next.js

d75f07b3 - turbo-persistence: skip BlockCache for uncompressed (mmap-backed) blocks (#92390)

turbo-persistence: skip BlockCache for uncompressed (mmap-backed) blocks (#92390) ### What? Skip the `BlockCache` entirely for uncompressed (mmap-backed) SST blocks, and add a per-file CRC verification bitmap so checksums are verified at most once per file open for any block. ### Why? Uncompressed blocks produce an `ArcBytes` with `Backing::Mmap` — just an `Arc::clone` of the mmap handle plus a pointer into it. No heap allocation, no decompression. The cost of constructing one (an atomic refcount increment + pointer copy) is far less than a `BlockCache` hash + concurrent-map lookup + potential insertion. Bypassing the cache for these blocks: - Reduces lock contention on the shared `BlockCache` - Avoids polluting the cache with entries that are essentially free to recreate - Eliminates redundant mmap reads (the old code peeked at the header, then re-read it inside `read_block_generic`) Key blocks (index block + all key blocks) tend to be small and are frequently accessed. In practice, small key blocks are often stored uncompressed, so this optimization applies on the hot path. ### How? **Single mmap read per block access.** `get_or_cache_block` calls `get_raw_block_slice` exactly once to read the block header (with all `#[cfg(feature = "strict_checks")]` bounds guards), then branches on `uncompressed_length`: - `== 0` (uncompressed): verify CRC via bitmap, return mmap-backed `ArcBytes` directly - `> 0` (compressed): check `BlockCache`; on miss, verify CRC via bitmap, decompress, insert **CRC verification bitmap.** `StaticSortedFile` now holds a `Box<[AtomicU64]>` with one bit per block, set once the block's CRC has been verified. This prevents re-computing the checksum on every access for uncompressed blocks (which bypass the cache) and for compressed blocks that were evicted and re-read. `Relaxed` ordering suffices since racing first-time verifications are idempotent. **`ArcBlockCacheReader` wrapper.** A `Copy` struct bundling `&BlockCache` + `&[AtomicU64]` that implements `ValueBlockCache<ArcBytes>`, threading the bitmap through the existing generic lookup machinery without changing the trait signature. **`BlockWeighter` safety net.** The mmap-backed branch now has a `debug_assert!(!val.is_mmap_backed())` to catch accidental insertion, with a negligible weight (64) as a defensive fallback in release builds. <!-- NEXT_JS_LLM_PR --> --------- Co-authored-by: Tobias Koppers <sokra@users.noreply.github.com> Co-authored-by: Claude <noreply@anthropic.com> Co-authored-by: Luke Sandberg <lukesandberg@users.noreply.github.com>