Flamegraph of precompute_features on 1Q ES showed 62% of CPU time in
zstd decompression, 6% in DBN FSM parsing, and only 2% in the actual
feature math — single-threaded zstd was the bottleneck, not compute.
Two fixes:
1. Per-quarter parallelism on the volume-bar trades loop (was sequential
`for file in &trade_files`); brings it in line with the OFI path that
already used par_iter.
2. Predecoded sidecar cache in `crates/ml-features/src/predecoded.rs`:
first call to a `.dbn.zst` writes a bincode'd Vec<Mbp10Snapshot> or
Vec<DbnTrade> under `<output_dir>/predecoded/`. Subsequent calls
deserialize the sidecar and skip zstd entirely. An mtime+size header
self-invalidates the sidecar when the source changes — no manual
flush needed when a quarter is re-downloaded.
Local 1Q ES results:
- cold (writes sidecar): 40.7s (was 39.3s; +1.4s for write)
- warm (HIT): 4.7s (8.7× faster)
- zstd in flat perf: 62% → 0% of CPU samples
- sidecar disk per Q: ~150MB
The sidecar layer also auto-dedupes within a single run: the OFI section
re-loads trades, but the second call hits the sidecar that the
volume-bar section wrote moments earlier.
CLI: `--rebuild-predecoded` purges sidecars for cold-path testing or
after a wire-format change to Mbp10Snapshot / DbnTrade. Sidecars also
self-invalidate on format-version mismatch so old caches are skipped
silently rather than mis-deserializing.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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