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authorkevin.xu <cming.xu@gmail.com>2020-04-27 21:51:31 +0800
committerGitHub <noreply@github.com>2020-04-27 21:51:31 +0800
commite896ca54db67524afc20b644d43c72185e72dc0e (patch)
tree2a16f3a62a5cafd098f1f028c621f1b655589d69 /pkg/syncevent/broadcaster.go
parent1f19624fa127d7d59cabe29593cc80b7fe6c81f8 (diff)
parent3c67754663f424f2ebbc0ff2a4c80e30618d5355 (diff)
Merge pull request #1 from google/master
catch up
Diffstat (limited to 'pkg/syncevent/broadcaster.go')
-rw-r--r--pkg/syncevent/broadcaster.go218
1 files changed, 218 insertions, 0 deletions
diff --git a/pkg/syncevent/broadcaster.go b/pkg/syncevent/broadcaster.go
new file mode 100644
index 000000000..4bff59e7d
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+++ b/pkg/syncevent/broadcaster.go
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+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package syncevent
+
+import (
+ "gvisor.dev/gvisor/pkg/sync"
+)
+
+// Broadcaster is an implementation of Source that supports any number of
+// subscribed Receivers.
+//
+// The zero value of Broadcaster is valid and has no subscribed Receivers.
+// Broadcaster is not copyable by value.
+//
+// All Broadcaster methods may be called concurrently from multiple goroutines.
+type Broadcaster struct {
+ // Broadcaster is implemented as a hash table where keys are assigned by
+ // the Broadcaster and returned as SubscriptionIDs, making it safe to use
+ // the identity function for hashing. The hash table resolves collisions
+ // using linear probing and features Robin Hood insertion and backward
+ // shift deletion in order to support a relatively high load factor
+ // efficiently, which matters since the cost of Broadcast is linear in the
+ // size of the table.
+
+ // mu protects the following fields.
+ mu sync.Mutex
+
+ // Invariants: len(table) is 0 or a power of 2.
+ table []broadcasterSlot
+
+ // load is the number of entries in table with receiver != nil.
+ load int
+
+ lastID SubscriptionID
+}
+
+type broadcasterSlot struct {
+ // Invariants: If receiver == nil, then filter == NoEvents and id == 0.
+ // Otherwise, id != 0.
+ receiver *Receiver
+ filter Set
+ id SubscriptionID
+}
+
+const (
+ broadcasterMinNonZeroTableSize = 2 // must be a power of 2 > 1
+
+ broadcasterMaxLoadNum = 13
+ broadcasterMaxLoadDen = 16
+)
+
+// SubscribeEvents implements Source.SubscribeEvents.
+func (b *Broadcaster) SubscribeEvents(r *Receiver, filter Set) SubscriptionID {
+ b.mu.Lock()
+
+ // Assign an ID for this subscription.
+ b.lastID++
+ id := b.lastID
+
+ // Expand the table if over the maximum load factor:
+ //
+ // load / len(b.table) > broadcasterMaxLoadNum / broadcasterMaxLoadDen
+ // load * broadcasterMaxLoadDen > broadcasterMaxLoadNum * len(b.table)
+ b.load++
+ if (b.load * broadcasterMaxLoadDen) > (broadcasterMaxLoadNum * len(b.table)) {
+ // Double the number of slots in the new table.
+ newlen := broadcasterMinNonZeroTableSize
+ if len(b.table) != 0 {
+ newlen = 2 * len(b.table)
+ }
+ if newlen <= cap(b.table) {
+ // Reuse excess capacity in the current table, moving entries not
+ // already in their first-probed positions to better ones.
+ newtable := b.table[:newlen]
+ newmask := uint64(newlen - 1)
+ for i := range b.table {
+ if b.table[i].receiver != nil && uint64(b.table[i].id)&newmask != uint64(i) {
+ entry := b.table[i]
+ b.table[i] = broadcasterSlot{}
+ broadcasterTableInsert(newtable, entry.id, entry.receiver, entry.filter)
+ }
+ }
+ b.table = newtable
+ } else {
+ newtable := make([]broadcasterSlot, newlen)
+ // Copy existing entries to the new table.
+ for i := range b.table {
+ if b.table[i].receiver != nil {
+ broadcasterTableInsert(newtable, b.table[i].id, b.table[i].receiver, b.table[i].filter)
+ }
+ }
+ // Switch to the new table.
+ b.table = newtable
+ }
+ }
+
+ broadcasterTableInsert(b.table, id, r, filter)
+ b.mu.Unlock()
+ return id
+}
+
+// Preconditions: table must not be full. len(table) is a power of 2.
+func broadcasterTableInsert(table []broadcasterSlot, id SubscriptionID, r *Receiver, filter Set) {
+ entry := broadcasterSlot{
+ receiver: r,
+ filter: filter,
+ id: id,
+ }
+ mask := uint64(len(table) - 1)
+ i := uint64(id) & mask
+ disp := uint64(0)
+ for {
+ if table[i].receiver == nil {
+ table[i] = entry
+ return
+ }
+ // If we've been displaced farther from our first-probed slot than the
+ // element stored in this one, swap elements and switch to inserting
+ // the replaced one. (This is Robin Hood insertion.)
+ slotDisp := (i - uint64(table[i].id)) & mask
+ if disp > slotDisp {
+ table[i], entry = entry, table[i]
+ disp = slotDisp
+ }
+ i = (i + 1) & mask
+ disp++
+ }
+}
+
+// UnsubscribeEvents implements Source.UnsubscribeEvents.
+func (b *Broadcaster) UnsubscribeEvents(id SubscriptionID) {
+ b.mu.Lock()
+
+ mask := uint64(len(b.table) - 1)
+ i := uint64(id) & mask
+ for {
+ if b.table[i].id == id {
+ // Found the element to remove. Move all subsequent elements
+ // backward until we either find an empty slot, or an element that
+ // is already in its first-probed slot. (This is backward shift
+ // deletion.)
+ for {
+ next := (i + 1) & mask
+ if b.table[next].receiver == nil {
+ break
+ }
+ if uint64(b.table[next].id)&mask == next {
+ break
+ }
+ b.table[i] = b.table[next]
+ i = next
+ }
+ b.table[i] = broadcasterSlot{}
+ break
+ }
+ i = (i + 1) & mask
+ }
+
+ // If a table 1/4 of the current size would still be at or under the
+ // maximum load factor (i.e. the current table size is at least two
+ // expansions bigger than necessary), halve the size of the table to reduce
+ // the cost of Broadcast. Since we are concerned with iteration time and
+ // not memory usage, reuse the existing slice to reduce future allocations
+ // from table re-expansion.
+ b.load--
+ if len(b.table) > broadcasterMinNonZeroTableSize && (b.load*(4*broadcasterMaxLoadDen)) <= (broadcasterMaxLoadNum*len(b.table)) {
+ newlen := len(b.table) / 2
+ newtable := b.table[:newlen]
+ for i := newlen; i < len(b.table); i++ {
+ if b.table[i].receiver != nil {
+ broadcasterTableInsert(newtable, b.table[i].id, b.table[i].receiver, b.table[i].filter)
+ b.table[i] = broadcasterSlot{}
+ }
+ }
+ b.table = newtable
+ }
+
+ b.mu.Unlock()
+}
+
+// Broadcast notifies all Receivers subscribed to the Broadcaster of the subset
+// of events to which they subscribed. The order in which Receivers are
+// notified is unspecified.
+func (b *Broadcaster) Broadcast(events Set) {
+ b.mu.Lock()
+ for i := range b.table {
+ if intersection := events & b.table[i].filter; intersection != 0 {
+ // We don't need to check if broadcasterSlot.receiver is nil, since
+ // if it is then broadcasterSlot.filter is 0.
+ b.table[i].receiver.Notify(intersection)
+ }
+ }
+ b.mu.Unlock()
+}
+
+// FilteredEvents returns the set of events for which Broadcast will notify at
+// least one Receiver, i.e. the union of filters for all subscribed Receivers.
+func (b *Broadcaster) FilteredEvents() Set {
+ var es Set
+ b.mu.Lock()
+ for i := range b.table {
+ es |= b.table[i].filter
+ }
+ b.mu.Unlock()
+ return es
+}