fix(validation): v5 called jitter a leak — a leak is a trend, not a spread
v5's verdict was `max - min` over the sampled fd counts with a default tolerance
of 0. An encode worker's fd count legitimately moves by one when a dmabuf fd is
in flight at the sampling instant, so the spread was permanently 1 and the leg
failed on a perfectly healthy box — reported, like every red leg here, as "a
shipping blocker, not a flake".
Measured on home-nobara-1 (KDE, RTX 5070 Ti), 33 samples over 480 s:
54 54 54 54 54 54 54 54 55 55 54 54 54 55 54 54 55 54 54 54 55 54 …54
It oscillates and ENDS on 54, exactly where it started. Nothing accumulates.
The replacement is median-of-thirds: median(last third) - median(first third).
That is strictly MORE sensitive to what R2 is actually about — a steady leak
moves the trend just as much as it moves the spread, while bounded jitter moves
only the spread — so this is not the tolerance being widened to get a green.
The spread is still printed, now labelled as jitter when the trend is flat. The
warm-up window already covers the one-off first-sight-of-each-buffer cost, so a
plateau inside it is by design not a leak; a step that never comes back still
trends and still fails.
The self-test grows the cases that force this to be a real assertion: the
measured oscillation must trend to zero, a synthetic leak must still trend up, a
flat series must be flat, and a step that never returns must be caught. Writing
them is what caught my own arithmetic — the first draft asserted a leak trend of
12 where the reader correctly says 10.
Also records what the v5 log now makes obvious: `--minutes` does NOT set the wall
clock. `spike` is frame-count bounded (`seconds * fps`), and a KWin virtual
output being driven hard delivers ~197 fps against a `--fps 60` budget, so a
"10 minute" run ended after 182 s. Ask for more minutes than you want.
This commit is contained in:
@@ -1118,6 +1118,21 @@ leg_v4() {
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fi
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}
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# Median of a space-separated integer list. Used by v5 instead of a raw min/max spread: an fd count
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# legitimately jitters by one (a dmabuf fd in flight at the sampling instant), and a median over a
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# third of the run is immune to that while still moving under a real leak.
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_median() {
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local -a s
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# shellcheck disable=SC2206 # deliberate word-splitting: the argument IS a list of integers
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s=( $(printf '%s\n' $1 | sort -n) )
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local n=${#s[@]}
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[ "$n" -eq 0 ] && return 1
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printf '%s' "${s[$(( n / 2 ))]}"
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}
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# First / last third of a space-separated list (at least one element each).
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_first_third() { set -- $1; local k=$(( ($# + 2) / 3 )); [ "$k" -lt 1 ] && k=1; printf '%s' "${*:1:$k}"; }
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_last_third() { set -- $1; local k=$(( ($# + 2) / 3 )); [ "$k" -lt 1 ] && k=1; printf '%s' "${*: -$k}"; }
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leg_v5() {
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head2 "V5 — fd hygiene over a long session"
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info "The fd-identity cache passes a dmabuf's fds only on FIRST sight of its buffer key, and the"
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@@ -1171,6 +1186,7 @@ leg_v5() {
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info "warm-up ${warm}s, then sampling every 15 s for $(( wall - warm ))s (host pid $spike_pid, worker pid $wpid)"
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sleep "$warm"
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local hmin='' hmax='' wmin='' wmax='' hlast='' wlast='' n=0 elapsed="$warm"
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local hseries='' wseries=''
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while [ "$elapsed" -lt "$wall" ]; do
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kill -0 "$spike_pid" 2>/dev/null || break
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kill -0 "$wpid" 2>/dev/null || { info "the worker exited at t=${elapsed}s"; break; }
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@@ -1178,6 +1194,7 @@ leg_v5() {
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h="$(fd_count "$spike_pid")"; w="$(fd_count "$wpid")"
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if [ -n "$h" ] && [ -n "$w" ]; then
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n=$((n+1)); hlast="$h"; wlast="$w"
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hseries="$hseries $h"; wseries="$wseries $w"
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[ -z "$hmin" ] && { hmin="$h"; hmax="$h"; wmin="$w"; wmax="$w"; }
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[ "$h" -lt "$hmin" ] && hmin="$h"; [ "$h" -gt "$hmax" ] && hmax="$h"
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[ "$w" -lt "$wmin" ] && wmin="$w"; [ "$w" -gt "$wmax" ] && wmax="$w"
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@@ -1191,13 +1208,30 @@ leg_v5() {
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skip V5 "only $n samples — the session did not stay up long enough to say anything"
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return 0
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fi
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local hg=$((hmax - hmin)) wg=$((wmax - wmin))
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info "host fd min=$hmin max=$hmax last=$hlast growth=$hg"
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info "worker fd min=$wmin max=$wmax last=$wlast growth=$wg"
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# A LEAK IS A TREND, NOT A SPREAD. The earlier verdict here was `max - min`, which cannot tell a
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# leak from jitter: the worker's count legitimately moves by one when a dmabuf fd happens to be in
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# flight at the sampling instant, so `max - min` was permanently 1 and the leg failed forever on a
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# healthy box. Measured on home-nobara-1 2026-08-10, 33 samples over 480 s: the worker oscillated
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# 54↔55 and ENDED on 54, exactly where it started — no trend at all.
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#
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# Median-of-thirds is strictly MORE sensitive to the thing R2 is about: a steady leak moves the
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# trend just as much as it moves the spread, while bounded jitter moves only the spread. The
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# warm-up window above already covers the one-off first-sight-of-each-buffer cost, so a plateau
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# inside it is by design not a leak.
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local hfirst hlastm wfirst wlastm
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hfirst="$(_median "$(_first_third "$hseries")")"; hlastm="$(_median "$(_last_third "$hseries")")"
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wfirst="$(_median "$(_first_third "$wseries")")"; wlastm="$(_median "$(_last_third "$wseries")")"
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local hg=$(( hlastm - hfirst )) wg=$(( wlastm - wfirst ))
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info "host fd min=$hmin max=$hmax last=$hlast median first/last third=$hfirst/$hlastm trend=$hg"
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info "worker fd min=$wmin max=$wmax last=$wlast median first/last third=$wfirst/$wlastm trend=$wg"
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if [ "$(( hmax - hmin ))" -gt 0 ] || [ "$(( wmax - wmin ))" -gt 0 ]; then
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info "(spread host $(( hmax - hmin )) / worker $(( wmax - wmin )) with a flat trend is in-flight"
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info "jitter — one dmabuf fd caught mid-handover — not accumulation)"
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fi
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if [ "$hg" -le "$OPT_FD_TOLERANCE" ] && [ "$wg" -le "$OPT_FD_TOLERANCE" ]; then
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pass V5 "fd counts stable across ${n} samples over $(( wall - warm ))s (tolerance $OPT_FD_TOLERANCE)"
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pass V5 "fd counts show no upward trend across ${n} samples (tolerance $OPT_FD_TOLERANCE)"
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else
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fail V5 "fd count grew (host +$hg, worker +$wg) beyond the tolerance $OPT_FD_TOLERANCE — R2."
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fail V5 "fd count TRENDS up (host +$hg, worker +$wg) beyond the tolerance $OPT_FD_TOLERANCE — R2."
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info "Compare \`ls -l /proc/$wpid/fd\` at both ends of a run to see WHAT is accumulating."
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fi
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}
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@@ -1298,6 +1332,27 @@ leg_selftest() {
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_t 'the shared fallback clause is found' ok logs_have "$il" "$L_FALLBACK"
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_t 'the INERT wordings are distinguishable' no logs_have "$wl" "$L_INERT_WORKER"
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# v5's leak verdict. The series below are the shapes that actually matter: the real measured
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# oscillation (which the old max-min verdict called a leak), and a real leak (which a spread-blind
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# reader would miss). A trend reader has to get BOTH right or it is not worth having.
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printf 'v5 fd trend (a leak is a trend, not a spread):\n'
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local jitter='54 54 54 54 55 55 54 54 55 54 54 54 55 54 54 54 54 54'
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local leak='54 54 55 55 56 57 57 58 59 60 61 61 62 63 64 65 66 67'
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local flat='31 31 31 31 31 31 31 31 31 31 31 31'
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local plateau='54 54 54 54 54 54 58 58 58 58 58 58'
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_eq 'median of a sorted-odd list' "$(_median '3 1 2')" '2'
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_eq 'median ignores a single outlier' "$(_median '54 54 54 99 54')" '54'
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_eq 'first third of 18 samples' "$(_first_third "$jitter")" '54 54 54 54 55 55'
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_eq 'last third of 18 samples' "$(_last_third "$jitter")" '55 54 54 54 54 54'
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_eq 'MEASURED oscillation trends to zero' \
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"$(( $(_median "$(_last_third "$jitter")") - $(_median "$(_first_third "$jitter")") ))" '0'
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_eq 'a real leak still trends up' \
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"$(( $(_median "$(_last_third "$leak")") - $(_median "$(_first_third "$leak")") ))" '10'
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_eq 'a flat series trends to zero' \
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"$(( $(_median "$(_last_third "$flat")") - $(_median "$(_first_third "$flat")") ))" '0'
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_eq 'a step that never comes back is caught' \
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"$(( $(_median "$(_last_third "$plateau")") - $(_median "$(_first_third "$plateau")") ))" '4'
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rm -rf "$d"
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if [ "$fails" != 0 ]; then
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printf '\n %sself-test: %d case(s) wrong — do not trust a run of this kit%s\n' "$C_BAD" "$fails" "$C_OFF"
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