perf(encode): reuse the match tables when a compressor's dictionary changes by polaz · Pull Request #618 · structured-world/structured-zstd

polaz · 2026-10-21T11:21:45Z

Summary

A compressor handed a different dictionary every few frames, as the COVER and FastCOVER searches do when they price candidates, builds each dictionary's attached tables once and allocates them once: frames that copy the dictionary set the dfast tables and the row index aside instead of dropping them, and a dictionary change reuses their allocation and the primed snapshot's buffers.
How much of a dictionary the match-finder tables index now comes from the CDict's cParams, as upstream's ZSTD_loadDictionaryContent does, instead of the frame's. Before, an index built for one frame size was reused for another, so a reused compressor could compress a frame differently from a fresh one (level 5 and level 4, already on main).

Changes

dict_attach: DictKeep keeps a table a backend detached, parked for the dictionary still in use or kept only for its allocation; a clone of it is empty, so snapshots never carry it, and it costs its matcher one pointer.
dfast: copied frames, evictions and width changes park the attached tables; the driver revives them before priming an attached dictionary of the same widths, index floor and length. A dictionary change keeps one allocation, emptied in place by the next build; a frame without a dictionary drops everything.
row: the same for its dictionary index, which a change of plan (attach or copy) used to drop. It is taken back only for the cParams it was built with: geometry, form, key width and tree depth.
driver: the dictionary's index reach is taken from the CDict's cParams at the reset; a dictionary change sets the dfast or row snapshot aside and the next capture copies into its buffers. The fast backend keeps cloning, which costs fewer instructions there.
Examples encode_loop_dict_swap and ffi_loop_dict_swap: a reused context handed a new dictionary every few frames, ours and libzstd's (a ZSTD_CDict per dictionary).

Measurements

x86 bench host, task-clock, median of three interleaved series; 300 rounds of 6 frames (40960 and 2000 bytes) over 5 dictionaries:

level	main	this branch	libzstd
1	311 ms	311 ms	210 ms
3	786 ms	704 ms	555 ms
5	1139 ms	997 ms	722 ms
9	1524 ms	1386 ms	1070 ms

--train-cover --maxdict=16384 over the crate's sources: 2751 ms on main, 2647 ms here. Compressed bytes and the trained dictionary are identical to main's.

Testing

Gates: fmt, clippy in every CI feature set, the codec's tests (macOS and the x86 host), the C ABI's, doc tests, the no_std build and the tests without legacy.
New tests: a reused compressor against a fresh one over seven levels, six dictionaries and frames either side of every attach cutoff, plus the level-5 case that failed on main and a parameter change that keeps the table sizes but moves the key width; table build and allocation counts for dfast and row.

Closes #542

Summary by CodeRabbit

Performance
- Improved repeated compression with dictionary-aware cache reuse across frames, dictionaries, compression settings, and frame sizes.
- Reduced unnecessary dictionary-table rebuilding and memory allocation while retaining compatible cached data.
- Improved memory accounting for active and retained compression resources.
Examples
- Added Rust and FFI benchmark examples for alternating inputs and dictionary swapping across repeated compression rounds.
Tests
- Added coverage confirming reused compressors produce output identical to newly initialized compressors.

…ary changes A compressor handed small and large frames with one dictionary rebuilt the dfast attach tables after every frame that copied the dictionary, and allocated them zeroed each time; a dictionary change also threw away the primed snapshot and cloned a new one for the next dictionary. - dict_attach: DictKeep keeps tables a backend detaches. A fully built one is parked while frames copy the dictionary or the window slides past it, and revived by the next frame that attaches the same dictionary at the same widths and index floor; otherwise it stays only for its allocation. A clone of it is empty, so snapshots never carry it. - dfast: copied frames, evictions and width changes park the tables; the driver revives them before priming an attached dictionary. A dictionary change keeps one allocation, emptied in place by the next build; a frame without a dictionary drops everything. A snapshot restore leaves each side's dictionary state as it was. - driver: a dictionary change sets the primed snapshot aside, and the next capture copies into its buffers (dfast, row, fast) instead of cloning; a frame without a dictionary drops it. Output is byte-identical: a new test alternates attaching and copying frames over two dictionaries against a fresh compressor per frame, and counts one table build per dictionary and one allocation in all. Part of #542

How much of a dictionary the match-finder tables index came from the frame's cParams, which an attached dictionary adjusts to the source size. A dictionary index a frame of one size built was then reused for a frame of another, so a reused compressor compressed a frame differently from a fresh one: at level 5 a 28 KiB frame after a 152-byte one under the same dictionary indexed less of it. Upstream indexes the last 1 << max(hashLog + 3, chainLog + 1) bytes into the CDict at the CDict's own cParams, the same for every frame (ZSTD_loadDictionaryContent). The driver now takes the reach from the CDict's cParams, once per frame at the reset. - regression test: the level-5 sequence above against a fresh compressor (red before), and a randomized one over seven levels, six dictionaries and frames either side of every attach cutoff - examples encode_loop_dict_swap and ffi_loop_dict_swap: a reused context handed a new dictionary every few frames, ours and libzstd's (a CDict per dictionary, as upstream's trainer prices candidates) Part of #542

The row backend dropped its attached dictionary index whenever the frame's dictionary plan changed, and the plan records whether the frame attaches, so a compressor alternating small and large frames rebuilt the index after every large one. With the index covering as much of the dictionary as upstream's CDict does, that rebuild cost a level-5 dictionary-swap loop 8% on the bench host. - row: a plan change, a copied frame and a window that slid past the dictionary park the index; the driver revives it before priming an attached dictionary of the same geometry, form, index floor and length. A dictionary change keeps one allocation, emptied in place by the next build; a frame without a dictionary drops everything - dict_attach: DictKeep holds its table in a slot allocated at the first park, so a holder costs its matcher one pointer and later parks allocate nothing - the reuse test runs on the row backend as well Part of #542

… one Capturing the fast backend's primed snapshot into the buffers of the one it replaces retired 1.2% more instructions on a level-1 dictionary-swap loop than cloning it, so that backend clones as before and a dictionary change no longer keeps its old snapshot around. Part of #542

chatgpt-codex-connector · 2026-10-21T11:22:00Z

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zstd/src/encoding/frame_compressor/tests.rs
zstd/src/encoding/match_generator/mod.rs
zstd/src/encoding/row/mod.rs

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📝 Walkthrough

The pull request adds reusable dictionary-table and snapshot storage for Dfast and Row matchers. It adds regression tests for output equivalence and benchmark examples that swap dictionaries across compression rounds.

Changes: Dictionary table reuse

Layer / File(s)	Summary
Dictionary cache lifecycle `zstd/src/encoding/dict_attach.rs`, `zstd/src/encoding/dict_attach/tests.rs`	`DictKeep` stores parked tables or spare allocations. Tests cover compatible revival, rejected tables, dictionary changes, release, and cloning.
Dfast and Row cache integration `zstd/src/encoding/dfast/mod.rs`, `zstd/src/encoding/row/mod.rs`	The matchers park dictionary tables when they cannot remain attached. They reuse storage when table shape and dictionary metadata match.
Dictionary priming and snapshot reuse `zstd/src/encoding/match_generator/...`	`MatchGeneratorDriver` derives dictionary reach from CDict parameters and applies the Dfast attachment check. Dfast and Row snapshot storage can be reused.
Regression coverage and swap examples `zstd/src/encoding/frame_compressor/tests.rs`, `ffi-bench/Cargo.toml`, `zstd/examples/*dict_swap.rs`	Regression tests compare reused compression with fresh compression. The manifest registers Rust and FFI examples that alternate dictionaries and input sizes.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant FrameCompressor
  participant MatchGeneratorDriver
  participant DfastMatchGenerator
  participant RowMatchGenerator
  participant DictKeep
  FrameCompressor->>MatchGeneratorDriver: reset with dictionary
  MatchGeneratorDriver->>DfastMatchGenerator: apply attachment decision
  MatchGeneratorDriver->>RowMatchGenerator: prime dictionary tables
  DfastMatchGenerator->>DictKeep: revive compatible parked tables
  RowMatchGenerator->>DictKeep: revive compatible parked tables

Possibly related PRs

structured-world/structured-zstd#494: Changes Dfast dictionary attach-versus-copy decisions and table filling, which overlaps with this pull request’s Dfast cache parking and revival.
structured-world/structured-zstd#511: Changes CDict parameters and preserves Row dictionary state across resets, which overlaps with this pull request’s Row cache reuse.

Merge Risk: ⚪ Minimal · up to 83690

No identified issue blocks merging after normal checks.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 74.29% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 70 functions across 9 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: reusing match tables when a compressor dictionary changes.
Linked Issues check		Issue `#542` requires reuse of compatible dictionary match-table allocations and snapshot storage without output changes. `DictKeep` parks and revives built tables, and reuses spare allocations. `dict_p…
Out of Scope Changes check		The changes stay within issue `#542`. Dfast, Row, and snapshot lifecycle changes implement dictionary table and snapshot reuse. The dictionary reach correction and shape checks prevent stale cached stat…

Full details: Linked Issues check

Explanation

Issue #542 requires reuse of compatible dictionary match-table allocations and snapshot storage without output changes. DictKeep parks and revives built tables, and reuses spare allocations. dict_prime restores compatible Dfast and Row snapshots in place. The new tests verify one allocation across dictionary changes, cache-shape guards, and byte-identical output against fresh compressors. The PR reports before/after timings for the dictionary loop and --train-cover.

Full details: Out of Scope Changes check

Explanation

The changes stay within issue #542. Dfast, Row, and snapshot lifecycle changes implement dictionary table and snapshot reuse. The dictionary reach correction and shape checks prevent stale cached state from changing output. The examples and regression tests support the requested performance and correctness behavior. No unrelated change is shown.

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Inline comments:
Review comments at @zstd/src/encoding/match_generator/mod.rs:
- Around line 1265-1273: Update reset_in_workspace to compute dfast_attach_next
before borrowing self.storage, using dict_hint and dfast_attaches_dictionary for
the decision. Remove the duplicated inline predicate and reuse the computed flag
in the reset logic so reset-time and prime-time dictionary layout decisions
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📒 Files selected for processing: 10

ffi-bench/Cargo.toml
zstd/examples/encode_loop_dict_swap.rs
zstd/examples/ffi_loop_dict_swap.rs
zstd/src/encoding/dfast/mod.rs
zstd/src/encoding/dict_attach.rs
zstd/src/encoding/dict_attach/tests.rs
zstd/src/encoding/frame_compressor/tests.rs
zstd/src/encoding/match_generator/dict_prime.rs
zstd/src/encoding/match_generator/mod.rs
zstd/src/encoding/row/mod.rs

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codecov · 2026-10-21T11:27:07Z

Codecov Report

❌ Patch coverage is 97.25275% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
zstd/src/encoding/dict_attach.rs	95.95%	4 Missing ⚠️
zstd/src/encoding/dfast/mod.rs	96.20%	3 Missing ⚠️
zstd/src/encoding/row/mod.rs	97.47%	3 Missing ⚠️

📢 Thoughts on this report? Let us know!

- The lazy backends' dictionary index is hashed with the CDict's minMatch and, in its tree form, sorted with 1 << searchLog compares. Parking it on a plan change and reviving it by table sizes alone took an index built for another minMatch back after set_parameters moved it, so the probe hashed with the new width into the old index and lost the dictionary's matches. - RowDictShape now holds everything the index depends on besides the dictionary, built in one place for the prime, the tree build and the revive; the revive and the park-on-mismatch compare all of it, and only a spare allocation is matched by table sizes. - The dfast reset takes its attach decision from the same helper the prime uses. Regression test: a_dictionary_index_is_not_reused_across_key_width_or_search_depth (row, chain and tree forms; it failed with 248-259 bytes against a fresh compressor's 23).

## 🤖 New release * `structured-zstd`: 0.2.3 -> 0.2.4 <details><summary>Changelog</summary> <blockquote> ## [0.2.4](v0.2.3...v0.2.4) - 2026-10-21 ### Added - *(decoding)* decode the v0.5, v0.6 and v0.7 frame formats ([#616](#616)) ### Performance - *(encode)* reuse the match tables when a compressor's dictionary changes ([#618](#618)) </blockquote> </details> --- This PR was generated with [release-plz](https://github.com/release-plz/release-plz/). Co-authored-by: sw-release-bot[bot] <255865126+sw-release-bot[bot]@users.noreply.github.com>

polaz added 4 commits October 21, 2026 12:35

chatgpt-codex-connector Bot reviewed Oct 21, 2026

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Comment thread zstd/src/encoding/match_generator/mod.rs

polaz merged commit 6dd7ba4 into main Oct 21, 2026
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polaz deleted the perf/#542-dict-table-reuse branch October 21, 2026 13:23

sw-release-bot Bot mentioned this pull request Oct 21, 2026

chore: release v0.2.4 #617

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perf(encode): reuse the match tables when a compressor's dictionary changes - #618

perf(encode): reuse the match tables when a compressor's dictionary changes#618
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