Commit 4bee88d4 authored by Robert Griesemer's avatar Robert Griesemer

math/big: more conservative use of lock for divisor table

Minor performance impact running sequentially:

benchmark                      old ns/op    new ns/op    delta
BenchmarkString10Base2               389          391   +0.51%
BenchmarkString100Base2             1530         1534   +0.26%
BenchmarkString1000Base2           11789        11787   -0.02%
BenchmarkString10000Base2         111443       112030   +0.53%
BenchmarkString100000Base2       1017483      1015347   -0.21%
BenchmarkString10Base8               339          344   +1.47%
BenchmarkString100Base8              753          756   +0.40%
BenchmarkString1000Base8            4618         4641   +0.50%
BenchmarkString10000Base8          43217        43534   +0.73%
BenchmarkString100000Base8        397518       400602   +0.78%
BenchmarkString10Base10              630          630   +0.00%
BenchmarkString100Base10            1975         1960   -0.76%
BenchmarkString1000Base10          10179        10174   -0.05%
BenchmarkString10000Base10         44527        44416   -0.25%
BenchmarkString100000Base10     14404694     14425308   +0.14%
BenchmarkString10Base16              283          288   +1.77%
BenchmarkString100Base16             597          598   +0.17%
BenchmarkString1000Base16           3189         3186   -0.09%
BenchmarkString10000Base16         29403        29364   -0.13%
BenchmarkString100000Base16       265657       265587   -0.03%

Note that due to other improvements (faster assembly routines,
better code generation by compiler), these benchmarks now run
up to 37% faster than they used to at the last time measured (1/9/2012).

Minor performance impact for StringPiParallel running in parallel:

Current CL but with Lock/Unlock commented out (removed):

BenchmarkStringPiParallel	    5000	    343581 ns/op
BenchmarkStringPiParallel-2	   10000	    184511 ns/op
BenchmarkStringPiParallel-3	   10000	    129768 ns/op
BenchmarkStringPiParallel-4	   10000	    102326 ns/op

Current CL:

BenchmarkStringPiParallel	    5000	    345169 ns/op
BenchmarkStringPiParallel-2	   10000	    185827 ns/op
BenchmarkStringPiParallel-3	   10000	    131168 ns/op
BenchmarkStringPiParallel-4	   10000	    102353 ns/op

Fixes #4218.

R=dvyukov, michael.jones, dave
CC=golang-dev
https://golang.org/cl/6643053
parent 3acce59b
......@@ -920,8 +920,10 @@ type divisor struct {
ndigits int // digit length of divisor in terms of output base digits
}
var cacheBase10 [64]divisor // cached divisors for base 10
var cacheLock sync.Mutex // protects cacheBase10
var cacheBase10 struct {
sync.Mutex
table [64]divisor // cached divisors for base 10
}
// expWW computes x**y
func (z nat) expWW(x, y Word) nat {
......@@ -937,34 +939,28 @@ func divisors(m int, b Word, ndigits int, bb Word) []divisor {
// determine k where (bb**leafSize)**(2**k) >= sqrt(x)
k := 1
for words := leafSize; words < m>>1 && k < len(cacheBase10); words <<= 1 {
for words := leafSize; words < m>>1 && k < len(cacheBase10.table); words <<= 1 {
k++
}
// create new table of divisors or extend and reuse existing table as appropriate
var table []divisor
var cached bool
switch b {
case 10:
table = cacheBase10[0:k] // reuse old table for this conversion
cached = true
default:
table = make([]divisor, k) // new table for this conversion
// reuse and extend existing table of divisors or create new table as appropriate
var table []divisor // for b == 10, table overlaps with cacheBase10.table
if b == 10 {
cacheBase10.Lock()
table = cacheBase10.table[0:k] // reuse old table for this conversion
} else {
table = make([]divisor, k) // create new table for this conversion
}
// extend table
if table[k-1].ndigits == 0 {
if cached {
cacheLock.Lock() // begin critical section
}
// add new entries as needed
var larger nat
for i := 0; i < k; i++ {
if table[i].ndigits == 0 {
if i == 0 {
table[i].bbb = nat(nil).expWW(bb, Word(leafSize))
table[i].ndigits = ndigits * leafSize
table[0].bbb = nat(nil).expWW(bb, Word(leafSize))
table[0].ndigits = ndigits * leafSize
} else {
table[i].bbb = nat(nil).mul(table[i-1].bbb, table[i-1].bbb)
table[i].ndigits = 2 * table[i-1].ndigits
......@@ -980,10 +976,10 @@ func divisors(m int, b Word, ndigits int, bb Word) []divisor {
table[i].nbits = table[i].bbb.bitLen()
}
}
if cached {
cacheLock.Unlock() // end critical section
}
if b == 10 {
cacheBase10.Unlock()
}
return table
......
......@@ -409,6 +409,20 @@ func TestScanPi(t *testing.T) {
}
}
func TestScanPiParallel(t *testing.T) {
const n = 2
c := make(chan int)
for i := 0; i < n; i++ {
go func() {
TestScanPi(t)
c <- 0
}()
}
for i := 0; i < n; i++ {
<-c
}
}
func BenchmarkScanPi(b *testing.B) {
for i := 0; i < b.N; i++ {
var x nat
......@@ -416,6 +430,28 @@ func BenchmarkScanPi(b *testing.B) {
}
}
func BenchmarkStringPiParallel(b *testing.B) {
var x nat
x, _, _ = x.scan(strings.NewReader(pi), 0)
if x.decimalString() != pi {
panic("benchmark incorrect: conversion failed")
}
n := runtime.GOMAXPROCS(0)
m := b.N / n // n*m <= b.N due to flooring, but the error is neglibible (n is not very large)
c := make(chan int, n)
for i := 0; i < n; i++ {
go func() {
for j := 0; j < m; j++ {
x.decimalString()
}
c <- 0
}()
}
for i := 0; i < n; i++ {
<-c
}
}
func BenchmarkScan10Base2(b *testing.B) { ScanHelper(b, 2, 10, 10) }
func BenchmarkScan100Base2(b *testing.B) { ScanHelper(b, 2, 10, 100) }
func BenchmarkScan1000Base2(b *testing.B) { ScanHelper(b, 2, 10, 1000) }
......@@ -516,7 +552,7 @@ func BenchmarkLeafSize64(b *testing.B) { LeafSizeHelper(b, 10, 64) }
func LeafSizeHelper(b *testing.B, base Word, size int) {
b.StopTimer()
originalLeafSize := leafSize
resetTable(cacheBase10[:])
resetTable(cacheBase10.table[:])
leafSize = size
b.StartTimer()
......@@ -533,7 +569,7 @@ func LeafSizeHelper(b *testing.B, base Word, size int) {
}
b.StopTimer()
resetTable(cacheBase10[:])
resetTable(cacheBase10.table[:])
leafSize = originalLeafSize
b.StartTimer()
}
......
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