# veilor-os — memory-pressure tuning for zram-only swap
#
# Rationale: veilor-os ships zram swap with NO disk swap (see THREAT-MODEL.md
# §"Lost or stolen laptop"). The kernel's default vm.* knobs assume a slow
# spinning disk and refuse to swap until physical RAM is nearly exhausted.
# Under a zram-only stack that policy is wrong on two axes:
#
#   1. zram is RAM-fast — there is no penalty for swapping early, only a
#      small CPU cost for zstd compress/decompress.
#   2. Once zram fills, there is no overflow (no disk swap by design), so
#      the kernel falls through to OOM. With default knobs the OOM trigger
#      is slow and reactive: by the time it fires, the system has spent
#      minutes in thrash (compositor/input frozen, mouse stuck) and the
#      kernel picks a victim by oom_score which is often plasmashell or
#      the terminal — i.e. the user's session goes down, not the runaway.
#
# What these knobs do:
#
#   vm.swappiness = 180
#     Tell the kernel to prefer evicting anonymous pages to (zram) swap
#     over reclaiming file-backed pages. Fedora's zram-generator upstream
#     recommends 180 for zram-only systems. Default 60 is tuned for HDD
#     swap and leaves zram unused until too late.
#
#   vm.watermark_scale_factor = 125
#     Start kswapd reclaim earlier (~1.25% of RAM headroom vs default
#     0.1%). On a 32 GiB box that's ~400 MiB head start before allocations
#     would otherwise stall in direct-reclaim. Trades a tiny amount of
#     usable RAM for much smoother latency under bursty allocators
#     (Chromium/Electron tab spawns, language server warm-up).
#
#   vm.page-cluster = 0
#     Read one page per swap-in instead of the default 8. Read-ahead is a
#     win on rotational media because seeks dominate; on zram the seek
#     cost is zero and grabbing 7 extra pages just wastes decompress
#     cycles and CPU cache. Setting to 0 is the documented zram tuning.
#
# Companion: systemd-oomd is enabled in the same change so PSI-based
# pre-OOM kills land on the right cgroup before the kernel OOM reaper
# fires. Without it, even with these knobs the system can still wedge
# briefly while the kernel waits for the global watermark.

vm.swappiness = 180
vm.watermark_scale_factor = 125
vm.page-cluster = 0