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Times the shape compare_ffi measures: a new FrameCompressor per frame, compress_independent_frame_into, one reused output buffer.
- Add Workspace, a single 64-byte-aligned allocation per context that tables and buffers are carved from (upstream ZSTD_cwksp): tables from the front, buffers from the back, grown only when a frame needs more. - The literal, sequence and sequence-code buffers of a block now live in it, sized once per frame from the block capacity (literals: the block, sequences and codes: block / 3, upstream ZSTD_maxNbSeq) instead of growing as separate Vecs a fresh context allocates and frees per frame. - Both frame starts (FrameCompressor::prepare_frame and the streaming context) lay the workspace out right after the matcher reset. Part of #478
- The workspace is laid out in two parts per frame: the match finder opens it with the bytes its tables need (reserving room behind them for the block buffers, sized from the block ceiling capped by the frame's window) and carves them; the context then carves the block buffers. - Matcher gains a hidden reset_in_workspace hook; its workspace type is unnameable outside the crate, so only the context can lay the workspace out and only for the matcher it resets. External matchers keep the default, which resets with their own allocations. - A table laid out on the same bytes of the same allocation as the previous frame keeps its contents (Table::bind reports it), which is what the Fast backend's epoch advance and snapshot restore rely on; any other layout starts the table empty and drops the cached dict table. - MatchGeneratorDriver reset on its own lays its tables out in a workspace of its own. Part of #478
The long and short tables are carved from the context workspace once their widths are settled; tables that continue the previous frame's keep their contents for the floor-advance reset, and a fresh layout tells the reset they hold no earlier frame. A matcher driven on its own still allocates them itself. Part of #478
- The Row backend's shared buffer (rows with their cursors and tags, or the chain / tree hash and link tables) is carved from the context workspace once the finder and widths are settled, laid out and emptied in the same step so the first block does not fill it again. - The workspace is allocated again at the need once it has stayed three times larger than the frames for more than 128 layouts (upstream ZSTD_WORKSPACETOOLARGE_FACTOR / _MAXDURATION), which is what now gives a tree level's tables back to a context that moved to rows. - Every new allocation starts a new generation with nothing known written, so a region at an address the allocator handed back is never taken for a continuation; a_new_allocation_starts_a_new_generation covers it. Part of #478
…orkspace The hash, chain and hash3 regions of the hash-chain / binary-tree backend are carved from the context workspace once configure has set their widths, with the seams set in the same step; continued tables keep their entries for the floor-advance reset. A matcher driven on its own still allocates them itself, at the exact size. Part of #478
The one-shot borrowed block loop cleared the matcher's borrowed window from a Drop guard holding a raw pointer taken with addr_of_mut! beside the loop's own &mut self.state. Every such &mut retags the whole state as Unique, which invalidates the raw pointer under Stacked Borrows, so the guard's access at drop was undefined. The guard now holds the state's &mut and the loop reaches the state through it; the frame-wide block capacity, pre-split tier and dictionary gate are resolved before the loop. Carries a_compressor_moved_across_levels_lays_its_workspace_out_again, which moves one compressor across levels whose match finders differ and checks every one-shot and streamed frame against a fresh compressor's. Under Miri it failed on the guard before the change. Part of #478
- FastKernelMatcher::reset takes a TableCarry (clear, advance the epoch, or leave it for a snapshot restore) in place of two booleans that were mutually exclusive by construction. - with_params is test-only now that the driver builds the matcher deferred; is_allocated had no reader left; the bare Workspace::table carve and Region::len serve only the layout tests. Part of #478
- Every backend's input history is a HistoryBuf carved from the context workspace after the tables, sized once per frame from how its input arrives: nothing for a raw frame or a slice scanned in place, the dictionary plus the input plus one block for a known size, the most the history ever holds for a stream that can fill the window - The history binds before the tables and carries its bytes into the new room wherever it lands; the workspace keeps an allocation it replaced until then, so a resident dictionary survives a reallocation - The reset decides whether a slice is scanned in place and the frame loop reads that decision back instead of taking it again - reserve_for_frame and reserve_history are gone: the layout sizes the history for the whole frame - replace_matcher moves the outgoing matcher's tables and history out of the compressor's workspace; before, a matcher taken out kept pointing into memory freed with the compressor (regression test included) Part of #478
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Included review availability: This review used your included allowance. Your plan provides up to 1 included review per hour; 0 remain after this review. 📝 WalkthroughWalkthroughThe encoder adds a per-context workspace for matcher tables, history, and compressed-block buffers. Frame setup lays out these regions based on the ingest plan and compression parameters. The change also adds fresh- and reused-compressor examples and regression tests. ChangesCompression workspace
Priority: ➖ Normal Estimated code review effort: 4 (Complex) | ~60 minutes Change: Refactor · Severity of issue fixed: Medium Sequence Diagram(s)sequenceDiagram
participant FrameCompressor
participant CompressState
participant MatchGeneratorDriver
participant Workspace
participant CompressedBlockScratch
FrameCompressor->>CompressState: prepare_frame with IngestPlan
CompressState->>MatchGeneratorDriver: reset_in_workspace
MatchGeneratorDriver->>Workspace: lay out matcher tables and history
CompressState->>CompressedBlockScratch: bind block buffers
CompressedBlockScratch->>Workspace: carve literal, sequence, and code regions
Merge Risk: ⚪ Minimal · up to The encoder now uses one reusable workspace for matcher tables, history, and block buffers, and input goes straight into matcher history. The review found no outstanding defect in the current changes, and the earlier memory-accounting issue has been fixed. Security Architecture ReviewSecurity architecture risk: 🟡 Moderate · up to The redesign affects how compression state is allocated, reused, and reset between frames. The inspected paths retain input-size controls and reset state before a new frame, but the breadth of the change and incomplete security coverage warrant design review. Retained concerns Security review detailsSecurity Blast Radius
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In @zstd/src/encoding/dfast/mod.rs:
- Line 425: Update heap_size for the dfast matcher to count only matcher-owned
history memory: replace the `self.history.capacity()` contribution with
`self.history.owned_bytes()`, keeping the existing window block and table
accounting unchanged.
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ffi-bench/Cargo.tomlzstd/examples/encode_loop_fresh_z000033.rszstd/src/encoding/blocks/compressed.rszstd/src/encoding/blocks/compressed/tests.rszstd/src/encoding/dfast/mod.rszstd/src/encoding/frame_compressor.rszstd/src/encoding/frame_compressor/tests.rszstd/src/encoding/hc/generator.rszstd/src/encoding/hc/hc_tests.rszstd/src/encoding/levels/fastest/tests.rszstd/src/encoding/match_generator/mod.rszstd/src/encoding/match_generator/tests.rszstd/src/encoding/match_table/storage.rszstd/src/encoding/match_table/storage/storage_tests.rszstd/src/encoding/mod.rszstd/src/encoding/row/mod.rszstd/src/encoding/simple/fast_kernel/hash_table.rszstd/src/encoding/simple/fast_matcher.rszstd/src/encoding/simple/fast_matcher/tests.rszstd/src/encoding/streaming_encoder.rszstd/src/encoding/workspace.rszstd/src/encoding/workspace/tests.rs
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One ingest path for every frame, owned or streamed, and for dictionary priming: bytes are read into the match finder's history, wait there uncommitted while the block boundary is chosen, and are claimed with commit_filled. The staged-buffer path and everything that fed it are gone. - Matcher: get_next_space and commit_space removed; fill_in_place, uncommitted_input and commit_filled are required. HistoryBuf is public so a matcher defined outside the crate can hold its input - FrameCompressor: one block loop with no staged copy; an uncompressed frame reads each block straight into the output behind its header - StreamingEncoder: input waits uncommitted in the matcher history (a raw frame assembles its block in the output buffer); the pending Vec and the restore-on-error copy are gone - driver: the recycled buffer pool is gone; the backends lose add_data and the unused recycle callbacks on reset - Fast backend: commits in place like the others, no pending block copy - MatchTable: an empty block range when nothing is committed, not a panic BREAKING CHANGE: Matcher no longer has get_next_space or commit_space; fill_in_place, uncommitted_input and commit_filled are required methods. Part of #478
One reused CompressionContext writes the corpus in fixed chunks, the path a Write sink takes, and prints a digest of the frame so two builds can be compared byte for byte. Part of #478
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- Dfast heap_size counted a workspace history as its own, so a context reported it twice - An uncompressed frame laid out the literal, sequence and code buffers it never uses (about 800 KiB on a fresh context) - A primed-dictionary restore replaced the dfast, row and binary-tree tables and history laid out in the workspace with fresh clones every frame; it now copies into them, and an attach-mode tree is emptied in place - The workspace was given back after 128 layouts of any size; it now counts only consecutive layouts that find it too large, as upstream does - A new workspace is allocated zeroed and a table of zeros laid out in that same layout is left unwritten, keeping its pages demand-zero Each defect carries a regression test that fails without its fix. Part of #478
The over-aligned zeroed request fell back to a plain allocation and a memset of all of it, so a fresh context faulted in its whole workspace, the untouched history room included. A byte-aligned zeroed request is a calloc, which takes a large allocation as fresh pages the kernel zeroes on first touch; the start is aligned by hand. Part of #478
A zeroed allocation pays off only when the tables are larger than the input the frame can write into them: then most of their pages are never touched and stay demand-zero. When the input covers them, the tables are written densely anyway, and a zeroed block the allocator reuses is cleared in full, history room included, which costs more than filling the tables alone. The match finder now tells the workspace which case it is from the frame's size, or the history it lays out when the size is unknown. Part of #478
The Fast table and the lazy backend's tree start as zeros too; the earlier change left the Fast table on the plain path and the lazy backend out entirely. The match finder now tells the workspace how many of its table bytes start as zeros (none for the row and chain layouts, whose empty slot is not zero), and only those count toward the choice. Part of #478
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| let too_large = total | ||
| .checked_mul(TOO_LARGE_FACTOR) | ||
| .is_some_and(|wasted| self.capacity - total >= wasted); |
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When the retained workspace is between three and four times the current layout (for example, capacity 3584 and need 1024), it should accumulate oversized layouts, but this comparison requires the unused portion to be three times the need, which is equivalent to requiring the full allocation to be at least four times the need. Such a context therefore retains a 3–4× allocation indefinitely instead of shrinking after the duration limit; compare self.capacity directly with total * TOO_LARGE_FACTOR.
AGENTS.md reference: AGENTS.md:L40-L44
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| fn reset_in_workspace( | ||
| &mut self, | ||
| level: CompressionLevel, | ||
| workspace: &mut crate::encoding::workspace::Workspace, | ||
| ) { |
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When a public MatchGeneratorDriver has first been used directly through Matcher::reset and is then passed to FrameCompressor::new_with_matcher, this method rebinds its tables and history into the compressor workspace but leaves self.own_workspace allocated. The compressor consequently retains both full workspaces for its lifetime even though no live region refers to the private one after rebinding; release that old workspace after the history and tables have moved.
AGENTS.md reference: AGENTS.md:L40-L44
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| pub(crate) fn workspace_bytes(&self, capacity: usize) -> usize { | ||
| region_bytes::<u8>(capacity.max(self.len)) |
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When a reused context follows a large streamed frame with a shorter frame or a smaller table shape, self.len still contains the previous frame's whole live history here, so the new layout reserves room for it and bind may copy that window to a new position; the backend reset immediately afterward clears everything except a resident dictionary. This can move or even grow the workspace by megabytes solely for bytes the new frame discards, so determine the retained dictionary prefix before layout and size/copy only that prefix.
AGENTS.md reference: AGENTS.md:L109-L110
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| .as_ref() | ||
| .map_or(0, |table| table.heap_size()) | ||
| + self.block_scratch.retained_heap_size() | ||
| + self.workspace.capacity() |
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For every nonempty workspace, grow requests capacity + ALIGN - 1 bytes so it can align the usable start manually, but this adds only the usable capacity to heap_size. Consequently FrameCompressor, CompressionContext, and a driver's private workspace underreport each such allocation by 63 bytes through ZSTD_sizeof_CCtx; expose and count the actual allocation size rather than the carveable capacity.
AGENTS.md reference: AGENTS.md:L63-L67
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Summary
A compression context now makes one allocation and carves everything the encoder needs per frame out of it, as upstream's
ZSTD_cwkspdoes. That covers the match finder's tables, its input history and the per-block literal, sequence and code buffers. Tables are carved from the front, buffers from the back, and every region is 64-byte aligned. The workspace grows when a frame needs more. Upstream's rule for giving it back is kept: a workspace left three times larger than the frames need for more than 128 frames is reallocated at the need.Before this change a fresh compressor held these as a spread of separate
Vecs. glibc gave their pages back at the end of every frame, so each frame faulted its whole working set in again.Input now reaches the match finder by one path for every frame: it is read straight into the matcher's history, waits there uncommitted while the block boundary is chosen, and is then claimed. The staged-buffer path that copied each block (and each pre-split remainder) through a scratch
Vecis gone.What changed
encoding::workspace:Workspace,Table<T>(a table in the workspace or owning a copy, one access path either way),HistoryBufandRegionVec<T>.replace_matchermoves the outgoing matcher's tables and history into allocations of their own. Before, a matcher taken out of a compressor kept pointing into memory freed with it. A regression test covers this.One ingest path:
Matcher:get_next_spaceandcommit_spaceare removed.fill_in_place,uncommitted_inputandcommit_filledare required.HistoryBufis public, so a matcher defined outside the crate can hold its input.FrameCompressor: one block loop with no staged copy. An uncompressed frame reads each block straight into the output behind its header and never touches the matcher.StreamingEncoder/CompressionContext: written input waits uncommitted in the matcher's history (a raw frame assembles its block in the output buffer). Thependingbuffer and its restore-on-error copy are gone; a failed drain already leaves the context failed.add_data, the unused recycle callbacks on reset, and a set of tests compiled out with#[cfg(any())]that referenced long-removed types.MatchTablereturns an empty block range when nothing is committed instead of panicking.encode_loop_stream_z000033) for measuring theCompressionContextpath.Measurements
runner1 (x86_64), bench profile. Harnesses are interleaved, prebuilt binaries, 50 frames of
z000033[..200000]unless noted.Page faults, fresh compressor per frame, streaming
compress(). Slope over 10 against 100 frames:The one-shot shape (
compress_independent_frame_into, fresh compressor) is flat at every level as well.Time, streaming, fresh compressor, full
z000033(branch start → workspace):Time, reused compressor (
main→ workspace): L5 227–237 → 196–208 ms, L9 388–391 → 349–350 ms. L2, L3, L13 and L19 are unchanged. L1 reads 60.2–60.4 against 60.7–61.8 ms; that is under the 1.5% this host resolves between two builds, so it is not a measured regression.Time, one ingest path (workspace → this PR, full
z000033, 50 frames, three interleaved rounds):CompressionContext, 64 KiB writesFrameCompressor, reader sourceThe one-shot slice paths scan the input in place and do not run the changed code. The reused L5 reading of +1.8% is binary layout: its instruction count is identical (496.06 M both, callgrind), and L13, which also runs no changed code, moved the same way.
Time, fresh context, 4 KiB through stdin with no size (the CLI,
main→ this PR; libzstd for reference): L13 35.5–36.5 → 16.7 ms (libzstd 59–60 ms), L19 118.9–119.9 on the workspace without zeroed pages → 30.4–30.7 ms (main31 ms, libzstd 133 ms). L1–L9 unchanged.Time, reused compressor with a dictionary (
z000033[..200000]withdict_tests/dictionary, 200 frames): L3 733–738 → 523–527 ms, L5 1,127–1,138 → 750–754 ms, L16 unchanged.Output is byte-identical at L1–L22 on
z000033[..200000], on the fullz000033, onz000033withdict_tests/dictionary, and with long-distance matching at L16–L22: 73 frames compared. The streaming path is byte-identical at L1–L22 as well.Testing
cargo nextest runfor the library (hash,std,dict-builder,ldm),ffi-bench(bench-internals,dict-builder) and the C ABI pass on aarch64 (M1) and x86_64.cargo clippy -D warningson the library (--all-targets, with and withoutbench-internals),ffi-bench, the C ABI, the wasm crate and the no-std / kernel-scalar configurations.cargo fmt --check,cargo test --doc, rustdoc with private items and-D warnings.BREAKING CHANGE:
Matcher::get_next_space,Matcher::commit_spaceandMatcher::reserve_for_frameare removed;Matcher::fill_in_place,Matcher::uncommitted_inputandMatcher::commit_filledare required, andfill_in_placehands the fill callback the matcher'sHistoryBufinstead of aVec<u8>.Closes #478
Summary by CodeRabbit
Performance
Bug Fixes