c6f65f0831
Companion to the memory-pressure tuning (7d2b94b). Memory was only
half the "expensive laptop typing like a Chromebook" story — once
zram-only OOM thrash was solved, a second symptom class emerged:
post-boot CPU/IO contention on machines with high core counts.
Live incident on a 24-thread Ryzen AI 9 HX 370 / 30 GiB workstation,
2026-05-13: ~16 min after login, load avg 6.5, typing in konsole and
the address bar lagged 100s of ms. RAM/swap uncontended (8 GiB/30 GiB
used, zero swap), so the memory tuning was holding. PSI showed
cpu some=0.34 — pure scheduler contention.
Root cause: every Fedora unit ships with CPUWeight=[not set] which
maps to weight=100. Under contention the kernel splits CPU evenly
between every leaf cgroup. With the post-boot storm running
concurrently (plasma-discover ~80%, packagekitd ~33%, fwupd ~20%,
dnf-makecache firing) the compositor (kwin_wayland, plasmashell) was
losing scheduling fights against package metadata.
Three fixes shipped together:
1. system-bg.slice — CPUWeight=20, IOWeight=50, MemoryHigh=4G. Five
service drop-ins assign packagekit, fwupd, fwupd-refresh,
dnf-makecache, dnf5-automatic into it with Nice=10 and
IOSchedulingClass=idle. Proportional, not a hard cap — idle
systems still get full speed.
2. user-.slice.d/10-boost.conf — CPUWeight=300, IOWeight=200 on every
logged-in user session. Combined with above gives a 15:1
interactive:background ratio under contention.
3. Boot-storm sources defused: skel autostart shadow disables the
discover update notifier auto-launch; dnf-makecache.timer
OnBootSec=20min pushes metadata refresh past peak session
bring-up.
One opt-in artifact: skel user-bg.slice (CPUWeight=30) for anyone
installing Syncthing, rclone, or a file indexer — drop a
Slice=user-bg.slice drop-in on the service to inherit the same
protection at the user level.
Verified live before opening this PR: load dropped 6.53 -> 3.55
within minutes of applying; cgroup placement confirmed via
systemd-cgls.
Follow-up filed in CHANGELOG (not in this PR): tuned-adm
"onyx-performance" profile silently falls back to balanced, and
EPP regresses to balance_performance on AC. Needs separate branch.
15 lines
549 B
SYSTEMD
15 lines
549 B
SYSTEMD
[Unit]
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Description=Background system services (low priority)
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Documentation=https://git.s8n.ru/veilor-org/veilor-os/src/branch/main/docs
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Before=slices.target
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# Holds dnf metadata refresh, PackageKit, fwupd, and other deferrable
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# system maintenance. CPUWeight=20 vs default 100 means these yield
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# 5:1 to the rest of system.slice under contention; idle systems still
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# get full speed. MemoryHigh=4G is a soft cap — kernel reclaims pages
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# rather than evicting interactive workloads when these grow.
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[Slice]
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CPUWeight=20
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IOWeight=50
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MemoryHigh=4G
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