telemetry: reuse the drained staging buffer - #2086
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nixlTelemetryStagingQueue allocated and freed one capacity-sized vector on every drain, once per flush interval for the life of the process. It now keeps two capacity-reserved buffers and alternates them: the drain discards the previous drain's contents and swaps live with drained under the mutex, so no allocation happens after construction. The drain contract therefore changes from an owning vector to a borrowed std::span valid until the next drain, and takePending() is renamed to drainPending() so every call site has to be revisited -- a span otherwise binds to the existing auto and silently acquires the new lifetime rule. Beyond removing the last allocation in the telemetry event path, this is what lets the NIX-1544 low-lock queue return a view straight over its retired buffer instead of copying it into an owning vector on every drain. Capacity, drop-newest overflow, all-or-none batches, drop accounting, drain order and shutdown behavior are unchanged. Staging residency becomes two capacity-sized buffers (128 KiB at the default 4096-event capacity) with no per-flush allocator traffic. Adds coverage for buffer alternation without reallocation, capacity surviving alternating drains, and a drain view carrying only its own events. Signed-off-by: Efraim Eygin <eeygin@nvidia.com>
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📝 WalkthroughWalkthroughThe telemetry staging queue now uses reusable live and drained buffers. ChangesTelemetry buffer reuse
Estimated code review effort: 3 (Moderate) | ~20 minutes Mergeability Score: ⚪ Minimal · up to This localized telemetry buffering change has no actionable merge-blocking risk remaining and is merge-ready after normal checks and review. Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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In `@test/gtest/unit/telemetry/telemetry_staging_queue_test.cpp`:
- Around line 192-200: Update the test around queue.drainPending() to capture
first.data() before the second drainPending() call, then compare the saved
address with second.data() instead of reading the invalidated first view.
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src/core/telemetry/telemetry.cppsrc/core/telemetry/telemetry_staging_queue.cppsrc/core/telemetry/telemetry_staging_queue.htest/gtest/unit/telemetry/telemetry_staging_queue_test.cpp
DrainViewCarriesOnlyTheCurrentDrain compared second.data() against first.data() after the second drain had already retired the first view. Reading a span's stored pointer does not dereference it and the buffer is still alive as the live buffer, so this was well defined, but the test that codifies the borrowed-view lifetime should not be the one holding a view across a drain. Capture the address first, matching how the alternation test already records its previous storage. Signed-off-by: Efraim Eygin <eeygin@nvidia.com>
The span was constructed from drained_ after the lock was released. That is correct for the single consumer the queue documents, since producers only ever touch live_, but it reads a member outside the lock that published it and invites the question on every re-read of this seam. Holding the lock across the return is enough: the returned span is initialized before the guard is destroyed, so pointer and size are captured in the same critical section as the swap. No extra local, and the critical section grows by two word reads. Signed-off-by: Efraim Eygin <eeygin@nvidia.com>
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/build |
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/ok to test d448833 |
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🤖 CI Triage Agent — TL;DR: The build compiled cleanly; the job failed in the "Allocate DL EP Environment" stage because Full analysisSummary: Slurm allocation for the GB200 test node timed out after 1 hour; the "Run DL EP elastic tests" stage never started. Root cause: In stage Implicated commit: unknown — not code-related (commit fcb116f built and installed successfully). File: Pipeline stage "Allocate DL EP Environment" (slurm Suggested fix: Retry the build once GB200 capacity frees up. If these timeouts are recurring, address at the infra level: raise/tune the Related: none found. |
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🤖 CI Triage Agent — TL;DR: The NIXL EP Docker image built and pushed successfully; the job failed in the "Allocate DL EP Environment" stage because the slurm Full analysisSummary: Stage "Allocate DL EP Environment" (node 156) failed after ~60 min when the slurm allocation on Root cause: GB200 resources on partition Implicated commit: unknown — not caused by commit fcb116f or PR #2086; this is an infrastructure/scheduling failure independent of the code change. File: N/A (CI/slurm allocation step, not a source file). Allocation reference: slurm job ID Suggested fix:
Related: none found. |
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🤖 CI Triage Agent — TL;DR: The Full analysisSummary: The GPU-environment allocation stage failed: Root cause: The Docker image build/install (stage 141, and the mislabeled "FAILURE" stage 123) actually succeeded end-to-end. The real failure is in stage 156, where the pipeline SSHes to Implicated commit: None — commit fcb116f (PR #2086) is not implicated; the compile, link, install, and image push all succeeded. File: N/A (Jenkins pipeline slurm-allocation step; slurm partition Suggested fix: Re-run the build — this is a transient resource-contention failure on the GB200 partition. If it recurs, (a) check GB200 node availability/queue depth on the Related: none |
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🤖 CI Triage Agent — TL;DR: The Full analysisSummary: Build #711 failed at the Root cause: The job requested one node on partition Implicated commit: none — infrastructure/capacity issue, not fcb116f or any source change. File: N/A (failure is in the Jenkins Suggested fix: Re-run the build when the Related: PR #2086 (trigger, unrelated telemetry change) — #2086 |
Was it previously dependent on the allocator state (rather than the state of the allocation)? |
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@ColinNV Allocator state: the size was fixed, so glibc's state was the only variable. Reworded in the description. Thanks for the review. |
…2092) ## What? `Tracing.RequestedBackendWithoutPluginIsInert` asked `makeTracer` for the `nvtx` backend and expected a null tracer. It now asks for a backend name no plugin can provide, matching what the sibling test `Tracing.MakeTracerUnknownBackendReturnsNull` already did; the name lives in one `kUnloadableBackend` constant used by both. ## Why? The old assertion rested on a property of the *environment*, not of the code: its own comment said "no `libtrace_backend_*.so` is registered in this unit binary". Whenever a real `libtrace_backend_nvtx.so` is discoverable, the plugin loads, `makeTracer` correctly returns a live tracer, and the test fails. That is the case in the dev container, where `NIXL_PLUGIN_DIR` points at an install tree containing the NVTX trace plugin, and it would be the case in any CI leg that installs NIXL before running the `unit` suite. The test has been permanently red locally for weeks and was written off as "pre-existing, unrelated" in the validation notes of three telemetry PRs (#1952, #2054, #2086). Beyond the recurring explanation, a genuine regression in the null-tracer / inert-`Span` path would have been indistinguishable from the known-red state. Coverage is unchanged: this test exists to prove that a null tracer leaves call sites on the safe default-constructed `Span` path, which never required NVTX specifically. Real NVTX behaviour stays covered by the e2e `TestTransferTracing` tests, which load the actual plugin. Tracking: NIX-1710. <details> <summary>Rejected alternative, and verification</summary> Clearing `NIXL_PLUGIN_DIR` inside the test via the existing `gtest::ScopedEnv` helper does not work reliably: `getPluginDir()` is read once in the `nixlPluginManager` constructor and cached in `plugin_dirs_` for the process lifetime, so the outcome would depend on whether an earlier test in the binary already touched the plugin manager. Verified both directions with the container's default `NIXL_PLUGIN_DIR`, i.e. with no workaround applied: - Baseline (this change stashed, rebuilt): exactly one failure, `RequestedBackendWithoutPluginIsInert`, reporting a live tracer pointer where `nullptr` was expected. - With the change: all 17 `Tracing.*` tests pass, and the full `unit` suite is 157 passed / 2 skipped / 3 failed — the three failures being `objCrtTestFixture.TransferBelowThreshold` and the two `ObjClientTests/objParamTestFixture.ReadTransfer` params, which need an object-storage endpoint this container does not provide. </details> <!-- This is an auto-generated comment: release notes by coderabbit.ai --> ## Summary by CodeRabbit * **Tests** * Improved tracing plugin-loading tests to consistently cover environments where the requested backend is unavailable. * Updated inert-tracer coverage to validate behavior when no tracing plugin can be loaded. <!-- end of auto-generated comment: release notes by coderabbit.ai --> Signed-off-by: Efraim Eygin <eeygin@nvidia.com>
3 participants
What?
nixlTelemetryStagingQueueallocated and freed one capacity-sized vector on every drain. It now keeps two capacity-reserved buffers and alternates them, so no allocation happens after construction.The drain contract changes from an owning
std::vectorto a borrowedstd::span<const nixlTelemetryEvent>valid until the next drain, andtakePending()is renamed todrainPending().Capacity, drop-newest overflow, all-or-none batches, drop accounting, drain order and shutdown behaviour are unchanged.
Why?
NIX-1641, from review feedback on the staging-queue extraction: the drain was the last allocation left in the telemetry event path, recurring once per flush interval for the life of the process. The drain is now constant work; it previously contained a fixed-size malloc/free pair whose latency depends on heap state rather than on how many events were staged.
It is also a prerequisite for NIX-1544 (low-lock queue): with a borrowed-view drain contract, that change can return a view straight over its retired buffer instead of copying into an owning vector, leaving it to be purely about removing the producer mutex.
How? — design, contract change, and validation
What the drain used to cost
takePending()calledreserve(capacity_)unconditionally, so every flush allocated a fixed 64 KiB (4096 events × 16 B at the defaultNIXL_TELEMETRY_BUFFER_SIZE) regardless of occupancy, and freed it when the returned vector died at the end offlushPendingEvents(). Both ends sat outside the critical section, so this never widened the window producers block on — the win is bounded, which is why the NIX-1544 prerequisite leads.Double buffering
The queue holds
live_(producers append here) anddrained_(handed to the consumer). The drain discards the previous drain's contents, swaps the two buffers, and returns a view overdrained_:Both vectors are
reserve()d to capacity at construction.clear()keeps capacity, andswap()is a constant-time exchange of the two vectors' internals — no element is copied or moved — so steady state is allocation-free. Clearing before the swap (not after) is what keeps the previous drain's storage available for reuse as the nextlive_. The lock is held across thereturnso the span's pointer and size are captured in the same critical section as the swap.Colin's original note suggested moving off
std::vectorto a raw buffer plus ausedindex. Kept as vectors deliberately:nixlTelemetryEventis trivially destructible, soclear()is already just a size reset andswap()is already a constant-time pointer exchange — the raw buffer would add hand-managed storage for no measurable gain.Residency grows from one to two capacity-sized buffers — 128 KiB at the default 4096-event capacity — in exchange for zero per-flush allocator traffic.
Why the rename
The single call site is
nixlTelemetry::flushPendingEvents(), which held the result inauto. Astd::spanbinds toautojust as happily as avectordoes, so keeping the name would have let the call site compile unchanged while silently acquiring the new lifetime rule. Renaming todrainPending()forces every call site to be looked at.Thread safety
Unchanged in structure: producers only ever mutate
live_, always under the mutex, and the swap happens under the same mutex, so a producer never observes a buffer mid-swap and the unlock publishes their writes to the consumer.The borrowed view is valid until the next drain, which is safe under the queue's documented single-consumer contract: the flush runs on a one-thread pool that re-arms only after the callback returns, so two drains cannot overlap; the destructor joins that pool before members are destroyed; and every exporter copies out of the
const &rather than retaining a pointer into the span.Tests
The 13 existing queue unit tests are adapted to the new name and view return. Three were added for the properties that make this correct rather than merely working:
DrainAlternatesBetweenTwoBuffersWithoutReallocating— exactly two distinct storage addresses across 200 drain cycles, which is the allocation-free proof;CapacitySurvivesAlternatingDrains— the logical capacity still holds after alternation (this is what fails if theclear()is dropped);DrainViewCarriesOnlyTheCurrentDrain— the second view is a different buffer carrying only the new events.Validation
--gtest_repeat=5.doca_test,doca_nixl_test,histogram_parity_testandtelemetry_benchmarkpass against a local DOCA 3.3.Summary by CodeRabbit
Performance
Reliability
Tests