runtime: unaligned jumps causing performance regression on Intel #37121
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For convenience, here's the benchmark (without testdata and other files in .zip): package flate
import (
"io/ioutil"
"testing"
)
func Benchmark_tokens_EstimatedBits(b *testing.B) {
tok := loadTestTokens(b)
b.ResetTimer()
// One "byte", one token iteration.
b.SetBytes(1)
for i := 0; i < b.N; i++ {
_ = tok.EstimatedBits()
}
}
// ...token.go (with tokens.EstimatedBits code)// Copyright 2009 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 flate
import (
"bytes"
"encoding/binary"
"fmt"
"io"
"math"
)
// EstimatedBits will return an minimum size estimated by an *optimal*
// compression of the block.
// The size of the block
func (t *tokens) EstimatedBits() int {
shannon := float32(0)
bits := int(0)
nMatches := 0
if t.nLits > 0 {
invTotal := 1.0 / float32(t.nLits)
for _, v := range t.litHist[:] {
if v > 0 {
n := float32(v)
shannon += -mFastLog2(n*invTotal) * n
}
}
// Just add 15 for EOB
shannon += 15
for i, v := range t.extraHist[1 : literalCount-256] {
if v > 0 {
n := float32(v)
shannon += -mFastLog2(n*invTotal) * n
bits += int(lengthExtraBits[i&31]) * int(v)
nMatches += int(v)
}
}
}
if nMatches > 0 {
invTotal := 1.0 / float32(nMatches)
for i, v := range t.offHist[:offsetCodeCount] {
if v > 0 {
n := float32(v)
shannon += -mFastLog2(n*invTotal) * n
bits += int(offsetExtraBits[i&31]) * int(v)
}
}
}
return int(shannon) + bits
}
// from https://stackoverflow.com/a/28730362
func mFastLog2(val float32) float32 {
ux := int32(math.Float32bits(val))
log2 := (float32)(((ux >> 23) & 255) - 128)
ux &= -0x7f800001
ux += 127 << 23
uval := math.Float32frombits(uint32(ux))
log2 += ((-0.34484843)*uval+2.02466578)*uval - 0.67487759
return log2
}
const (
lengthShift = 22
offsetMask = 1<<lengthShift - 1
typeMask = 3 << 30
matchType = 1 << 30
literalCount = 286
maxMatchLength = 258 // The longest match for the compressor
maxStoreBlockSize = 65535
debugDecode = false
baseMatchOffset = 1 // The smallest match offset
baseMatchLength = 3 // The smallest match length per the RFC section 3.2.5
maxMatchOffset = 1 << 15 // The largest match offset
offsetCodeCount = 30
endBlockMarker = 256
)
// The length code for length X (MIN_MATCH_LENGTH <= X <= MAX_MATCH_LENGTH)
// is lengthCodes[length - MIN_MATCH_LENGTH]
var lengthCodes = [256]uint8{
0, 1, 2, 3, 4, 5, 6, 7, 8, 8,
9, 9, 10, 10, 11, 11, 12, 12, 12, 12,
13, 13, 13, 13, 14, 14, 14, 14, 15, 15,
15, 15, 16, 16, 16, 16, 16, 16, 16, 16,
17, 17, 17, 17, 17, 17, 17, 17, 18, 18,
18, 18, 18, 18, 18, 18, 19, 19, 19, 19,
19, 19, 19, 19, 20, 20, 20, 20, 20, 20,
20, 20, 20, 20, 20, 20, 20, 20, 20, 20,
21, 21, 21, 21, 21, 21, 21, 21, 21, 21,
21, 21, 21, 21, 21, 21, 22, 22, 22, 22,
22, 22, 22, 22, 22, 22, 22, 22, 22, 22,
22, 22, 23, 23, 23, 23, 23, 23, 23, 23,
23, 23, 23, 23, 23, 23, 23, 23, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
25, 25, 26, 26, 26, 26, 26, 26, 26, 26,
26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
26, 26, 26, 26, 27, 27, 27, 27, 27, 27,
27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
27, 27, 27, 27, 27, 28,
}
// lengthCodes1 is length codes, but starting at 1.
var lengthCodes1 = [256]uint8{
1, 2, 3, 4, 5, 6, 7, 8, 9, 9,
10, 10, 11, 11, 12, 12, 13, 13, 13, 13,
14, 14, 14, 14, 15, 15, 15, 15, 16, 16,
16, 16, 17, 17, 17, 17, 17, 17, 17, 17,
18, 18, 18, 18, 18, 18, 18, 18, 19, 19,
19, 19, 19, 19, 19, 19, 20, 20, 20, 20,
20, 20, 20, 20, 21, 21, 21, 21, 21, 21,
21, 21, 21, 21, 21, 21, 21, 21, 21, 21,
22, 22, 22, 22, 22, 22, 22, 22, 22, 22,
22, 22, 22, 22, 22, 22, 23, 23, 23, 23,
23, 23, 23, 23, 23, 23, 23, 23, 23, 23,
23, 23, 24, 24, 24, 24, 24, 24, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 25, 25,
25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
26, 26, 27, 27, 27, 27, 27, 27, 27, 27,
27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
27, 27, 27, 27, 28, 28, 28, 28, 28, 28,
28, 28, 28, 28, 28, 28, 28, 28, 28, 28,
28, 28, 28, 28, 28, 28, 28, 28, 28, 28,
28, 28, 28, 28, 28, 29,
}
var offsetCodes = [256]uint32{
0, 1, 2, 3, 4, 4, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7,
8, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 9, 9,
10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10,
11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11,
12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12,
12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12,
13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
}
// offsetCodes14 are offsetCodes, but with 14 added.
var offsetCodes14 = [256]uint32{
14, 15, 16, 17, 18, 18, 19, 19, 20, 20, 20, 20, 21, 21, 21, 21,
22, 22, 22, 22, 22, 22, 22, 22, 23, 23, 23, 23, 23, 23, 23, 23,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28,
28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28,
28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28,
28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28, 28,
29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29,
29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29,
29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29,
29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29, 29,
}
// The number of extra bits needed by length code X - LENGTH_CODES_START.
var lengthExtraBits = [32]int8{
/* 257 */ 0, 0, 0,
/* 260 */ 0, 0, 0, 0, 0, 1, 1, 1, 1, 2,
/* 270 */ 2, 2, 2, 3, 3, 3, 3, 4, 4, 4,
/* 280 */ 4, 5, 5, 5, 5, 0,
}
// offset code word extra bits.
var offsetExtraBits = [64]int8{
0, 0, 0, 0, 1, 1, 2, 2, 3, 3,
4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
9, 9, 10, 10, 11, 11, 12, 12, 13, 13,
/* extended window */
14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20,
}
type token uint32
type tokens struct {
nLits int
extraHist [32]uint16 // codes 256->maxnumlit
offHist [32]uint16 // offset codes
litHist [256]uint16 // codes 0->255
n uint16 // Must be able to contain maxStoreBlockSize
tokens [maxStoreBlockSize + 1]token
}
func (t *tokens) Reset() {
if t.n == 0 {
return
}
t.n = 0
t.nLits = 0
for i := range t.litHist[:] {
t.litHist[i] = 0
}
for i := range t.extraHist[:] {
t.extraHist[i] = 0
}
for i := range t.offHist[:] {
t.offHist[i] = 0
}
}
func (t *tokens) indexTokens(in []token) {
t.Reset()
for _, tok := range in {
if tok < matchType {
t.AddLiteral(tok.literal())
continue
}
t.AddMatch(uint32(tok.length()), tok.offset())
}
}
func (t *tokens) AddLiteral(lit byte) {
t.tokens[t.n] = token(lit)
t.litHist[lit]++
t.n++
t.nLits++
}
// AddMatch adds a match to the tokens.
// This function is very sensitive to inlining and right on the border.
func (t *tokens) AddMatch(xlength uint32, xoffset uint32) {
if debugDecode {
if xlength >= maxMatchLength+baseMatchLength {
panic(fmt.Errorf("invalid length: %v", xlength))
}
if xoffset >= maxMatchOffset+baseMatchOffset {
panic(fmt.Errorf("invalid offset: %v", xoffset))
}
}
t.nLits++
lengthCode := lengthCodes1[uint8(xlength)] & 31
t.tokens[t.n] = token(matchType | xlength<<lengthShift | xoffset)
t.extraHist[lengthCode]++
t.offHist[offsetCode(xoffset)&31]++
t.n++
}
// AddMatchLong adds a match to the tokens, potentially longer than max match length.
// Length should NOT have the base subtracted, only offset should.
func (t *tokens) AddMatchLong(xlength int32, xoffset uint32) {
if debugDecode {
if xoffset >= maxMatchOffset+baseMatchOffset {
panic(fmt.Errorf("invalid offset: %v", xoffset))
}
}
oc := offsetCode(xoffset) & 31
for xlength > 0 {
xl := xlength
if xl > 258 {
// We need to have at least baseMatchLength left over for next loop.
xl = 258 - baseMatchLength
}
xlength -= xl
xl -= 3
t.nLits++
lengthCode := lengthCodes1[uint8(xl)] & 31
t.tokens[t.n] = token(matchType | uint32(xl)<<lengthShift | xoffset)
t.extraHist[lengthCode]++
t.offHist[oc]++
t.n++
}
}
func (t *tokens) AddEOB() {
t.tokens[t.n] = token(endBlockMarker)
t.extraHist[0]++
t.n++
}
func (t *tokens) Slice() []token {
return t.tokens[:t.n]
}
// VarInt returns the tokens as varint encoded bytes.
func (t *tokens) VarInt() []byte {
var b = make([]byte, binary.MaxVarintLen32*int(t.n))
var off int
for _, v := range t.tokens[:t.n] {
off += binary.PutUvarint(b[off:], uint64(v))
}
return b[:off]
}
// FromVarInt restores t to the varint encoded tokens provided.
// Any data in t is removed.
func (t *tokens) FromVarInt(b []byte) error {
var buf = bytes.NewReader(b)
var toks []token
for {
r, err := binary.ReadUvarint(buf)
if err == io.EOF {
break
}
if err != nil {
return err
}
toks = append(toks, token(r))
}
t.indexTokens(toks)
return nil
}
// Returns the type of a token
func (t token) typ() uint32 { return uint32(t) & typeMask }
// Returns the literal of a literal token
func (t token) literal() uint8 { return uint8(t) }
// Returns the extra offset of a match token
func (t token) offset() uint32 { return uint32(t) & offsetMask }
func (t token) length() uint8 { return uint8(t >> lengthShift) }
// Returns the offset code corresponding to a specific offset
func offsetCode(off uint32) uint32 {
if false {
if off < uint32(len(offsetCodes)) {
return offsetCodes[off&255]
} else if off>>7 < uint32(len(offsetCodes)) {
return offsetCodes[(off>>7)&255] + 14
} else {
return offsetCodes[(off>>14)&255] + 28
}
}
if off < uint32(len(offsetCodes)) {
return offsetCodes[uint8(off)]
}
return offsetCodes14[uint8(off>>7)]
} |
dr2chase
added a commit
to dr2chase/benchmarks
that referenced
this issue
Feb 7, 2020
|
I get a startling regression on a Mac laptop -- 70% slowdown -- will look further to see if I am making some obvious mistake. |
|
@dr2chase I can reproduce your result on my Mac laptop too: Tentatively marking this as a release blocker, until we understand what's causing this and whether this is representative of many Go programs. /cc @cagedmantis @toothrot |
cagedmantis
changed the title
|
Charming. Turning off asynchronous preemption eliminates a whole lot of the regression, but not all of it. (Why did I try this? Because attempting to collect profiles made both of them equally, terribly slow.) |
|
I have to quit for the night, what I find in 1.14 vs 1.13 (ignoring likely interactions with preemption) is an extra instruction in the GOSSAFUNC output, comparing final output for I doubt we're going to fix that extra instruction, but the other slowdown seems worth further look. The inner loops are and 1.14 |
|
It might be worthwhile quickly eliminating #35881 as the source of the regression. Is your CPU listed in the description of #35881? Are you running a recent version of the microcode? If so, you could try
|
|
Adding |
|
I did a as-careful-as-possible laptop benchmark, and with asynchronous preemption OFF, the branch alignment fix makes 1.13 and 1.14 effectively equal (on my laptop). We still don't entirely understand what's wrong with preemption in this benchmark, will keep looking, will also see how the latest version of the branch alignment batch compares. |
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|
The alignment issue depends on your CPU's microcode version, which Apple has updated in the last few months. No idea how that works for Linux users. For example: |
|
On Linux the microcode version can be found in /proc/cpuinfo. |
|
Re the large slowdown on Darwin: there's something strange going on with signals here. i.e., just turning on the profiler leads to an almost 4x slowdown. 1.14 is different because it uses signals all the time. So we always see the slow behavior unless both the profiler and the async preempt are off: Not sure what's going on here yet, but something about Darwin and signals is bad. So this really isn't new with 1.14. It just appears more often due to more use of signals. |
|
I've split out the Darwin-specific weirdness into #37174 . |
|
Change https://golang.org/cl/219229 mentions this issue: |
|
The remaining performance issue is related to unaligned jumps. We will not be making changes to our jumps before the 1.14 release, but likely will address it in a point release. I'll remove the |
toothrot
changed the title
|
@cherrymui @dr2chase Is this still an issue for 1.15? If so, what should we set the milestone to? Thanks. |
|
|
Oops, no, because of a finger-fumble that is not correct. Shut down Chrome and the IDE, try again. |
|
Okay, now it's closed. There's a 0.3% slowdown (2 parts in 673), which is close enough. |
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What version of Go are you using (
go version)?And Go 1.14-RC1.
Does this issue reproduce with the latest release?
Yes.
What operating system and processor architecture are you using (
go env)?go envOutputWhat did you do?
Isolated code: reproducer.zip
go test -bench=. -test.benchtime=10sMost of the code is needed for the test setup, only
(*tokens).EstimatedBitsandmFastLog2is run during the benchmark.What did you expect to see?
Equivalent performance.
What did you see instead?
8% performance regression.
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