpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMem.cpp:
- Around line 207-211: Coordinate the helper loop using nextHelperOffset with
the commit loop so commitRange cannot publish a chunk while a helper may still
run hostPrefaultChunk’s zeroing fallback on it. Track claimed chunk completion
and wait before committing or publishing that chunk, while reusing
helper-prefaulted work instead of zeroing it again.
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cpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMem.cppcpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMem.hcpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMemBacking.cppcpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMemBacking.hcpp/tests/unit_tests/batch_manager/kvCacheManagerV2HostMemTest.cpp
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…parallel, populate in 4 MiB calls Two changes to how HostMem fills the host tier: - Populate in 4 MiB MADV_POPULATE_WRITE calls instead of one call per 512 MiB fill step. Each call holds mmap_lock for reading, and mprotect, registration and allocations elsewhere in the process wait behind it. - The initial fill (the constructor's) opens its whole range once and 7 helper threads populate it ahead of the per-step commits, which then mostly register. Grows run on the fill thread alone, as before: parallel populates slow the executor while it serves. Registration, the watermark and waitUsable() are unchanged. Signed-off-by: Thor Johnsen <tjohnsen@nvidia.com>
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… once A helper could still be populating a step after commit() had handed it out. Where MADV_POPULATE_WRITE is unavailable, populating means zeroing, which would wipe data written into that step. Helpers and the commit loop now share a per-step state: whichever claims a step first populates it, and commit() waits for a helper's claim to end and then only registers the step. This also drops commit()'s second, redundant populate of steps the helpers already filled. Signed-off-by: Thor Johnsen <tjohnsen@nvidia.com>
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Description
A KVCMv2 host tier is filled by one worker thread per
HostMem. It goes 512 MiB at a time:mprotectthe step open, prefault it with oneMADV_POPULATE_WRITEcall, then register it. This PR changes two things:MADV_POPULATE_WRITEcall holds the process'smmap_lockfor reading until it returns. Everything that needs the lock for writing waits behind it: the other pool'smprotectand registrations, and allocations on the executor. One call per 512 MiB step makes those waits long.resize()) don't use helpers: they overlap serving, and parallel prefaults stall the executor.Each step of the initial fill is prefaulted by whichever thread claims it first. The helpers and the commit loop share one state per step (pending / claimed / prefaulted). The commit loop either claims a pending step and prefaults it itself, as before, or waits for a helper to finish the step and then only registers it (
HostMemBacking::commit(offset, size, /*populated=*/true)). So no step is prefaulted twice, and nothing prefaults a step after it has been handed out. That matters whereMADV_POPULATE_WRITEis unavailable (kernel < 5.14, or built without the macro): there, prefaulting falls back to zeroing the memory. Without the claim, a helper that fell behind the commit loop could zero KV data already written into a published step. (The second commit adds this; CodeRabbit found the race.)Not changed: registration (in order, on the fill thread), the watermark, and
waitUsable().HostMemBacking::commit()is nowopen()+populate()+ the same registration loop, or registration alone for a range the caller already opened and prefaulted.hostPrefaultChunk()is unchanged.Helpers are
std::jthreads, so they are stopped and joined on every exit fromcommitRange(). A helper that fails hands its step back and stops.commit()then prefaults whatever is left and reports any error, as before. If the initial fill fails, everything above the watermark is decommitted, because the helpers may have prefaulted past the failed step.Behavior change to review: opening the range up front weakens the guard against reading ahead of the watermark, during the initial fill only. A reader that skips
waitUsable()and lands past the watermark reads zeros instead of faulting.Results
8×B200, DeepSeek-V4-Flash, TP8/EP8, KV cache rebalancing on with
cooldown_secs: 0, the reporter's trace from https://nvbugs/6900756 (729 requests). main = this PR's merge base. All runs are on one node and all completed 729/729 with health 200.The two parts act at different times. The prefault call size matters during serving, where grows run on the fill thread alone. The helper count matters only during the initial fill, which ends before the server is ready. Both values were tuned separately with experiment-only overrides: call size with 16 initial-fill threads, thread count with 8 MiB calls. The final combination, 4 MiB and 8 threads, was not run as one build.
Prefault call size, 16 initial-fill threads:
Initial-fill threads, 8 MiB calls: 8 threads 18.0 s, 16 threads 18.2 s (2 runs), 32 threads 19.2 s. Latency doesn't change with the thread count.
cooldown_secs: 0) fall outside it. They each grow pools by 0.1–20 GiB and pause the executor for 0.6–4.4 s each, on main and with this change alike.What the numbers show:
mprotectand ~3 s more in registration, waiting behind the other pool's prefault. Prefaulting itself takes ~10 s either way. With smaller calls those waits shrink: each fill drops from ~20 s to 12–14 s, and the executor keeps ~4 it/s during them instead of ~2. 4 MiB is the smallest size that doesn't slow the initial fill (2 MiB adds ~1.5 s to it).adjust().Second commit: each step prefaulted once
Same setup, one node (a different one from the tables above), 2 runs each, interleaved. "First commit" =
d82badde5, "both commits" =1c1422574.The initial fill gets ~6 s shorter: the commit loop no longer prefaults steps the helpers have already prefaulted, and no longer
mprotects them again. Serving is unchanged (grows don't use helpers). The server is still ready ~47 s after the initial fill ends.Test Coverage
KvCacheManagerV2HostMemTest.ParallelInitialFillThenGrow: an initial fill of 2.5 chunks (with helpers), then a grow to 4.25 chunks (without). It checks the contents survive and every unit is registered (cudaHostGetFlags).DeferredFillIsUsableAfterWaitUsable, now run the helper path.KvCacheManagerV2HostMemTest.DeferredParallelFillKeepsEarlyWrites: a 4-chunk initial fill withwaitForFill=false; each chunk is written as soon aswaitUsable()lets it be, and all of it must survive the rest of the fill.HostMemBackingTest.CommitOfPopulatedRangeKeepsContents:open()+populate(), write, thencommit(..., /*populated=*/true); the contents survive and every unit is registered.kvCacheManagerV2HostMemTest21/21 andkvCacheManagerV2HostMemBackingTest11/11 pass on B200.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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Dev Engineer Review
HostMemBacking::populateusesMADV_POPULATE_WRITEcalls of up to 4 MiB. Initial fills use seven NUMA-bound helper threads plus the committing thread. Growth remains single-threaded.waitUsable()may read zeros beyond the watermark instead of faulting.MADV_POPULATE_WRITEis unavailable.waitUsable()are unchanged. Review-finding counts are unavailable. The supplied CI reports include failures or unstable status, but do not identify the failing tests.QA Engineer Review
kvCacheManagerV2HostMemTest.cppaddsDeferredParallelFillKeepsEarlyWritesandParallelInitialFillThenGrow.kvCacheManagerV2HostMemBackingTest.cppaddsCommitOfPopulatedRangeKeepsContents.cpp/tests/unit_tests/batch_manager/CMakeLists.txt. No matching entry was found in the integrationtest-db/or manual-QA lists.kvCacheManagerV2HostMemTestcases passing on B200. CI reports also include failed or unstable pipelines without failing-test details. Coverage verdict: needs follow-up.Per-File QA Perspective
cpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMem.cpp: Verify parallel initial fill, single-threaded growth, helper creation failure, step handoff, and failure cleanup.cpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMem.h:commitRangeandstartFillnow accept a helper count. Verify callers select helpers only for initial fill.cpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMemBacking.cpp:commitsupports ranges populated before registration;openand chunkedpopulateare separate operations. Verify partial-failure handling and decommit cleanup.cpp/tensorrt_llm/batch_manager/kv_cache_manager_v2/utils/hostMemBacking.h: Addsopenandpopulateand adds apopulatedparameter tocommit. Verify concurrent use on disjoint ranges and error handling.cpp/tests/unit_tests/batch_manager/kvCacheManagerV2HostMemTest.cpp: Adds tests for preserving early writes during parallel fill and preserving data and registration through growth. These unit tests are not listed in the integration CI or manual-QA lists.cpp/tests/unit_tests/batch_manager/kvCacheManagerV2HostMemBackingTest.cpp: Adds coverage for retaining contents when committing a populated range and checking host registration. This unit test is not listed in the integration CI or manual-QA lists.