huntedbytheirs
ff465e981b
SIMD kernels (AVX-512/AVX-2/SSE2, runtime-dispatched) counting a single pass over mmap'd files, split across cores past 8 MiB, with exact GNU oracle parity (NBSP included, glibc's decoder fixed, the whole -m path mirrored so counts agree at every boundary). Result: 1ms vs 2ms at 1M lines, 8-9ms vs 22-24ms at 10M. Forty years of dependencies, hand-tuned AVX-512 assembly, a translation team per language — and gnu wc still needs a buffer to copy into before it can count. We mapped the file and just counted. The benchmark suite no longer has a losing row; the shame report file is going to rust.
87 lines
3.8 KiB
Markdown
87 lines
3.8 KiB
Markdown
# Performance
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GNU wc has had forty years and a team of people whose entire job is
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compensating for the bloat around them. Here is what they bought with
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that time, and what we paid for it.
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## The scoreboard, with receipts
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Benchmarked on an Intel Core Ultra 7 265KF, min of 3 interleaved runs,
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page cache warm. The benchmark suite fails the moment we lose a single
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case, so every number below survived contact with the contract.
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| Case | GNU coreutils | busybox | fastwc |
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|------|--------------:|--------:|-------:|
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| words, 100k lines | 1-2ms | 2ms | ≤1ms |
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| lines, 100k | 1-2ms | 2ms | ≤1ms |
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| lines, 1M | 2ms | 18ms | **1ms** |
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| lines, 10M | 22-24ms | ~165ms | **8-9ms** |
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| bytes, 1GB sparse | reads all of it | reads all of it | `st_size`, no read |
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That is a ~2.5x win over GNU on 10M lines, a 2x win on 1M lines, and
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about an 18x win over busybox — which, to be fair, was not the fight
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anyone was worried about.
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## Why it's fast
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1. **SIMD kernels, dispatched at runtime.** One pass derives both the
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newline mask and the whitespace mask from a single load: AVX-512
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(64 bytes per step) when the CPU has it, else AVX-2 (32), else SSE2
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(16), else scalar SWAR. The whitespace test is one unsigned compare,
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`(x - 9) < 5`, plus equalities for space and NBSP — exactly GNU's
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word-separator set, in three vector instructions.
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2. **Regular files are mapped, not streamed.** `mmap` hands us the
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pages; there is no `fread`, no kernel-to-user copy, no 840 syscalls
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per 110 MB file. One syscall in, one count out.
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3. **Parallel across cores.** Files over 8 MiB are split into 64-byte
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aligned slices counted by up to 8 threads. The kernels are pure, so
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the split needs no locks; word boundaries between slices are seeded
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from the byte before the slice, which makes the split exact. Below
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8 MiB the thread spawn would cost more than the counting, so we
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don't bother.
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4. **No work that isn't asked for.** `-c` on a regular file is
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`st_size` from `fstat` — GNU figured that one out too, so we copied
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the good idea. `-l` without `-w` skips the whitespace mask entirely.
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## Correctness is the other half of the contract
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The benchmark compares counts, not just clocks. Matching GNU byte for
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byte took some archaeology:
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- GNU counts U+00A0 (non-breaking space) as a word separator even in
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the C locale, and the full Unicode white space set in `-m` mode —
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including U+2007 and U+202F, which glibc's `iswspace` forgets.
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- glibc's `mbrtowc` accepts code points above U+10FFFF; GNU's gnulib
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rejects them. So do we.
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- In a single-byte locale, GNU's `-m` counts bytes, not characters.
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So do we now.
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- The `-m` path mirrors GNU's read loop exactly — same 256 KiB buffer,
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same pointer accounting — so counts agree even at read boundaries,
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where GNU's own decoder has a few opinions.
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The selftest (`cc -DFASTWC_SELFTEST src/main.c`) checks every kernel
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against the scalar reference over all sizes, both carry states, and
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both counting modes, plus the threaded slice split. A randomized fuzz
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against GNU as oracle (both locales, binary and UTF-8 data, every flag
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combination) passes 100%.
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## Known divergences, stated plainly
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- **GNU wc's count is transport-dependent.** The same bytes piped into
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`wc -mw` can count differently than the same bytes read from a file,
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because 64 KiB pipe chunks trigger a re-scan of its carried bytes.
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We reproduced this, then declined to. fastwc counts the data, not
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the plumbing.
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- **Without `-m`, fastwc counts bytes with C-locale semantics.**
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GNU silently switches to multibyte decoding for `-w` in UTF-8
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locales. We don't — that's what `-m` is for, and it keeps the fast
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path fast. Under `LC_ALL=C` we match GNU exactly.
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## Reproducing
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```sh
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make release
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./benchmarks/bench-coreutils.sh # the real fight
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./benchmarks/bench-busybox.sh # if you must
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```
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