Refactor: retain the host_build_graph SM mirror on the runner - #2013
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📝 WalkthroughWalkthroughThe change adds retained, aligned SM mirror buffers per pipeline slot. Host orchestration acquires them through HostApi. Onboard and simulator runners grow and release the buffers. Documentation records init-on-write behavior and measurement results. ChangesShared-memory mirror lifecycle
Estimated code review effort: 3 (Moderate) | ~25 minutes Merge Risk: ⚪ Minimal · up to The PR changes the shared-memory mirror to runner-owned reuse and releases it at Worker finalization; one investigation note incorrectly describes that lifetime as process-long. Correcting that documentation is a small follow-up, with no supplied evidence of a runtime or data-safety defect. Sequence Diagram(s)sequenceDiagram
participant HostOrchestration
participant HostApi
participant PlatformRunner
participant RetainedSmMirror
HostOrchestration->>HostApi: acquire_sm_mirror(sm_size, CHIP_ALIGN_SIZE, &host_sm)
HostApi->>PlatformRunner: forward pipeline slot, size, and alignment
PlatformRunner->>RetainedSmMirror: grow slot storage when capacity is insufficient
RetainedSmMirror-->>PlatformRunner: aligned uninitialized address
PlatformRunner-->>HostApi: address or allocation error
HostApi-->>HostOrchestration: mirror result
PlatformRunner->>RetainedSmMirror: release_sm_mirrors() during finalization
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🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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In `@docs/investigations/2026-08-host-orch-phase-tail-is-page-faults.md`:
- Around line 269-270: Update the mirror lifetime wording to state that each
pipeline-slot buffer is retained until Worker finalization, or otherwise match
the actual runner lifetime documented by HostApiOps; do not claim it lasts for
the entire process lifetime.
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📒 Files selected for processing (15)
docs/investigations/2026-08-host-orch-phase-tail-is-page-faults.mddocs/investigations/README.mdsrc/a2a3/platform/sim/host/device_runner.cppsrc/a2a3/runtime/host_build_graph/docs/RUNTIME_LOGIC.mdsrc/a2a3/runtime/host_build_graph/host/runtime_maker.cppsrc/a5/platform/sim/host/device_runner.cppsrc/a5/runtime/host_build_graph/docs/RUNTIME_LOGIC.mdsrc/a5/runtime/host_build_graph/host/runtime_maker.cppsrc/common/platform/include/common/host_api.hsrc/common/platform/onboard/host/c_api_shared.cppsrc/common/platform/onboard/host/device_runner_base.cppsrc/common/platform/onboard/host/device_runner_base.hsrc/common/platform/sim/host/c_api_shared.cppsrc/common/platform/sim/host/device_runner_base.cppsrc/common/platform/sim/host/device_runner_base.h
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@coderabbitai Addressed:
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Every bind allocated the host mirror of the runtime shared memory from the heap and freed it on the way out. The mirror is dimensioned for the run's configured task count, which is 82 MB at the dsv4 case's 16384 — far above every glibc reuse threshold, so each bind was one mmap and one guaranteed munmap of that size. A bind's own faults then cost 14-33 us each instead of ~1.7 us, because the unmap holds mmap_lock for write and excludes every faulting thread in the address space (docs/investigations/2026-08-host-orch-phase-tail-is-page-faults.md). The mirror is now the platform runner's, one buffer per pipeline slot, grown to the high-water mark and held across binds until Worker finalization — the same shape hw-native-sys#1988 gave the Graph Definition staging block, reached through a new HostApi::acquire_sm_mirror. The buffer is an uninitialized owning block rather than a container: the layout is init-on-write, so a value-initializing resize would fault in every page of a capacity whose live prefix is a fraction of it, and would copy bytes that mean nothing between binds. First touch still commits the mirror, so a run pays only for the bytes it writes. Reuse needs no new invariant. A bind already cleared only the fixed-size header and relied on init-on-write for the per-slot segments, which is why compact_live_image can ship each segment's used prefix: prepare_task writes the slot state and clears the completion flag as it claims a slot, and TaskPayload::init is the single payload-init point. The one shipped range no device-side reader reaches is the alignment padding the fanin cursor rounds past, which lies outside every payload's fanin_count. The buffer is released at Worker finalization on both the ordinary and the force-reset path, since it is host memory that no device failure invalidates. Measured on dsv4, interleaved twice: mallinfo2's hblkhd stops returning to its pre-bind value on 6 of 6 binds, which is the map/unmap going away, and the cold-bind fault count matches the per-bind allocation it replaces. host_orch's warm-bind fault count and the control-plane duration resolve in neither direction — the mirror is ~6 THP faults of a ~1200-fault bind. This closes the investigation entry's "Where a fix would go" list, so the entry is amended to say so: items 1-3 shipped as hw-native-sys#1981 and item 4 as hw-native-sys#1988 plus this change, and none of them reached the ~1100 faults the submitting thread takes per bind. hw-native-sys#1981 already reported those unmoved when the recording's allocations went away; this is the second measurement of the same thing. Attributing them needs mincore() on a buffer's pages before the write, not another guess at which allocation it is. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Summary
host_build_graphbind allocated the host mirror of the runtime shared memory from the heap and freed it on the way out. The mirror is dimensioned for the run's configured task count — 82 MB at the dsv4 case's 16384 — far above every glibc reuse threshold, so each bind was onemmapand one guaranteedmunmapof that size. That unmap holdsmmap_lockfor write and excludes every faulting thread in the address space, which is the mechanismdocs/investigations/2026-08-host-orch-phase-tail-is-page-faults.mdprices at 14–33 µs per fault against 1.7 µs off-tree.PTO_PIPELINE_MAX_DEPTHslot, grown to the high-water mark and held across binds — the same shape Refactor: hbg records and ships Definitions in one retained block #1988 gave the Graph Definition staging block — reached through a newHostApi::acquire_sm_mirror. Wired on both the onboard and the sim runner, and released at Worker finalization on both the ordinary and the force-reset path, since it is host memory that no device failure invalidates.resizewould fault in every page of a capacity whose live prefix is a fraction of it, and would copy bytes that mean nothing between binds. First touch still commits the mirror, so a run pays only for the bytes it writes.compact_live_imageship each segment's used prefix:prepare_taskwrites the slot state and clears the completion flag as it claims a slot, andTaskPayload::initis the single payload-init point. The one shipped range no device-side reader reaches is the alignment padding the fanin cursor rounds past, outside every payload'sfanin_count.RUNTIME_LOGIC.md§3.2 records the contract in both.What this is worth, measured
Interleaved A/B on dsv4 FLASH decode (
base, retained, base, retained, three rounds over two ranks,mallinfo2+smaps_rollupsampled per bind):hblkhdatbind_end, 6 of 6 bindshost_orchminflt, the two cold bindshost_orchminflt, four warm binds (median)hblkhdno longer returning to its pre-bind value is the whole result: the 82 MB map/unmap per bind is gone, deterministically, on every bind of every run.It is not a speedup, and this PR does not claim one.
host_orch's warm-bind fault count resolves in neither direction — base spans [1160, 1256] over its eight warm binds, retained [181, 1268] — because the mirror is ~6 THP faults of a ~1200-fault bind. Control-plane duration likewise: minimum-of-sums 1.724 → 1.795 ms on one repetition, 1.532 → 1.010 ms on the other.What this closes, and what is actually left
This was the last item on the investigation entry's "Where a fix would go" list, so the entry and its index are amended to say the list is closed: items 1–3 shipped as #1981 (the recorder thread owns its recording storage — hazard map stood up once per thread, flat arrays
clear()ed at capacity, node slots and their tensor buffers never released), item 4 as #1988 plus this change.None of them reached the ~1100 minor faults the submitting thread takes per bind. #1981 already reported those unmoved when the recording's allocations went away; this arm is the second measurement of the same thing. So the fault decomposition in that entry attributes the tail to allocations that no longer happen, and it no longer explains the steady state — the amendment now says so rather than leaving four implemented items reading as open work. Attributing the ~1100 needs
mincore()on a buffer's pages before the write that would fault them (thepage-faultsperf event carries no ADDR, already checked), not another guess at which allocation it is.Also worth stating because it is the obvious next reach and would not help: node/tensor storage is already retained at its high-water mark, so pre-reserving it to
GRAPH_MAX_NODES×CORE_MAX_TENSOR_ARGS(4 MB per recorder thread, ~32 MB across the prewarmed pool) would only move each thread's first recording off the growth path. A warm bind already allocates nothing there.Testing
Full matrix, green on this diff at base
fc42cd1dd:ctest -LE requires_hardware— 119/119pytest tests/ut— 1935 passed / 14 skipped (one wall-clock teardown test,test_second_child_failure_reaps_first, flakes at load average 40–75; passes alone, in-file, and in a full run at lower load — unrelated to this diff)-m "not sdma" --exclude-level 4— 149 passed / 1 skippedRebased afterwards onto
f830f13c3(#2004 and #2012 both landed in hbg mid-review); that rebase conflicted only in the four files this diff touches, was resolved onto the newsm_layout::segment_offsets/task_capacitynames, and the tree builds. The matrix above was not re-run on the final base — CI covers it.