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runtime: fix documented alignment of 32KiB and 64KiB size classes

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As Cherry pointed out on golang.org/cl/299909, the page allocator
doesn't guarantee any alignment for multi-page allocations, so object
alignments are thus implicitly capped at page alignment.

Change-Id: I6f5df27f269b095cde54056f876fe4240f69c5c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/301292
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
This commit is contained in:
Matthew Dempsky 2021-03-12 16:58:10 -08:00
parent 8e725f8452
commit a8a85281ca
2 changed files with 8 additions and 4 deletions
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6
src/runtime/mksizeclasses.go
View File

@ -239,7 +239,7 @@ func computeDivMagic(c *class) {
func printComment(w io.Writer, classes []class) {
fmt.Fprintf(w, "// %-5s %-9s %-10s %-7s %-10s %-9s %-9s\n", "class", "bytes/obj", "bytes/span", "objects", "tail waste", "max waste", "min align")
prevSize := 0
var minAligns [32]int
var minAligns [pageShift + 1]int
for i, c := range classes {
if i == 0 {
continue
@ -249,6 +249,10 @@ func printComment(w io.Writer, classes []class) {
tailWaste := spanSize - c.size*(spanSize/c.size)
maxWaste := float64((c.size-prevSize-1)*objects+tailWaste) / float64(spanSize)
alignBits := bits.TrailingZeros(uint(c.size))
if alignBits > pageShift {
// object alignment is capped at page alignment
alignBits = pageShift
}
for i := range minAligns {
if i > alignBits {
minAligns[i] = 0

6
src/runtime/sizeclasses.go
View File

@ -62,7 +62,7 @@ package runtime
// 56 12288 24576 2 0 11.45% 4096
// 57 13568 40960 3 256 9.99% 256
// 58 14336 57344 4 0 5.35% 2048
// 59 16384 16384 1 0 12.49% 16384
// 59 16384 16384 1 0 12.49% 8192
// 60 18432 73728 4 0 11.11% 2048
// 61 19072 57344 3 128 3.57% 128
// 62 20480 40960 2 0 6.87% 4096
@ -70,7 +70,7 @@ package runtime
// 64 24576 24576 1 0 11.45% 8192
// 65 27264 81920 3 128 10.00% 128
// 66 28672 57344 2 0 4.91% 4096
// 67 32768 32768 1 0 12.50% 32768
// 67 32768 32768 1 0 12.50% 8192
// alignment bits min obj size
// 8 3 8
@ -79,7 +79,7 @@ package runtime
// 64 6 512
// 128 7 768
// 4096 12 28672
// 32768 15 32768
// 8192 13 32768
const (
_MaxSmallSize = 32768