Fix: measure a bind phase's faults over the span its clock covers - #2022
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📝 WalkthroughWalkthroughThe investigation documentation revises page-fault latency conclusions and identifies ChangesHost orchestration investigation
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🔵 Low · up to The change only affects diagnostic bind-phase measurements, but the current implementation can report fault counts and durations over slightly different boundaries, which may mislead performance analysis. It is mergeable with explicit owner awareness or a follow-up to align the boundaries or document the metrics as approximate; no production behavior change is indicated. Poem
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In `@docs/investigations/2026-08-host-orch-phase-tail-is-page-faults.md`:
- Around line 28-33: The Answer paragraph below the warning must no longer
present mmap/munmap traffic as the production fault-cost mechanism. Clearly
label its 14–33 µs measurement as applying only to the off-tree reproducer,
while retaining mprotect as the mechanism for the documented host-orchestration
path.
- Around line 423-434: Revise the residual-fault conclusion in the investigation
document to state that the tunable arm showed no measurable latency impact after
the fault reduction, rather than asserting the faults cost no time or cannot be
a performance defect. Replace the categorical “do not treat” wording with “not
currently established as a performance defect,” and apply the same qualification
in the investigation README’s corresponding summary.
- Around line 389-392: Update the allocator arm description in the table to use
the exact settings MALLOC_MMAP_THRESHOLD_, MALLOC_TRIM_THRESHOLD_, and
MALLOC_TOP_PAD_ with their stated values, or the equivalent GLIBC_TUNABLES keys,
and add the glibc version used in the experiment.
In `@docs/investigations/README.md`:
- Line 87: Update the issue mapping in the index entry to match the detailed
investigation: include `#2013` with item 4, and identify the flat-region form of
item 3 as `#2015`, while preserving the existing mappings for items 1–2 and the
rest of the entry.
In `@src/a2a3/runtime/host_build_graph/host/runtime_maker.cpp`:
- Around line 159-176: Update bind_phase_begin and host_phase_record_bind so
counter sampling and duration timestamps use matching start/end boundaries,
preventing scheduling gaps between those operations from skewing phase deltas;
preserve the existing behavior when host_phase_breakdown_enabled() is disabled.
Apply the same fix in `@src/a5/runtime/host_build_graph/host/runtime_maker.cpp`
around lines 159 - 176: The same counter-versus-duration boundary mismatch
applies to the a5 implementation.
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- Create a new PR with the fixes
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A bind phase reported its duration as the interval from its own start
timestamp, but its minor-fault count as the delta since the *previous*
phase was recorded. Those are the same span only when no code runs
between one phase closing and the next one opening, which is not the case
anywhere on this path -- and is furthest from true for host_orch, whose
count started back at BindRuntimeInit, before run_host_orchestration is
called at all, and so also covered the shared-memory mirror acquisition
and ChipOrchestratorState::init.
A count that covers a wider span than the clock describes work the phase
does not contain, which makes the two numbers on one line disagree about
what they measure. Both ends of a phase now close together:
- bind_phase_begin() takes the counter snapshot and the timestamp
together, and every phase start goes through it.
- the close reads the counters, takes the timestamp one call later, and
hands that instant to host_phase_record_bind through a new end_ns
argument. Leaving the end to the record path would have covered the
attribute formatting plus the four early returns above its own clock,
so the duration would report work the counts do not.
- recording a phase no longer re-marks, so a stretch belonging to no
phase is dropped from the counts exactly as it is already dropped
from the durations, rather than being attributed to whichever phase
happens to open next.
`end_ns` defaults to 0, which keeps the clock where it was for a caller
with no counters to align against.
Measured on dsv4 at six rounds, twice: this moves no phase's count.
host_orch reads 1003-1145 before and 950-1153 after, so the setup region
the old span wrongly included contributes approximately none of its
faults. The fix is to what the numbers mean, not to their values.
The investigation entry is amended with what these counts were used to
establish, and with the framing they refute. Two arms point opposite ways
-- glibc keeping freed memory removes 86% of the faults and shows no
measurable latency change, while hw-native-sys#2015 removes 6% and buys 29-43% -- so
the tail is not count x price. Only the price is a lever, and the in-tree
mmap_lock writer that sets it is mprotect, which glibc uses to open a
non-main arena and which no strace in the entry had traced; the entry's
own mmap/munmap attribution describes its off-tree reproducer. The
residual faults are therefore not currently established as a performance
defect, which is recorded explicitly because they have been chased as one
three times.
No behavior changes outside the SIMPLER_HBG_BIND_BREAKDOWN_ENABLE
diagnostic: bind_phase_begin only reads getrusage when the breakdown is
enabled, and the counters feed an attribute string.
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@coderabbitai all five addressed and pushed. Two of them changed the diff rather than the prose:
The other three: the Answer's superseded block now separates the exclusion (which holds, and carries the 14–33 µs) from the cpput 119/119, both arches build, |
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Tip For best results, initiate chat on the files or code changes. 🧩 Analysis chain🏁 Script executed: #!/bin/bash
set -euo pipefail
printf '%s\n' '=== PR diff summary ==='
git diff --stat HEAD^ HEAD
printf '%s\n' '=== Runtime-maker symbol outlines ==='
ast-grep outline src/a2a3/runtime/host_build_graph/host/runtime_maker.cpp --items all --match 'bind_phase_begin|record_bind_phase|host_phase_record_bind'
ast-grep outline src/a5/runtime/host_build_graph/host/runtime_maker.cpp --items all --match 'bind_phase_begin|record_bind_phase|host_phase_record_bind'
printf '%s\n' '=== Relevant code occurrences ==='
rg -n -C 8 'bind_phase_begin|record_bind_phase|host_phase_record_bind|read_process_counters' \
src/a2a3/runtime/host_build_graph/host/runtime_maker.cpp \
src/a5/runtime/host_build_graph/host/runtime_maker.cpp
printf '%s\n' '=== Investigation and index diff ==='
git diff HEAD^ HEAD -- \
docs/investigations/2026-08-host-orch-phase-tail-is-page-faults.md \
docs/investigations/README.mdLength of output: 88139 🏁 Script executed: #!/bin/bash
set -euo pipefail
printf '%s\n' '=== Trace API declarations and implementations ==='
rg -n -C 8 'host_phase_record_bind\s*\(' \
src/a2a3/runtime/host_build_graph/runtime/host_phase_trace.h \
src/a2a3/runtime/host_build_graph/host/host_phase_trace.cpp \
src/a5/runtime/host_build_graph/runtime/host_phase_trace.h \
src/a5/runtime/host_build_graph/host/host_phase_trace.cpp
printf '%s\n' '=== Explicit close timestamp assignment ==='
rg -n -C 5 'const int64_t end_ns|bind_now_ns\(\)' \
src/a2a3/runtime/host_build_graph/host/runtime_maker.cpp \
src/a5/runtime/host_build_graph/host/runtime_maker.cpp
printf '%s\n' '=== Residual-fault section heading and conclusion ==='
rg -n -C 3 'residual faults are not a performance item|not currently established as a performance defect|showed no measurable latency change' \
docs/investigations/2026-08-host-orch-phase-tail-is-page-faults.mdLength of output: 17001
The five prior findings are addressed in the implementation and documentation:
One documentation inconsistency remains. The section heading at Line 427 says: ### The residual faults are not a performance itemThe section body correctly says that the residual fault count is “not currently established as a performance defect.” Please qualify the heading to the same standard. For example: ### The residual faults are not currently established as a performance defect🐇 💭 🧠 Learnings usedYou are interacting with an AI system. |
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The close boundary now matches the open boundary. Passing The revised residual-fault conclusion also states the evidence accurately. The documentation now separates the valid exclusion from the superseded 🧠 Learnings usedIf you found this review helpful, would you consider giving us a shout-out on X? Thank you for using CodeRabbit! You are interacting with an AI system. |
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The paragraph correcting dsv4's `args` and `host_view_close` rows cited measurements taken on `f830f13c3` plus a then-uncommitted child-memory change, so its baseline cannot be reproduced from any commit in the tree. Its `host_view_close` figure was also measured before #1973 removed the `halHostRegister` side, leaving it an order of magnitude high: a reader A/B-ing against 0.014-0.028 ms today would read a 10x regression where there is none. Both figures now come from the merged tree at `dcf7559e8`, 12 binds over two ranks at `--rounds 6`: `args` 0.036-0.075 ms, `host_view_close` 0.0012-0.0030 ms with `count=0 bytes=0`. #2022 established that giving a phase's counters the span its clock covers changes what the numbers mean and not their values, so those remain the current figures. The same run's peak host RSS is recorded beside them, since the rows they correct are per-byte costs over what a bind stages: 1.31 GiB across the whole process tree under `--skip-golden`, 23.4 GiB when the fixture is streamed in, against the ~45.5 GB per rank the pinned row cost. The pinned table itself is untouched. It is anchored to `777d4171` on purpose and says so.
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Aug 27, 2026
…ate (#2036) Every per-bind fault count in the host-orch investigation is a warm-up figure under a steady-state label, and the correction is not a smaller number: the quantity being divided was never per-bind. Measured at f40cacf, with #1988, #2013, #2015, #2019 and #2022 all in, host_orch's minflt per bind in arrival order over three runs at different round counts: --rounds 2 (4 binds) 992, 949 (cold), then 165, 130 --rounds 5 (10 binds) 989, 983 (cold), then 114, 173, 54, 3, 13, 13, 11, 8 --rounds 8 (16 binds) 931, 837 (cold), then 112, 216, 164, 10, 2, 1, 1, 8, 57, 10, 8, 11, 0, 0 `args` follows the same curve (699310 cold, then 0-2) and so does graph_upload (246 cold, 0 after). The tail decays over roughly six binds and then reaches zero, on three independent runs. So nothing "survived" the four changes the entry says it did, and the mechanism left undetermined for three rounds turned out not to need determining. The entry's counts came from three-to-six-round runs divided by the bind count, so each one averaged two cold binds and three or four still-decaying ones. The reusable half of that mistake goes to the measurement guide, because the guide's own framing invited it: dropping one cold bind per rank is what the parser does, and the decay behind that bind is what it does not. Two traps added -- reading a warm bind as a steady-state one, and dividing a total by the bind count -- and the Reference-numbers preamble no longer calls its post-cold binds steady-state, since a four-round session spends most of them inside the decay. Also records where the control plane stands at that commit, with the caveat the same trap implies: 0.529 ms min / 0.570 median over 8 warm binds, of which host_orch 0.341/0.364, graph_upload 0.129/0.133, arena_h2d 0.056/0.058 -- and two of those 8 are still decaying, so it needs --rounds 12 to be clean. What survives from the amendment above it: the tunable arm and #2015 do point in opposite directions, so inside the warm-up window the count is not a lever and the price is.
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Summary
host_orchwas furthest off: its clock starts att_orch_ns, its count started back atBindRuntimeInit— beforerun_host_orchestrationis entered at all — so it was also charged the shared-memory mirror acquisition andChipOrchestratorState::init.bind_phase_begin()takes the counter snapshot and the timestamp together, and all ten phase starts go through it. At the close, the counters are read (the attribute string needs them), the timestamp is taken one call later, and that instant is handed tohost_phase_record_bindthrough a newend_ns— leaving the end to the record path would have covered thesnprintfplus the four early returns above its own clock, so the duration would report work the counts do not.end_nsdefaults to 0, keeping the clock where it was for the non-breakdown caller, which has no counters to align against.SIMPLER_HBG_BIND_BREAKDOWN_ENABLEdiagnostic:bind_phase_beginreadsgetrusageonly when the breakdown is enabled, and the counters feed an attribute string.This changes what the numbers mean, not their values. Measured on dsv4 at six rounds, twice: no phase's count moves.
host_orchreads 1003–1145 before and 950–1153 after — so the setup region the old span wrongly included contributes approximately none of its faults (new/acquiredoes not touch what it maps, and the tensormap stand-up is init-on-write).The investigation entry is amended, and one of its framings is refuted
The counts this fixes are what
docs/investigations/2026-08-host-orch-phase-tail-is-page-faults.mdhas been steering by since #1981, and its opening framing of the tail as count × price does not survive two arms that point opposite ways:MALLOC_MMAP_THRESHOLD_andMALLOC_TRIM_THRESHOLD_at 1 GiB,MALLOC_TOP_PAD_at 256 MiB; glibc 2.36)Removing 86% of the faults shows no measurable latency change; removing 6% buys a third of the phase. The count is not a lever — only the price is. The tunable arm is not new evidence; it is the arm the entry already had, read as "the tunables are not a fix" instead of "the count does not buy time".
The in-tree
mmap_lockwriter ismprotect, and no strace in the entry had traced it. glibc reserves a non-main arena withmmap(PROT_NONE)and opens it up withmprotect(grow_heap), which takesmmap_lockfor write and excludes every faulting thread exactly asmunmapdoes — the entry's ownmmap/munmapattribution describes its off-tree reproducer. Onestrace -ff -e trace=mprotect,madvise,brkover three rounds, in the non-main-arena band:mprotect(PROT_READ|PROT_WRITE)madvise(MADV_DONTNEED)Those arenas only grow. That is why #2015 — which sizes every recorder array from its contract at thread stand-up and never grows one again — removes the writer, and why it is the first change on this path to buy time.
Consequence, recorded explicitly because it has been chased as a latency problem three times: the residual ~1100 resident-page re-faults per bind (survived #1981, #1988, #2013 and #2015; mechanism undetermined) are not currently established as a performance defect — the tunable arm removed 86% of them and showed no measurable latency change, which is the strongest statement the measurement supports. It does not show they cannot cost time in another workload or allocator state, only that nothing here has priced them. Userspace tools are exhausted —
mincoreand pagemap both report presence, not writability, and no interface exposes a PTE's write bit — so pricing them at all needsbpftraceonhandle_mm_faultunder root.The amendment also carries the per-site fault table with its shelf life stated (two of its top three sites were deleted by #2019 and #2015 within days — keep the method, not the numbers), everything refuted while attributing them (
MADV_DONTNEED, fork-COW, KSM, AutoNUMA, THP, first-write, allocation shape), and three tooling traps that each produced a wrong conclusion first:perfwithout-k mono(whose obvious sanity check still passes),--no-buildid-cacheplus a later rebuild, and readingmincore/pagemap as writability.Testing
ctest -LE requires_hardware— 119/119clang-format,markdownlint-cli2,tests/lint/check_retired_names.py— cleanSIMPLER_HBG_BIND_BREAKDOWN_ENABLEis set, which no scene test setsRebased onto
dcf7559e8(#2020 and #2021 landed mid-review); both rebases were clean.