Audiobook-text aligners are single-threaded
Investigating why an audiobook-to-text alignment pipeline appeared stalled at single-core utilization despite parallel transcription preceding it.
Audiobook alignment tools (storyteller-style pipelines wrapping forced-aligners) typically parallelize the transcription stage but run the sync/alignment stage strictly single-threaded — chapter-by-chapter sequence matching has cross-chapter ordering dependencies that defeat naive parallelization. Container CPU pegged at 100% with N-1 idle host cores is the expected steady-state, not a misconfiguration, and there is usually no setting to fan it out. Additionally, expect benign 'Could not find chapter #X in transcription' warnings for epub front matter (cover, copyright, TOC, dedication) that have no audio counterpart — these are skipped, not errors.
Before assuming an alignment stage is broken, verify (a) container CPU is at exactly one core, not zero, and (b) initial 'not found' warnings only fire on the first few epub items — both indicate normal operation.