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Let the fan settle downward on a re-assert

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A symmetric deadband catches the fan on the way up and holds it there:
on iz-pve0 under load, 17 of 20 samples ran at 38% while the curve
wanted 34-36%, and nothing would bring it down until the request fell a
full 8 points. Damping was turning into a permanently louder machine.

A re-assert may now settle the speed downward to the current request.
Upward moves still require a full deadband crossing, so the drift this
was built to prevent cannot come back - covered by a test.

Verified under a live migration: settled 38 -> 36 on the first
re-assert, then one change in ten minutes while the request wandered
36-42, CPU stable at 66-69c and disks a degree cooler.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Zeb Hering
2026-08-28 22:33:38 -07:00
parent be440e95a3
commit 4dffbb070c
3 changed files with 39 additions and 5 deletions
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18
fan_speed.sh
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@@ -117,6 +117,14 @@ selftest() {
# leak is what let the setpoint drift 32 -> 36 -> 31 on iz-pve0.
check "re-assert re-pushes held value" 32 "$(next_fan_speed 34 32)"
echo "settling downward (damping must not make the machine permanently louder):"
check "holds above the want between re-asserts" 38 "$(settle_fan_speed 34 38 0)"
check "settles down to the want on re-assert" 34 "$(settle_fan_speed 34 38 1)"
check "small rise still held on re-assert" 38 "$(settle_fan_speed 42 38 1)"
check "real rise still adopted" 46 "$(settle_fan_speed 46 38 1)"
# The old upward leak: a re-assert must never drift the setpoint up.
check "no upward drift on re-assert" 32 "$(settle_fan_speed 36 32 1)"
echo "trend verdict (load is not a fault - only cooling that cannot keep up is):"
TREND_SAMPLES=5
HIGH_FAN_SPEED=50
@@ -293,12 +301,12 @@ one_pass() {
# Hold the current speed unless the change is worth making - see the deadband
# note in monitor.sh. Chasing every degree is what made this louder than stock.
local target now held=""
target=$(next_fan_speed "$DECIMAL_CURRENT_FAN_SPEED" "$APPLIED_FAN_SPEED")
[ "$target" = "$DECIMAL_CURRENT_FAN_SPEED" ] || held=" held, want $DECIMAL_CURRENT_FAN_SPEED"
local target now due=0 held=""
now=$(date +%s)
if [ "$target" != "$APPLIED_FAN_SPEED" ] ||
[ $((now - APPLIED_FAN_SPEED_AT)) -ge "$FAN_REASSERT_INTERVAL" ]; then
[ $((now - APPLIED_FAN_SPEED_AT)) -ge "$FAN_REASSERT_INTERVAL" ] && due=1
target=$(settle_fan_speed "$DECIMAL_CURRENT_FAN_SPEED" "$APPLIED_FAN_SPEED" "$due")
[ "$target" = "$DECIMAL_CURRENT_FAN_SPEED" ] || held=" held, want $DECIMAL_CURRENT_FAN_SPEED"
if [ "$target" != "$APPLIED_FAN_SPEED" ] || [ "$due" = 1 ]; then
apply_user_fan_control "$target"
APPLIED_FAN_SPEED=$target
APPLIED_FAN_SPEED_AT=$now