perf(systemd): cap glibc malloc arenas on the display service - #469
perf(systemd): cap glibc malloc arenas on the display service#469ChuckBuilds wants to merge 2 commits into
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Measured on a live rig 2.5 hours after start:
RSS 1030 MB
Private_Dirty 988 MB
anonymous mappings > 10 MB 23
largest 104, 79, 66, 63, 63 MB, on 64 MB-aligned addresses
threads 9
cores 3 -> glibc ceiling = 8 x 3 = 24 arenas
23 against a ceiling of 24, all 64 MB-aligned: these are glibc's per-thread
malloc arenas, not live objects. The data the process was actually holding
accounts for perhaps 15 MB -- the widest scroll strip observed was 35,746 x 64,
about 7 MB as RGB and the same again for its numpy mirror.
It is bloat rather than a leak: sampled four times over 135 seconds, RSS sat
between 990 and 1030 MB rather than climbing. glibc gives each allocating
thread its own arena, grows them to hold peak demand, and never gives them
back. A process that builds and drops large images across several threads is
exactly the shape that produces this.
The device had 59 MB free at the time, on 1845 MB total.
MALLOC_ARENA_MAX=2 trades a little allocator concurrency for that resident
memory. It is a tuning knob rather than a fix for a defect, so the rationale
and the measurements sit next to it in the unit file, and a test asserts they
stay there -- a bare environment variable invites removal by whoever meets it
next.
Two things this is NOT, both checked rather than assumed:
- Not an OOM problem today. A grep for "oom" in the service journal returned
24 matches, all of which were the radar logging zoom=9 and zoom=7. The kernel
OOM killer has not fired: dmesg has zero matches.
- Not currently capped by the unit's MemoryMax=85% either. That directive is in
this file but absent from the unit actually installed on the rig, which
reports MemoryMax=infinity, so nothing is enforcing a ceiling there.
The saving is unmeasured on hardware: applying it needs a service restart,
which blanks the panel, so that is the user's call rather than something to do
mid-audit. If p99 frame time regresses -- it sits at 18.4 ms against a 16.7 ms
budget for 60 FPS, so there is not much headroom -- raise the value rather than
remove it.
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📝 WalkthroughWalkthroughThe systemd service now sets ChangesMalloc arena configuration
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: ⚪ Minimal · up to The service now caps glibc malloc arenas at 2, and no actionable merge-blocking risk remains. The test could be tightened to require that exact value so future configuration changes do not silently reduce the intended memory savings. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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🧪 Generate unit tests (beta)
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Actionable comments posted: 1
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Inline comments:
In `@test/test_systemd_malloc_arenas.py`:
- Around line 53-57: Update the MALLOC_ARENA_MAX assertion in the test to
require exactly 2, rejecting all other values while preserving the existing
diagnostic context.
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systemd/ledmatrix.servicetest/test_systemd_malloc_arenas.py
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Review follow-up. The range check accepted 1, 3 and 4, so a change to 4 -- which hands most of the resident saving back -- passed a test whose whole purpose is to notice that. Pinned to the value the unit ships, in one named constant. Raising it is still a legitimate response to a frame-time regression, but it should be a visible edit here rather than silent drift, and the failure message says so. Mutation-checked: changing the unit to 4 now fails.
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Right — pinned. Pushed The range accepted 1, 3 and 4, so a change to 4 — which hands most of the resident saving back — would pass a test whose entire purpose is to notice that. It now asserts against a single named constant ( Mutation-checked: changing the unit to 4 now fails. |
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Superseded by #476, which carries this commit unchanged (cherry-picked with |
Found while looking for a memory leak. It isn't one — it's allocator bloat, and there's a one-line lever for it.
Measured on a live rig, 2.5 hours after start
23 against a ceiling of 24, all 64 MB-aligned. Those are glibc's per-thread malloc arenas, not live objects.
For comparison, the data the process was actually holding accounts for perhaps 15 MB — the widest scroll strip observed was 35,746 × 64, about 7 MB as RGB plus the same again for its numpy mirror.
It is bloat, not a leak. Sampled four times across 135 seconds, RSS sat between 990 and 1030 MB rather than climbing. glibc gives each allocating thread its own arena, grows them to peak demand, and never returns them. A process that builds and drops large images across several threads is exactly that shape.
The device had 59 MB free, on 1845 MB total.
The change
MALLOC_ARENA_MAX=2— trades a little allocator concurrency for that resident memory.This is a tuning knob, not a defect fix, so the measurements sit next to it in the unit file and a test asserts they stay there. A bare environment variable invites removal by whoever meets it next and can't tell whether it still applies to their hardware.
Two things this is NOT — both checked, not assumed
zoom=9/zoom=7. The kernel OOM killer has never fired (dmesg: zero).MemoryMax=85%either. That directive is in this file but absent from the unit installed on the rig, which reportsMemoryMax=infinity. Worth knowing separately: the repo's intent isn't reaching installed devices.Honest limits
The saving is unmeasured on hardware. Applying it needs a service restart, which blanks the panel — the user's call, not something to do mid-audit. I'd expect a large drop based on the arena arithmetic, but I haven't proven it here.
If p99 frame time regresses, raise the value rather than remove it. There isn't much headroom: frame time sits at 18.4 ms against a 16.7 ms budget for 60 FPS.
Verification
4 tests: the setting exists, is in the useful range (1–4), the rationale with measurements sits beside it, and the unit still parses as INI (a malformed unit leaves the panel dark).
Mutation-checked — removing the setting fails 2, setting it to 24 fails the range check, and stripping the rationale fails the third.
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Summary by CodeRabbit
Performance
Documentation
Tests