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perf(byte-storage): write inputs under 128 B as a literals-only block (LAB-12921) - #156
store now writes every input under 128 B as one literals-only LZ4 block, straight into the envelope, instead of only inputs under 64 B. From 64 B to 127 B it skips the encoder's 8 KiB hash-table clear and its match search: a 64 B store drops from 10,398 to 515 Ir/op and takes about a fifth of the wall time. The price is stored size on compressible values from 64 B to 127 B: up to 117 bytes per value (table below).
Closes LAB-12921
Why 128 B
Paired, interleaved wall-clock timings of ByteStorage::store on the benches/common MessagePack corpus. Two builds of the crate are linked into one binary: base bd72f85, and a variant that writes a literals-only block for every input up to 1 KiB. The arms rotate order every round, 400 rounds of about 100 µs batches, pinned to one core. Each figure is the median of the per-round paired ratio with a bootstrap 95% CI. The fill arm times the same 8 KiB fill(0) of a 64-byte-aligned [u16; 4096] that lz4_flex's clear runs. Hardware performance counters were not available, so instruction counts come from perf_ir (cachegrind) below.
Size
A/A
8 KiB clear, share of a base store
Literal block up to 1 KiB vs base
Envelope bytes, base → literal
64 B
−0.09% [−0.25, +0.07]
36.5%
−69.8%
90 → 90
128 B
+0.00% [−0.14, +0.14]
31.8%
−71.8%
157 → 157
256 B
−0.02% [−0.19, +0.12]
23.7%
−76.7%
221 → 286
512 B
−0.06% [−0.17, +0.07]
15.5%
−82.0%
356 → 544
1 KiB
−0.02% [−0.15, +0.10]
8.8%
−82.8%
612 → 1,059
A second run on another core agreed within 2.2 percentage points on every share and saving (8.4% share at 1 KiB). The clear alone is at least 8% of a store at every size up to 1 KiB, so this proceeds. Skipping the encoder saves far more than the clear, because the match search goes too, but from about 130 B it costs stored size. Averaged over 256 windows of a 256 KiB stream of the same corpus:
Size
Mean envelope, encoder
Mean envelope, literal block
Growth
64 B
90.8
91.0
+0.2%
96 B
122.8
123.0
+0.2%
124 B
150.7
150.9
+0.2%
128 B
155.1
156.0
+0.6%
136 B
160.6
164.2
+2.2%
144 B
164.8
171.9
+4.3%
160 B
173.4
188.0
+8.5%
256 B
223.8
285.8
+27.7%
512 B
353.0
543.1
+53.9%
1 KiB
596.4
1,057.0
+77.2%
A record in this corpus is about 130 bytes long, so a slice under 128 B sits inside one record and the corpus cannot show the cost there. Real small values do pay it. Envelope bytes, encoder (base) against literal block (head):
Value
Base
Head
Growth
JSON user record, 104 B
114
132
+15.8% (+18 B)
JSON feature flags, 88 B
77
116
+50.6% (+39 B)
MessagePack list of 5 maps with repeated keys, 111 B
88
139
+58.0% (+51 B)
one UUID three times, 108 B
74
136
+83.8% (+62 B)
23 B zero-padded to 100 B
60
126
+110% (+66 B)
127 B of one byte
36
153
+325% (+117 B)
random bytes, 100 B
127
127
0
These are examples: envelope lengths move by a few bytes with the checksum, whose bytes MessagePack writes in 1 or 2 bytes each. A sweep of every repeated-byte input from 64 to 127 B, the most compressible case, reached at most 4.5 times and +117 bytes; at 127 B the literal envelope is 151 to 158 bytes. Past one record, the corpus shows the literal block costing more with every byte (+4% at 144 B, +28% at 256 B), on top of the same kind of loss. 128 B is also the hard cap of the existing one-pass envelope writer: original_size must fit a positive fixint and the block a bin8 (the <= 0x80 const assert), so going higher needs a wider writer as well as a size decision. The threshold therefore moves to 128 B and no further, and the code change is the constant.
The shipped change, head against base, same method:
Size
A/A
Head vs base
Envelope bytes
64 B
+0.14% [−0.29, +0.80]
−80.5% [−80.6, −80.3]
90 → 90
96 B
+0.07% [−0.28, +0.72]
−82.6% [−82.7, −82.5]
124 → 124
127 B
−0.20% [−0.76, +0.35]
−82.4% [−82.5, −82.3]
153 → 153
128 B
+0.25% [−0.11, +0.63]
+0.64% [+0.07, +0.96]
157 → 157
1 KiB
+0.16% [−0.38, +0.49]
+0.60% [−0.07, +1.00]
612 → 612
At 128 B and 1 KiB the code path is unchanged and perf_ir reads the same instruction count on both sides, so the +0.6% is code layout between the two builds. The second run read +1.07% and +0.48%.
Instruction counts
perf_ir Ir/op, (Ir[1000] - Ir[0]) / 1000, x86_64-linux, rustc 1.98.1, valgrind 3.22.0. Base is the merge-base bd72f85, head is 4ea0376 (the later commit changes only tests and docs), both from scripts/perf-ir-vs-base.sh origin/main on one machine, which passes on every case.
Case
Base
Head
Change
store/40
533
533
0
store/64
10,398
515
−95.05%
store/128 (new)
11,082
11,082
0
store/1024
22,781
22,781
0
retrieve/40
1,005
1,005
0
retrieve/64
1,109
1,007
−9.20%
retrieve/128 (new)
1,304
1,304
0
retrieve/1024
5,739
5,739
0
retrieve_data/64
943
841
−10.82%
retrieve runs the same code; at 64 B its envelope now holds a literal block, which decodes in fewer instructions than the encoder's block with a match. Every other case moved by at most ±0.01%.
benches/perf_ir_baselines.json is re-recorded with make perf-ir-update on the machine and toolchain the budgets were recorded with (the unchanged cases read the same figures to the instruction). That ratchets store/64, adds the 128 B cases, and corrects budgets that earlier merges left stale: store/40 (1,186 → 533), retrieve/40 (2,637 → 1,005), retrieve_data/40 (2,471 → 839), and the retrieve* cases at 64 B, 1 KiB, 64 KiB and 1 MiB. No budget rose.
Compatibility
Compressed bytes are not canonical (protocol spec/wire-format.md, "Compressed-byte reproducibility"), and every LZ4 block decoder reads a literals-only block. The envelope format is unchanged. The protocol vectors with 64 B and 74 B inputs were already literals-only blocks, and head's store writes their bin envelopes byte for byte. A compressible input from 64 B to 127 B now grows instead of shrinking. The worst case is a repeated byte: 127 B of one byte went from a 36-byte envelope to 153 bytes (64 B: 39 → 93). Its block is at most 129 bytes and the envelope at most 407 bytes (was 343).
Tests
cargo test --all-features (263 passed), cargo clippy --all-features --all-targets -- -D warnings, cargo fmt --check. The existing threshold tests read LITERAL_BLOCK_THRESHOLD, so they now cover 0 to 127 B on the literal side and 128 B on the encoder side. Tests that straddled the old threshold with 63 B and 64 B (store_and_encode_into_write_rmp_serde_bytes, store_allocates_only_its_output_and_block, the one-pass decode test's store sizes) now read LITERAL_BLOCK_THRESHOLD - 1 and LITERAL_BLOCK_THRESHOLD, so they still cover both sides. Two tests used 100 B as an interior size of the encoder range; that size is now 200 B.
Docs
The LITERAL_BLOCK_THRESHOLD rustdoc gives the corpus reason for 128 B. The README's store note, its small-values practice, and the perf_ir size list now say 128 B; its claim that make perf-ir has no case below 64 B or above 64 KiB was stale and is corrected. SECURITY.md (the per-thread scratch range and the no-encoder range) and the 32-bit bound test's header say 128 B.
Sibling paths
wasm32 and other 32-bit targets (safe encoder): the constant is not target-gated, so they take the literal path below 128 B too. Not measured there.
estimate_compression: it calls the same compress_block, so it reports the literal block below 128 B, matching store (estimate_compression_reports_what_store_writes passes).
128 B to 4 KiB (the table-reuse range): still pays the 8 KiB clear on every call, 31% of a 128 B store and 9% of a 1 KiB store. lz4_flex 0.13 and 0.14 clear the table on every compress_into_with_table call and offer no narrower clear, so removing it needs a change to the dependency, which this PR does not make.
retrieve: not changed.
cachekit-py, on its next core bump:tests/unit/test_serializer_lazy_loading.py asserts estimate_compression_ratio({"a": [1] * 100}) > 1. That value packs to 106 B, which now takes the literal path and reports a ratio just under 1, so the test needs an input of 128 B or more. A test helper's docstring there also says "Under 64 B". Both are fixed in cachekit-py when it bumps core.
Protocol spec:spec/wire-format.md's note on the canonical writer says "shorter than 64 bytes"; it is updated to 128 bytes when this lands.
The LZ4 encoder zeroes its 8 KiB hash table on every call and then
searches for matches; below 128 B a cached value rarely holds a repeat,
so both bought nothing. On the bench corpus the envelope grows 0.2% on
average from 64 B to 127 B, and a 64 B store drops from 10,398 to 515
Ir/op. Adds store/retrieve/retrieve_data cases at 128 B, the first size
the encoder takes, and ratchets the stale Ir budgets.
Storage now uses a literals-only compression path for inputs smaller than 128 bytes; inputs of 128 bytes or more use the standard encoder.
Performance coverage and benchmark results now include 64-byte and 128-byte inputs, alongside existing payload sizes.
Documentation
Updated the guidance on compression thresholds, allocation behaviour, and trade-offs for small inputs.
Walkthrough
The literals-only LZ4 threshold increases from 64 to 128 bytes. The change updates envelope handling, boundary-test coverage, benchmark cases and baselines, and threshold documentation.
Changes
LZ4 threshold update
Layer / File(s)
Summary
Threshold behaviour and boundary coverage src/byte_storage.rs, tests/compress_bound_32_bits.rs
Inputs under 128 bytes use the literals-only block path. The documented maximum envelope size changes to 407 bytes, and the compression test size case changes from 100 to 200 bytes.
Benchmark coverage and threshold documentation benches/perf_ir.rs, benches/perf_ir_baselines.json, README.md, SECURITY.md
Benchmarks add 64-byte and 128-byte cases, and the retrieve, retrieve_data and store baselines are updated. The README and SECURITY.md describe the 128-byte threshold and related measurements.
cachekit-io/cachekit-core#128: Adds the literals-only LZ4 block path that this change extends to inputs under 128 bytes.
cachekit-io/cachekit-core#131: Adds the envelope length and buffer interfaces used by the literals-only envelope path.
cachekit-io/cachekit-core#150: Adds the one-pass envelope decoder used to decode envelopes written by the literals-only path.
Suggested reviewers: kodus-27b
Merge Risk: Minimal · up to 4ea03
No actionable merge-blocking risk is established for the threshold update; merge after normal checks pass.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
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Docstring Coverage
Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
Check name
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Title check
The title names the main change: inputs under 128 B are now written as a single literals-only LZ4 block in the store path of byte-storage. It matches the raw summary (LITERAL_BLOCK_THRESHOLD raised …
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Description check
The description is specific and closely tied to the changeset. It explains that store now writes inputs under 128 B as a literals-only LZ4 block, which matches the LITERAL_BLOCK_THRESHOLD change from …
Full details: Title check
Explanation
The title names the main change: inputs under 128 B are now written as a single literals-only LZ4 block in the store path of byte-storage. It matches the raw summary (LITERAL_BLOCK_THRESHOLD raised from 64 to 128 bytes in src/byte_storage.rs) and the PR objectives. It is specific and not vague.
Full details: Description check
Explanation
The description is specific and closely tied to the changeset. It explains that store now writes inputs under 128 B as a literals-only LZ4 block, which matches the LITERAL_BLOCK_THRESHOLD change from 64 to 128 bytes in src/byte_storage.rs. It also covers the benchmark and perf_ir baseline updates, the README and SECURITY.md documentation changes, the test adjustments, and the compatibility impact on envelope sizes. The description is detailed and on topic, so the check passes.
Full details: Docstring Coverage
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Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 3 functions across 3 files. (3 skipped: 3 unsupported.)
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Write docstrings for the functions missing them to satisfy the coverage threshold.
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README.md
SECURITY.md
benches/perf_ir.rs
benches/perf_ir_baselines.json
src/byte_storage.rs
tests/compress_bound_32_bits.rs
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…ated figures
Tests that straddled the old 64 B threshold with 63/64 now read
LITERAL_BLOCK_THRESHOLD, so they still cover both sides of it, and the
round-trip test's interior size moves above it. The rustdoc drops its
instruction counts and the README points to the rustdoc instead of
repeating the corpus figures.
…small values
The bench corpus cannot show the cost below 128 B, because a slice that
short sits inside one record. State the measured growth on small JSON
and MessagePack values, padding and byte runs, its 117-byte bound, and
that 128 is the literal envelope writer's own cap. ByteStorage::store
and StorageEnvelope::new now say what happens under 128 B, and the
private comments name the constant instead of its value.
Envelope lengths vary by a few bytes with the checksum's MessagePack
encoding, so a single 127-byte example is not an invariant: a sweep of
every repeated-byte input from 64 to 127 B reached about 4.5 times and
151 to 158 bytes. The 117-byte increment is the sweep's maximum.
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storenow writes every input under 128 B as one literals-only LZ4 block, straight into the envelope, instead of only inputs under 64 B. From 64 B to 127 B it skips the encoder's 8 KiB hash-table clear and its match search: a 64 B store drops from 10,398 to 515 Ir/op and takes about a fifth of the wall time. The price is stored size on compressible values from 64 B to 127 B: up to 117 bytes per value (table below).Closes LAB-12921
Why 128 B
Paired, interleaved wall-clock timings of
ByteStorage::storeon thebenches/commonMessagePack corpus. Two builds of the crate are linked into one binary: basebd72f85, and a variant that writes a literals-only block for every input up to 1 KiB. The arms rotate order every round, 400 rounds of about 100 µs batches, pinned to one core. Each figure is the median of the per-round paired ratio with a bootstrap 95% CI. The fill arm times the same 8 KiBfill(0)of a 64-byte-aligned[u16; 4096]that lz4_flex'sclearruns. Hardware performance counters were not available, so instruction counts come fromperf_ir(cachegrind) below.A second run on another core agreed within 2.2 percentage points on every share and saving (8.4% share at 1 KiB). The clear alone is at least 8% of a store at every size up to 1 KiB, so this proceeds. Skipping the encoder saves far more than the clear, because the match search goes too, but from about 130 B it costs stored size. Averaged over 256 windows of a 256 KiB stream of the same corpus:
A record in this corpus is about 130 bytes long, so a slice under 128 B sits inside one record and the corpus cannot show the cost there. Real small values do pay it. Envelope bytes, encoder (base) against literal block (head):
These are examples: envelope lengths move by a few bytes with the checksum, whose bytes MessagePack writes in 1 or 2 bytes each. A sweep of every repeated-byte input from 64 to 127 B, the most compressible case, reached at most 4.5 times and +117 bytes; at 127 B the literal envelope is 151 to 158 bytes. Past one record, the corpus shows the literal block costing more with every byte (+4% at 144 B, +28% at 256 B), on top of the same kind of loss. 128 B is also the hard cap of the existing one-pass envelope writer:
original_sizemust fit a positive fixint and the block abin8(the<= 0x80const assert), so going higher needs a wider writer as well as a size decision. The threshold therefore moves to 128 B and no further, and the code change is the constant.The shipped change, head against base, same method:
At 128 B and 1 KiB the code path is unchanged and
perf_irreads the same instruction count on both sides, so the +0.6% is code layout between the two builds. The second run read +1.07% and +0.48%.Instruction counts
perf_irIr/op,(Ir[1000] - Ir[0]) / 1000, x86_64-linux, rustc 1.98.1, valgrind 3.22.0. Base is the merge-basebd72f85, head is4ea0376(the later commit changes only tests and docs), both fromscripts/perf-ir-vs-base.sh origin/mainon one machine, which passes on every case.store/40store/64store/128(new)store/1024retrieve/40retrieve/64retrieve/128(new)retrieve/1024retrieve_data/64retrieveruns the same code; at 64 B its envelope now holds a literal block, which decodes in fewer instructions than the encoder's block with a match. Every other case moved by at most ±0.01%.benches/perf_ir_baselines.jsonis re-recorded withmake perf-ir-updateon the machine and toolchain the budgets were recorded with (the unchanged cases read the same figures to the instruction). That ratchetsstore/64, adds the 128 B cases, and corrects budgets that earlier merges left stale:store/40(1,186 → 533),retrieve/40(2,637 → 1,005),retrieve_data/40(2,471 → 839), and theretrieve*cases at 64 B, 1 KiB, 64 KiB and 1 MiB. No budget rose.Compatibility
Compressed bytes are not canonical (protocol
spec/wire-format.md, "Compressed-byte reproducibility"), and every LZ4 block decoder reads a literals-only block. The envelope format is unchanged. The protocol vectors with 64 B and 74 B inputs were already literals-only blocks, and head'sstorewrites theirbinenvelopes byte for byte. A compressible input from 64 B to 127 B now grows instead of shrinking. The worst case is a repeated byte: 127 B of one byte went from a 36-byte envelope to 153 bytes (64 B: 39 → 93). Its block is at most 129 bytes and the envelope at most 407 bytes (was 343).Tests
cargo test --all-features(263 passed),cargo clippy --all-features --all-targets -- -D warnings,cargo fmt --check. The existing threshold tests readLITERAL_BLOCK_THRESHOLD, so they now cover 0 to 127 B on the literal side and 128 B on the encoder side. Tests that straddled the old threshold with 63 B and 64 B (store_and_encode_into_write_rmp_serde_bytes,store_allocates_only_its_output_and_block, the one-pass decode test'sstoresizes) now readLITERAL_BLOCK_THRESHOLD - 1andLITERAL_BLOCK_THRESHOLD, so they still cover both sides. Two tests used 100 B as an interior size of the encoder range; that size is now 200 B.Docs
The
LITERAL_BLOCK_THRESHOLDrustdoc gives the corpus reason for 128 B. The README'sstorenote, its small-values practice, and theperf_irsize list now say 128 B; its claim thatmake perf-irhas no case below 64 B or above 64 KiB was stale and is corrected.SECURITY.md(the per-thread scratch range and the no-encoder range) and the 32-bit bound test's header say 128 B.Sibling paths
estimate_compression: it calls the samecompress_block, so it reports the literal block below 128 B, matchingstore(estimate_compression_reports_what_store_writespasses).compress_into_with_tablecall and offer no narrower clear, so removing it needs a change to the dependency, which this PR does not make.retrieve: not changed.tests/unit/test_serializer_lazy_loading.pyassertsestimate_compression_ratio({"a": [1] * 100}) > 1. That value packs to 106 B, which now takes the literal path and reports a ratio just under 1, so the test needs an input of 128 B or more. A test helper's docstring there also says "Under 64 B". Both are fixed in cachekit-py when it bumps core.spec/wire-format.md's note on the canonical writer says "shorter than 64 bytes"; it is updated to 128 bytes when this lands.