Rudder was measured slowing after about 1,400 words while a Tauri build
ran. The word count is incidental — it is a proxy for how long the build had
been going. The speech model runs on the CPU (backend/rudder-engine starts it
with --device cpu), and a Rust build saturates every core, so at equal priority
it starves the one process whose latency a person can hear.
Three changes, and the direction of the first is forced rather than chosen:
1. internal/desktop-build.mjs nices the whole build tree to 10, through the one
run() helper every step goes through, so cargo, rustc and the linker inherit
it. We nice the COMPETITOR because an unprivileged process cannot raise its
own priority on Linux — a negative nice needs CAP_SYS_NICE — so "promote the
ASR" would fail with EPERM. If nice(1) is missing the build still runs and
says so, because a silent fallback would look exactly like the fix working.
2. cargo is capped at cores-1. nice decides who wins a CONTESTED core; it does
not stop the build taking all of them, and a fully subscribed machine still
adds scheduling latency to the audio path. An explicit CARGO_BUILD_JOBS is
honoured rather than overridden.
3. yieldCPU reports its error instead of discarding it. It was
`_ = syscall.Setpriority(...)`, which cannot fail visibly — a call that
silently does nothing is indistinguishable from one that worked, and this one
guards something audible.
Also, one real O(n) in the widget, found while checking studio's answer that the
collapsed pill already shows the last words: sendActivity split the ENTIRE draft
on every activity tick to take five words. Measured at 3,000 words: 1.61ms per
tick against 0.051ms for a bounded tail, 31x, with byte-identical output.
Verify the priority work with `ps -o ni=` on the running cargo processes, never
by any of these calls returning.
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