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runtime/internal/maps: only conditionally clear groups when sparse

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We only want to do the work of clearing slots
if they are full. But we also don't want to do too
much work to figure out whether a slot is full or not,
especially if clearing a slot is cheap.
 1) We decide group-by-group instead of slot-by-slot.
    If any slot in a group is full, we zero the whole group.
 2) If groups are unlikely to be empty, don't bother
    testing for it.
 3) If groups are 50%/50% likely to be empty, also don't
    bother testing, as it confuses the branch predictor. See #75097.
 4) But if a group is really large, do the test anyway, as
    clearing is expensive.

Fixes #75097

Change-Id: I9191865dd3e0fe887751cffe6082ac27d8d8439c
Reviewed-on: https://go-review.googlesource.com/c/go/+/697876
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Youlin Feng <fengyoulin@live.com>
This commit is contained in:
Keith Randall 2025-08-20 17:33:14 -07:00
parent 8098b99547
commit de5d7eccb9
1 changed files with 30 additions and 5 deletions
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35
src/internal/runtime/maps/table.go
View file

@ -604,12 +604,37 @@ func (t *table) Clear(typ *abi.MapType) {
if t.used == 0 && t.growthLeft == mgl { // no current entries and no tombstones
return
}
for i := uint64(0); i <= t.groups.lengthMask; i++ {
g := t.groups.group(typ, i)
if g.ctrls().anyFull() {
typedmemclr(typ.Group, g.data)
// We only want to do the work of clearing slots
// if they are full. But we also don't want to do too
// much work to figure out whether a slot is full or not,
// especially if clearing a slot is cheap.
// 1) We decide group-by-group instead of slot-by-slot.
// If any slot in a group is full, we zero the whole group.
// 2) If groups are unlikely to be empty, don't bother
// testing for it.
// 3) If groups are 50%/50% likely to be empty, also don't
// bother testing, as it confuses the branch predictor. See #75097.
// 4) But if a group is really large, do the test anyway, as
// clearing is expensive.
fullTest := uint64(t.used)*4 <= t.groups.lengthMask // less than ~0.25 entries per group -> >3/4 empty groups
if typ.SlotSize > 32 {
// For large slots, it is always worth doing the test first.
fullTest = true
}
if fullTest {
for i := uint64(0); i <= t.groups.lengthMask; i++ {
g := t.groups.group(typ, i)
if g.ctrls().anyFull() {
typedmemclr(typ.Group, g.data)
}
g.ctrls().setEmpty()
}
} else {
for i := uint64(0); i <= t.groups.lengthMask; i++ {
g := t.groups.group(typ, i)
typedmemclr(typ.Group, g.data)
g.ctrls().setEmpty()
}
g.ctrls().setEmpty()
}
t.used = 0
t.growthLeft = mgl