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perf(stream): run the subtitle/thumbnail prewarm at idle I/O priority

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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.
This commit is contained in:
Deivid Soto 2026-06-02 11:51:26 +02:00
parent 8a47132f15
commit 1c8cc1c409
3 changed files with 51 additions and 5 deletions
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25
internal/library/mediainfo/sidecar.go
View file

@ -1,6 +1,7 @@
package mediainfo
import (
"bytes"
"context"
"errors"
"fmt"
@ -177,9 +178,17 @@ func ExtractSubtitlesVTTMulti(ctx context.Context, ffmpegPath, mediaPath string,
cmd := exec.CommandContext(ctx, ffmpegPath, args...)
var stderr strings.Builder
cmd.Stderr = &stderr
// A non-zero exit can still leave good per-track files (e.g. one corrupt
// stream), so don't bail on err — read whatever landed and judge by that.
runErr := cmd.Run()
// Run it at IDLE I/O priority: this single ~14 min sequential read of a huge
// remux must not starve live streaming off the same disk/NFS.
var runErr error
if startErr := cmd.Start(); startErr != nil {
runErr = startErr
} else {
setIdleIOPriority(cmd.Process.Pid)
// A non-zero exit can still leave good per-track files (e.g. one corrupt
// stream), so don't bail on err — read whatever landed and judge by that.
runErr = cmd.Wait()
}
out := make(map[int][]byte, len(indices))
for idx, f := range tmp {
@ -230,9 +239,15 @@ func ExtractThumbnailJPEG(ctx context.Context, ffmpegPath, mediaPath string, pos
"pipe:1",
}
cmd := exec.CommandContext(ctx, ffmpegPath, args...)
var stderr strings.Builder
var stdout, stderr bytes.Buffer
cmd.Stdout = &stdout
cmd.Stderr = &stderr
out, err := cmd.Output()
if err := cmd.Start(); err != nil {
return nil, fmt.Errorf("ffmpeg thumbnail start: %w", err)
}
setIdleIOPriority(cmd.Process.Pid) // background prewarm yields I/O to live playback
err := cmd.Wait()
out := stdout.Bytes()
if err != nil {
return nil, fmt.Errorf("ffmpeg thumbnail extract: %w: %s", err, strings.TrimSpace(stderr.String()))
}