blob: b92d1fed5f834ae23a60b3138c840dca2b8831b9 [file] [log] [blame]
// Copyright 2016 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.
// +build ignore
// Generate tables for small malloc size classes.
//
// See malloc.go for overview.
//
// The size classes are chosen so that rounding an allocation
// request up to the next size class wastes at most 12.5% (1.125x).
//
// Each size class has its own page count that gets allocated
// and chopped up when new objects of the size class are needed.
// That page count is chosen so that chopping up the run of
// pages into objects of the given size wastes at most 12.5% (1.125x)
// of the memory. It is not necessary that the cutoff here be
// the same as above.
//
// The two sources of waste multiply, so the worst possible case
// for the above constraints would be that allocations of some
// size might have a 26.6% (1.266x) overhead.
// In practice, only one of the wastes comes into play for a
// given size (sizes < 512 waste mainly on the round-up,
// sizes > 512 waste mainly on the page chopping).
// For really small sizes, alignment constraints force the
// overhead higher.
package main
import (
"bytes"
"flag"
"fmt"
"go/format"
"io"
"log"
"os"
)
// Generate msize.go
var stdout = flag.Bool("stdout", false, "write to stdout instead of sizeclasses.go")
func main() {
flag.Parse()
var b bytes.Buffer
fmt.Fprintln(&b, "// Code generated by mksizeclasses.go; DO NOT EDIT.")
fmt.Fprintln(&b, "//go:generate go run mksizeclasses.go")
fmt.Fprintln(&b)
fmt.Fprintln(&b, "package runtime")
classes := makeClasses()
printComment(&b, classes)
printClasses(&b, classes)
out, err := format.Source(b.Bytes())
if err != nil {
log.Fatal(err)
}
if *stdout {
_, err = os.Stdout.Write(out)
} else {
err = os.WriteFile("sizeclasses.go", out, 0666)
}
if err != nil {
log.Fatal(err)
}
}
const (
// Constants that we use and will transfer to the runtime.
maxSmallSize = 32 << 10
smallSizeDiv = 8
smallSizeMax = 1024
largeSizeDiv = 128
pageShift = 13
// Derived constants.
pageSize = 1 << pageShift
)
type class struct {
size int // max size
npages int // number of pages
mul int
shift uint
shift2 uint
mask int
}
func powerOfTwo(x int) bool {
return x != 0 && x&(x-1) == 0
}
func makeClasses() []class {
var classes []class
classes = append(classes, class{}) // class #0 is a dummy entry
align := 8
for size := align; size <= maxSmallSize; size += align {
if powerOfTwo(size) { // bump alignment once in a while
if size >= 2048 {
align = 256
} else if size >= 128 {
align = size / 8
} else if size >= 32 {
align = 16 // heap bitmaps assume 16 byte alignment for allocations >= 32 bytes.
}
}
if !powerOfTwo(align) {
panic("incorrect alignment")
}
// Make the allocnpages big enough that
// the leftover is less than 1/8 of the total,
// so wasted space is at most 12.5%.
allocsize := pageSize
for allocsize%size > allocsize/8 {
allocsize += pageSize
}
npages := allocsize / pageSize
// If the previous sizeclass chose the same
// allocation size and fit the same number of
// objects into the page, we might as well
// use just this size instead of having two
// different sizes.
if len(classes) > 1 && npages == classes[len(classes)-1].npages && allocsize/size == allocsize/classes[len(classes)-1].size {
classes[len(classes)-1].size = size
continue
}
classes = append(classes, class{size: size, npages: npages})
}
// Increase object sizes if we can fit the same number of larger objects
// into the same number of pages. For example, we choose size 8448 above
// with 6 objects in 7 pages. But we can well use object size 9472,
// which is also 6 objects in 7 pages but +1024 bytes (+12.12%).
// We need to preserve at least largeSizeDiv alignment otherwise
// sizeToClass won't work.
for i := range classes {
if i == 0 {
continue
}
c := &classes[i]
psize := c.npages * pageSize
new_size := (psize / (psize / c.size)) &^ (largeSizeDiv - 1)
if new_size > c.size {
c.size = new_size
}
}
if len(classes) != 68 {
panic("number of size classes has changed")
}
for i := range classes {
computeDivMagic(&classes[i])
}
return classes
}
// computeDivMagic computes some magic constants to implement
// the division required to compute object number from span offset.
// n / c.size is implemented as n >> c.shift * c.mul >> c.shift2
// for all 0 <= n <= c.npages * pageSize
func computeDivMagic(c *class) {
// divisor
d := c.size
if d == 0 {
return
}
// maximum input value for which the formula needs to work.
max := c.npages * pageSize
if powerOfTwo(d) {
// If the size is a power of two, heapBitsForObject can divide even faster by masking.
// Compute this mask.
if max >= 1<<16 {
panic("max too big for power of two size")
}
c.mask = 1<<16 - d
}
// Compute pre-shift by factoring power of 2 out of d.
for d%2 == 0 {
c.shift++
d >>= 1
max >>= 1
}
// Find the smallest k that works.
// A small k allows us to fit the math required into 32 bits
// so we can use 32-bit multiplies and shifts on 32-bit platforms.
nextk:
for k := uint(0); ; k++ {
mul := (int(1)<<k + d - 1) / d // āŒˆ2^k / dāŒ‰
// Test to see if mul works.
for n := 0; n <= max; n++ {
if n*mul>>k != n/d {
continue nextk
}
}
if mul >= 1<<16 {
panic("mul too big")
}
if uint64(mul)*uint64(max) >= 1<<32 {
panic("mul*max too big")
}
c.mul = mul
c.shift2 = k
break
}
// double-check.
for n := 0; n <= max; n++ {
if n*c.mul>>c.shift2 != n/d {
fmt.Printf("d=%d max=%d mul=%d shift2=%d n=%d\n", d, max, c.mul, c.shift2, n)
panic("bad multiply magic")
}
// Also check the exact computations that will be done by the runtime,
// for both 32 and 64 bit operations.
if uint32(n)*uint32(c.mul)>>uint8(c.shift2) != uint32(n/d) {
fmt.Printf("d=%d max=%d mul=%d shift2=%d n=%d\n", d, max, c.mul, c.shift2, n)
panic("bad 32-bit multiply magic")
}
if uint64(n)*uint64(c.mul)>>uint8(c.shift2) != uint64(n/d) {
fmt.Printf("d=%d max=%d mul=%d shift2=%d n=%d\n", d, max, c.mul, c.shift2, n)
panic("bad 64-bit multiply magic")
}
}
}
func printComment(w io.Writer, classes []class) {
fmt.Fprintf(w, "// %-5s %-9s %-10s %-7s %-10s %-9s\n", "class", "bytes/obj", "bytes/span", "objects", "tail waste", "max waste")
prevSize := 0
for i, c := range classes {
if i == 0 {
continue
}
spanSize := c.npages * pageSize
objects := spanSize / c.size
tailWaste := spanSize - c.size*(spanSize/c.size)
maxWaste := float64((c.size-prevSize-1)*objects+tailWaste) / float64(spanSize)
prevSize = c.size
fmt.Fprintf(w, "// %5d %9d %10d %7d %10d %8.2f%%\n", i, c.size, spanSize, objects, tailWaste, 100*maxWaste)
}
fmt.Fprintf(w, "\n")
}
func printClasses(w io.Writer, classes []class) {
fmt.Fprintln(w, "const (")
fmt.Fprintf(w, "_MaxSmallSize = %d\n", maxSmallSize)
fmt.Fprintf(w, "smallSizeDiv = %d\n", smallSizeDiv)
fmt.Fprintf(w, "smallSizeMax = %d\n", smallSizeMax)
fmt.Fprintf(w, "largeSizeDiv = %d\n", largeSizeDiv)
fmt.Fprintf(w, "_NumSizeClasses = %d\n", len(classes))
fmt.Fprintf(w, "_PageShift = %d\n", pageShift)
fmt.Fprintln(w, ")")
fmt.Fprint(w, "var class_to_size = [_NumSizeClasses]uint16 {")
for _, c := range classes {
fmt.Fprintf(w, "%d,", c.size)
}
fmt.Fprintln(w, "}")
fmt.Fprint(w, "var class_to_allocnpages = [_NumSizeClasses]uint8 {")
for _, c := range classes {
fmt.Fprintf(w, "%d,", c.npages)
}
fmt.Fprintln(w, "}")
fmt.Fprintln(w, "type divMagic struct {")
fmt.Fprintln(w, " shift uint8")
fmt.Fprintln(w, " shift2 uint8")
fmt.Fprintln(w, " mul uint16")
fmt.Fprintln(w, " baseMask uint16")
fmt.Fprintln(w, "}")
fmt.Fprint(w, "var class_to_divmagic = [_NumSizeClasses]divMagic {")
for _, c := range classes {
fmt.Fprintf(w, "{%d,%d,%d,%d},", c.shift, c.shift2, c.mul, c.mask)
}
fmt.Fprintln(w, "}")
// map from size to size class, for small sizes.
sc := make([]int, smallSizeMax/smallSizeDiv+1)
for i := range sc {
size := i * smallSizeDiv
for j, c := range classes {
if c.size >= size {
sc[i] = j
break
}
}
}
fmt.Fprint(w, "var size_to_class8 = [smallSizeMax/smallSizeDiv+1]uint8 {")
for _, v := range sc {
fmt.Fprintf(w, "%d,", v)
}
fmt.Fprintln(w, "}")
// map from size to size class, for large sizes.
sc = make([]int, (maxSmallSize-smallSizeMax)/largeSizeDiv+1)
for i := range sc {
size := smallSizeMax + i*largeSizeDiv
for j, c := range classes {
if c.size >= size {
sc[i] = j
break
}
}
}
fmt.Fprint(w, "var size_to_class128 = [(_MaxSmallSize-smallSizeMax)/largeSizeDiv+1]uint8 {")
for _, v := range sc {
fmt.Fprintf(w, "%d,", v)
}
fmt.Fprintln(w, "}")
}