Reverts 523af46. That revert was made on the premise that Phase 2.5
caused a ~60% mux throughput regression on NFS bidirectional workloads.
The premise was wrong: at the time of measurement the producer was
capped at ~8 MB/s by a 50 ms thread::sleep poll in
Pipeline::send_with_halt (fixed in v0.21.7's io/pipeline change), so
the comparison was measuring the polling cap on both sides.
With the polling cap removed, direct passthrough exposes the kernel's
default dirty-page writeback pathology on NFS: writes accumulate, the
kernel periodically bursts a flush, app writes block for the burst.
Observed empirically on Civil War UHD remux 2026-05-14: 5-45 MB/s
spiking around a ~21 MB/s sustained mean, dominated by burst-flush
back-pressure cycles.
Phase 2.5 decouples the mux thread from the file syscall:
* mux writes complete instantly into a 128 MiB byte-bounded SPSC ring,
* a dedicated writer thread executes the real File writes, seeks,
and sync_file_range calls; can sit in a kernel burst without
blocking the mux pipeline,
* backpressure via Condvar notify/wait, no polling primitive,
* the ActiveClusterBuffer fast-path preserves the original MKV
cluster-backpatch optimisation so in-window seeks don't drain
the current writeback chunk.
Halt-safety is preserved: every blocking writeback syscall on the
writer thread still routes through bounded_syscall with a 60 s
deadline. A wedged NFS server cannot trap the writer indefinitely;
the muxer keeps queueing into the ring; the kernel page cache and
the ring together absorb the stall.
MattJackson
2a33364253