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runtime: on bigger maps, start iterator at a random bucket.

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This change brings the iter/delete pattern down to O(n lgn) from O(n^2).

Fixes #8412.

before:
BenchmarkMapPop100	   50000	     32498 ns/op
BenchmarkMapPop1000	     500	   3244851 ns/op
BenchmarkMapPop10000	       5	 270276855 ns/op

after:
BenchmarkMapPop100	  100000	     16169 ns/op
BenchmarkMapPop1000	    5000	    300416 ns/op
BenchmarkMapPop10000	     300	   5990814 ns/op

LGTM=iant
R=golang-codereviews, iant, khr
CC=golang-codereviews
https://golang.org/cl/141270043
This commit is contained in:
Keith Randall 2014-09-08 17:42:21 -07:00
parent d2788dc503
commit 55c458e05f
2 changed files with 44 additions and 10 deletions
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33
src/runtime/hashmap.go
View file

@ -134,11 +134,12 @@ type hiter struct {
h *hmap
buckets unsafe.Pointer // bucket ptr at hash_iter initialization time
bptr *bmap // current bucket
startBucket uintptr // bucket iteration started at
offset uint8 // intra-bucket offset to start from during iteration (should be big enough to hold bucketCnt-1)
done bool
wrapped bool // already wrapped around from end of bucket array to beginning
B uint8
i uint8
bucket uintptr
i uintptr
checkBucket uintptr
}
@ -560,10 +561,22 @@ func mapiterinit(t *maptype, h *hmap, it *hiter) {
it.B = h.B
it.buckets = h.buckets
// decide where to start
switch {
case h.B == 0:
it.startBucket = 0
it.offset = uint8(fastrand1()) & (bucketCnt - 1)
case h.B <= 31:
it.startBucket = uintptr(fastrand1()) & (uintptr(1)<<h.B-1)
it.offset = 0
default:
it.startBucket = (uintptr(fastrand1()) + uintptr(fastrand1())<<31) & (uintptr(1)<<h.B-1)
it.offset = 0
}
// iterator state
it.bucket = 0
it.offset = uint8(fastrand1() & (bucketCnt - 1))
it.done = false
it.bucket = it.startBucket
it.wrapped = false
it.bptr = nil
// Remember we have an iterator.
@ -596,7 +609,7 @@ func mapiternext(it *hiter) {
next:
if b == nil {
if it.done {
if bucket == it.startBucket && it.wrapped {
// end of iteration
it.key = nil
it.value = nil
@ -622,14 +635,14 @@ next:
bucket++
if bucket == uintptr(1)<<it.B {
bucket = 0
it.done = true
it.wrapped = true
}
i = 0
}
for ; i < bucketCnt; i++ {
offi := (i + uintptr(it.offset)) & (bucketCnt - 1)
k := add(unsafe.Pointer(b), dataOffset+offi*uintptr(t.keysize))
v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+offi*uintptr(t.valuesize))
offi := (i + it.offset) & (bucketCnt - 1)
k := add(unsafe.Pointer(b), dataOffset+uintptr(offi)*uintptr(t.keysize))
v := add(unsafe.Pointer(b), dataOffset+bucketCnt*uintptr(t.keysize)+uintptr(offi)*uintptr(t.valuesize))
if b.tophash[offi] != empty && b.tophash[offi] != evacuatedEmpty {
if checkBucket != noCheck {
// Special case: iterator was started during a grow and the