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internal/zstd: use circular buffer for backreference window

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goos: linux
goarch: amd64
pkg: internal/zstd
        │ /tmp/BenchmarkLarge.old │       /tmp/BenchmarkLarge.new       │
        │         sec/op          │   sec/op     vs base                │
Large-8              10.172m ± 1%   7.243m ± 1%  -28.79% (p=0.000 n=10)

        │ /tmp/BenchmarkLarge.old │       /tmp/BenchmarkLarge.new        │
        │           B/s           │     B/s       vs base                │
Large-8              16.74Mi ± 1%   23.50Mi ± 1%  +40.40% (p=0.000 n=10)

        │ /tmp/BenchmarkLarge.old │       /tmp/BenchmarkLarge.new        │
        │          B/op           │     B/op      vs base                │
Large-8              51.82Ki ± 0%   32.33Ki ± 1%  -37.60% (p=0.000 n=10)

        │ /tmp/BenchmarkLarge.old │    /tmp/BenchmarkLarge.new     │
        │        allocs/op        │ allocs/op   vs base            │
Large-8                0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10)
This commit is contained in:
Alexander Yastrebov 2023-09-15 01:06:27 +02:00
parent 1eb0485950
commit 16cb1e13ff
4 changed files with 172 additions and 37 deletions
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6
src/internal/zstd/block.go
View file

@ -393,17 +393,17 @@ func (r *Reader) copyFromWindow(rbr *reverseBitReader, offset, match uint32) err
lenBlock := uint32(len(r.buffer))
if lenBlock < offset {
lenWindow := uint32(len(r.window))
lenWindow := r.window.len()
windowOffset := offset - lenBlock
if windowOffset > lenWindow {
return rbr.makeError("offset past window")
}
from := lenWindow - windowOffset
if from+match <= lenWindow {
r.buffer = append(r.buffer, r.window[from:from+match]...)
r.buffer = r.window.appendTo(r.buffer, from, from+match)
return nil
}
r.buffer = append(r.buffer, r.window[from:]...)
r.buffer = r.window.appendTo(r.buffer, from, lenWindow)
copied := lenWindow - from
offset -= copied
match -= copied

90
src/internal/zstd/window.go Normal file
View file

@ -0,0 +1,90 @@
// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package zstd
// window stores up to size bytes of data.
// It is implemented as a circular buffer:
// sequential save calls append to the data slice until
// its length reaches configured size and after that,
// save calls overwrite previously saved data at off
// and update off such that it always points at
// the byte stored before others.
type window struct {
size int
data []byte
off int
}
// reset clears stored data and configures window size.
func (w *window) reset(size int) {
w.data = w.data[:0]
w.off = 0
w.size = size
}
// len returns the number of stored bytes.
func (w *window) len() uint32 {
return uint32(len(w.data))
}
// save stores up to size last bytes from the buf.
func (w *window) save(buf []byte) {
if w.size == 0 {
return
}
if len(buf) == 0 {
return
}
if len(buf) >= w.size {
from := len(buf) - w.size
w.data = append(w.data[:0], buf[from:]...)
w.off = 0
return
}
// Update off to point to the oldest remaining byte.
free := w.size - len(w.data)
if free == 0 {
n := copy(w.data[w.off:], buf)
if n == len(buf) {
w.off += n
} else {
w.off = copy(w.data, buf[n:])
}
} else {
if free >= len(buf) {
w.data = append(w.data, buf...)
} else {
w.data = append(w.data, buf[:free]...)
w.off = copy(w.data, buf[free:])
}
}
}
// appendTo appends stored bytes between from and to indices to the buf.
// Index from must be less or equal to index to and to must be less or equal to w.len().
func (w *window) appendTo(buf []byte, from, to uint32) []byte {
dataLen := uint32(len(w.data))
from += uint32(w.off)
to += uint32(w.off)
wrap := false
if from > dataLen {
from -= dataLen
wrap = !wrap
}
if to > dataLen {
to -= dataLen
wrap = !wrap
}
if wrap {
buf = append(buf, w.data[from:]...)
return append(buf, w.data[:to]...)
} else {
return append(buf, w.data[from:to]...)
}
}

72
src/internal/zstd/window_test.go Normal file
View file

@ -0,0 +1,72 @@
// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package zstd
import (
"bytes"
"fmt"
"testing"
)
func makeSequence(start, n int) (seq []byte) {
for i := 0; i < n; i++ {
seq = append(seq, byte(start+i))
}
return
}
func TestWindow(t *testing.T) {
for size := 0; size <= 3; size++ {
for i := 0; i <= 2*size; i++ {
a := makeSequence('a', i)
for j := 0; j <= 2*size; j++ {
b := makeSequence('a'+i, j)
for k := 0; k <= 2*size; k++ {
c := makeSequence('a'+i+j, k)
t.Run(fmt.Sprintf("%d-%d-%d-%d", size, i, j, k), func(t *testing.T) {
testWindow(t, size, a, b, c)
})
}
}
}
}
}
// testWindow tests window by saving three sequences of bytes to it.
// Third sequence tests read offset that can become non-zero only after second save.
func testWindow(t *testing.T, size int, a, b, c []byte) {
var w window
w.reset(size)
w.save(a)
w.save(b)
w.save(c)
var tail []byte
tail = append(tail, a...)
tail = append(tail, b...)
tail = append(tail, c...)
if len(tail) > size {
tail = tail[len(tail)-size:]
}
if w.len() != uint32(len(tail)) {
t.Errorf("wrong data length: got: %d, want: %d", w.len(), len(tail))
}
var from, to uint32
for from = 0; from <= uint32(len(tail)); from++ {
for to = from; to <= uint32(len(tail)); to++ {
got := w.appendTo(nil, from, to)
want := tail[from:to]
if !bytes.Equal(got, want) {
t.Errorf("wrong data at [%d:%d]: got %q, want %q", from, to, got, want)
}
}
}
}

41
src/internal/zstd/zstd.go
View file

@ -59,8 +59,7 @@ type Reader struct {
huffmanTableBits int
// The window for back references.
windowSize int // maximum required window size
window []byte // window data
window window
// A buffer available to hold a compressed block.
compressedBuf []byte
@ -112,7 +111,6 @@ func (r *Reader) Reset(input io.Reader) {
// repeatedOffset3
// huffmanTable
// huffmanTableBits
// windowSize
// window
// compressedBuf
// literals
@ -236,10 +234,10 @@ retry:
// Figure out the maximum amount of data we need to retain
// for backreferences.
var windowSize int
if singleSegment {
// No window required, as all the data is in a single buffer.
r.windowSize = 0
windowSize = 0
} else {
// Window descriptor. RFC 3.1.1.1.2.
windowDescriptor := r.scratch[0]
@ -248,7 +246,7 @@ retry:
windowLog := exponent + 10
windowBase := uint64(1) << windowLog
windowAdd := (windowBase / 8) * mantissa
windowSize := windowBase + windowAdd
windowSize = int(windowBase + windowAdd)
// Default zstd sets limits on the window size.
if fuzzing && (windowLog > 31 || windowSize > 1<<27) {
@ -259,8 +257,6 @@ retry:
if windowSize > 8<<20 {
windowSize = 8 << 20
}
r.windowSize = int(windowSize)
}
// Frame_Content_Size. RFC 3.1.1.4.
@ -293,7 +289,7 @@ retry:
r.repeatedOffset2 = 4
r.repeatedOffset3 = 8
r.huffmanTableBits = 0
r.window = r.window[:0]
r.window.reset(windowSize)
r.seqTables[0] = nil
r.seqTables[1] = nil
r.seqTables[2] = nil
@ -368,7 +364,7 @@ func (r *Reader) readBlock() error {
// Maximum block size is smaller of window size and 128K.
// We don't record the window size for a single segment frame,
// so just use 128K. RFC 3.1.1.2.3, 3.1.1.2.4.
if blockSize > 128<<10 || (r.windowSize > 0 && blockSize > r.windowSize) {
if blockSize > 128<<10 || (r.window.size > 0 && blockSize > r.window.size) {
return r.makeError(relativeOffset, "block size too large")
}
@ -414,7 +410,7 @@ func (r *Reader) readBlock() error {
}
if !lastBlock {
r.saveWindow(r.buffer)
r.window.save(r.buffer)
} else {
if !r.frameSizeUnknown && r.remainingFrameSize != 0 {
return r.makeError(relativeOffset, "not enough uncompressed bytes for frame")
@ -449,29 +445,6 @@ func (r *Reader) setBufferSize(size int) {
r.buffer = r.buffer[:size]
}
// saveWindow saves bytes in the backreference window.
// TODO: use a circular buffer for less data movement.
func (r *Reader) saveWindow(buf []byte) {
if r.windowSize == 0 {
return
}
if len(buf) >= r.windowSize {
from := len(buf) - r.windowSize
r.window = append(r.window[:0], buf[from:]...)
return
}
keep := r.windowSize - len(buf) // must be positive
if keep < len(r.window) {
remove := len(r.window) - keep
copy(r.window[:], r.window[remove:])
r.window = r.window[:keep]
}
r.window = append(r.window, buf...)
}
// zstdError is an error while decompressing.
type zstdError struct {
offset int64