Four targeted changes to maximize mux throughput regardless of storage
backend (local SSD, local HDD, NFS, network share) and surface enough
log data to diagnose 'mux slow' reports without a re-rip:
1. POSIX_FADV_SEQUENTIAL on FileSectorSource::open (Linux only).
Widens the kernel readahead window for sequential ISO reads. One
syscall at open, free on every storage type.
2. POSIX_FADV_DONTNEED on the ISO read side after every 32 MiB chunk.
Mirrors the writeback DONTNEED that already runs on the write
side. Keeps the read-side page cache bounded during multi-GB ISO
reads — eliminates the OOM-pressure / eviction-storm risk on
long mux runs. Linux only; per-drop trace at target="mux".
3. WritebackFile::create_with_size_hint(path, size_bytes) calls
fallocate(FALLOC_FL_KEEP_SIZE) on Linux to pre-reserve extents
for the output. Reported file size stays 0 (writes grow it
naturally) but the on-disk extent allocation is contiguous —
reduces extent fragmentation for big sequential muxes. Wired
into mkv:// and m2ts:// output paths via DiscTitle::size_bytes.
No-op on macOS/Windows; old create() kept with #[allow(dead_code)]
for callers without a size hint.
4. Adaptive WRITEBACK_CHUNK_BYTES in the Linux writeback pipeline.
Tracks sync_file_range(WAIT_AFTER) elapsed_ms in a rolling
16-sample window. p95 > 200 ms → double chunk size (cap 256 MiB).
p95 < 20 ms → halve (floor 4 MiB). One algorithm, both
fast-storage (small chunks, responsive) and slow-storage (big
chunks, fewer commit round-trips) optimized. Per-chunk trace +
per-32-chunk debug snapshot + info-on-resize so an operator can
see where the autoscaler settled.
All four are universal — no storage-type detection, no env vars to
flip, no per-deploy tuning required. Total +201/-6 across four files.
matthew
ec2741d5e1