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internal/runtime/maps: reuse deleted slots on insert

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While walking the probe sequence, Put keeps track of the first deleted
slot it encountered. If it reaches the end of the probe sequence without
finding a match, then it will prefer to use the deleted slot rather than
a new empty slot.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: I19356ef6780176506f57b42990ac15dc426f1b14
Reviewed-on: https://go-review.googlesource.com/c/go/+/618016
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Michael Pratt 2024-10-04 15:20:48 -04:00 committed by Gopher Robot
parent 7de87ebd59
commit efa43c57b1
4 changed files with 137 additions and 22 deletions
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36
src/internal/runtime/maps/export_test.go
View File

@ -36,12 +36,48 @@ func (m *Map) GroupCount() uint64 {
return n
}
// Return a key from a group containing no empty slots, or nil if there are no
// full groups.
//
// Also returns nil if a group is full but contains entirely deleted slots.
func (m *Map) KeyFromFullGroup() unsafe.Pointer {
var lastTab *table
for _, t := range m.directory {
if t == lastTab {
continue
}
lastTab = t
for i := uint64(0); i <= t.groups.lengthMask; i++ {
g := t.groups.group(i)
match := g.ctrls().matchEmpty()
if match != 0 {
continue
}
// All full or deleted slots.
for j := uint32(0); j < abi.SwissMapGroupSlots; j++ {
if g.ctrls().get(j) == ctrlDeleted {
continue
}
return g.key(j)
}
}
}
return nil
}
func (m *Map) TableFor(key unsafe.Pointer) *table {
hash := m.typ.Hasher(key, m.seed)
idx := m.directoryIndex(hash)
return m.directory[idx]
}
func (t *table) GrowthLeft() uint64 {
return uint64(t.growthLeft)
}
// Returns the start address of the groups array.
func (t *table) GroupsStart() unsafe.Pointer {
return t.groups.data

7
src/internal/runtime/maps/map.go
View File

@ -77,9 +77,10 @@ import (
// slot as empty, as there could be more slots used later in a probe sequence
// and this deletion would cause probing to stop too early. Instead, we mark
// such slots as "deleted" with a tombstone. If the group still has an empty
// slot, we don't need a tombstone and directly mark the slot empty. Currently,
// tombstone are only cleared during grow, as an in-place cleanup complicates
// iteration.
// slot, we don't need a tombstone and directly mark the slot empty. Insert
// prioritizes reuse of tombstones over filling an empty slots. Otherwise,
// tombstones are only completely cleared during grow, as an in-place cleanup
// complicates iteration.
//
// Growth
//

62
src/internal/runtime/maps/map_test.go
View File

@ -213,6 +213,68 @@ func TestTableKeyUpdate(t *testing.T) {
}
}
// Put should reuse a deleted slot rather than consuming an empty slot.
func TestTablePutDelete(t *testing.T) {
// Put will reuse the first deleted slot it encounters.
//
// This is awkward to test because Delete will only install ctrlDeleted
// if the group is full, otherwise it goes straight to empty.
//
// So first we must add to the table continuously until we happen to
// fill a group.
m, _ := maps.NewTestMap[uint32, uint32](8)
key := uint32(0)
elem := uint32(256 + 0)
for {
key += 1
elem += 1
m.Put(unsafe.Pointer(&key), unsafe.Pointer(&elem))
// Normally a Put that fills a group would fill it with the
// inserted key, so why search the whole map for a potentially
// different key in a full group?
//
// Put may grow/split a table. Initial construction of the new
// table(s) could result in a full group consisting of
// arbitrary keys.
fullKeyPtr := m.KeyFromFullGroup()
if fullKeyPtr != nil {
// Found a full group.
key = *(*uint32)(fullKeyPtr)
elem = 256 + key
break
}
}
// Key is in a full group. Deleting it will result in a ctrlDeleted
// slot.
m.Delete(unsafe.Pointer(&key))
// Re-insert key. This should reuse the deleted slot rather than
// consuming space.
tabWant := m.TableFor(unsafe.Pointer(&key))
growthLeftWant := tabWant.GrowthLeft()
m.Put(unsafe.Pointer(&key), unsafe.Pointer(&elem))
tabGot := m.TableFor(unsafe.Pointer(&key))
growthLeftGot := tabGot.GrowthLeft()
if tabGot != tabWant {
// There shouldn't be a grow, as replacing a deleted slot
// doesn't require more space.
t.Errorf("Put(%d) grew table got %v want %v map %v", key, tabGot, tabWant, m)
}
if growthLeftGot != growthLeftWant {
t.Errorf("GrowthLeft got %d want %d: map %v tab %v", growthLeftGot, growthLeftWant, m, tabGot)
}
}
func TestTableIteration(t *testing.T) {
m, _ := maps.NewTestMap[uint32, uint64](8)

54
src/internal/runtime/maps/table.go
View File

@ -161,8 +161,6 @@ func (t *table) Get(key unsafe.Pointer) (unsafe.Pointer, bool) {
// TODO(prattmic): We could avoid hashing in a variety of special
// cases.
//
// - One group maps with simple keys could iterate over all keys and
// compare them directly.
// - One entry maps could just directly compare the single entry
// without hashing.
// - String keys could do quick checks of a few bytes before hashing.
@ -243,6 +241,11 @@ func (t *table) getWithKey(hash uintptr, key unsafe.Pointer) (unsafe.Pointer, un
func (t *table) PutSlot(m *Map, hash uintptr, key unsafe.Pointer) (unsafe.Pointer, bool) {
seq := makeProbeSeq(h1(hash), t.groups.lengthMask)
// As we look for a match, keep track of the first deleted slot we
// find, which we'll use to insert the new entry if necessary.
var firstDeletedGroup groupReference
var firstDeletedSlot uint32
for ; ; seq = seq.next() {
g := t.groups.group(seq.offset)
match := g.ctrls().matchH2(h2(hash))
@ -265,15 +268,28 @@ func (t *table) PutSlot(m *Map, hash uintptr, key unsafe.Pointer) (unsafe.Pointe
match = match.removeFirst()
}
// No existing slot for this key in this group. Is this the end
// of the probe sequence?
match = g.ctrls().matchEmpty()
if match != 0 {
// Finding an empty slot means we've reached the end of
// the probe sequence.
var i uint32
// If we found a deleted slot along the way, we can
// replace it without consuming growthLeft.
if firstDeletedGroup.data != nil {
g = firstDeletedGroup
i = firstDeletedSlot
t.growthLeft++ // will be decremented below to become a no-op.
} else {
// Otherwise, use the empty slot.
i = match.first()
}
// If there is room left to grow, just insert the new entry.
if t.growthLeft > 0 {
i := match.first()
slotKey := g.key(i)
typedmemmove(t.typ.Key, slotKey, key)
slotElem := g.elem(i)
@ -287,24 +303,24 @@ func (t *table) PutSlot(m *Map, hash uintptr, key unsafe.Pointer) (unsafe.Pointe
return slotElem, true
}
// TODO(prattmic): While searching the probe sequence,
// we may have passed deleted slots which we could use
// for this entry.
//
// At the moment, we leave this behind for
// rehash to free up.
//
// cockroachlabs/swiss restarts search of the probe
// sequence for a deleted slot.
//
// TODO(go.dev/issue/54766): We want this optimization
// back. We could search for the first deleted slot
// during the main search, but only use it if we don't
// find an existing entry.
t.rehash(m)
return nil, false
}
// No empty slots in this group. Check for a deleted
// slot, which we'll use if we don't find a match later
// in the probe sequence.
//
// We only need to remember a single deleted slot.
if firstDeletedGroup.data == nil {
// Since we already checked for empty slots
// above, matches here must be deleted slots.
match = g.ctrls().matchEmptyOrDeleted()
if match != 0 {
firstDeletedGroup = g
firstDeletedSlot = match.first()
}
}
}
}