The prewarm's single big read (a ~14 min sequential pass over a 60GB remux to
demux subtitles) shares the same disk/NFS as live streaming. Lower the prewarm
ffmpeg processes to the Linux IDLE I/O class (ioprio_set) so that background
read yields bandwidth to a user who's actually watching — the prewarm slows
down under contention instead of starving playback, and runs full speed when the
disk is idle.
Applied only to the prewarm-only extractors (ExtractSubtitlesVTTMulti,
ExtractThumbnailJPEG) via Start → setIdleIOPriority(pid) → Wait; the on-demand
/sub + /thumbnail handlers keep normal priority (a user is waiting on those).
Linux-only syscall behind a build tag; a no-op stub elsewhere. Best-effort —
errors ignored, never required for correctness.
Verified: the prewarm ffmpeg shows 'idle' under ionice -p; on-demand stays normal.
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