Package syncevent is intended to subsume ~all uses of channels in the sentry
(including //pkg/waiter), as well as //pkg/sleep.
Compared to channels:
- Delivery of events to a syncevent.Receiver allows *synchronous* execution of
an arbitrary callback, whereas delivery of events to a channel requires a
goroutine to receive from that channel, resulting in substantial scheduling
overhead. (This is also part of the motivation for the waiter package.)
- syncevent.Waiter can wait on multiple event sources without the high O(N)
overhead of select. (This is the same motivation as for the sleep package.)
Compared to the waiter package:
- syncevent.Waiters are intended to be persistent (i.e. per-kernel.Task), and
syncevent.Broadcaster (analogous to waiter.Queue) is a hash table rather than
a linked list, such that blocking is (usually) allocation-free.
- syncevent.Source (analogous to waiter.Waitable) does not include an equivalent
to waiter.Waitable.Readiness(), since this is inappropriate for transient
events (see e.g. //pkg/sentry/kernel/time.ClockEventSource).
Compared to the sleep package:
- syncevent events are represented by bits in a bitmask rather than discrete
sleep.Waker objects, reducing overhead and making it feasible to broadcast
events to multiple syncevent.Receivers.
- syncevent.Receiver invokes an arbitrary callback, which is required by the
sentry's epoll implementation. (syncevent.Waiter, which is analogous to
sleep.Sleeper, pairs a syncevent.Receiver with a callback that wakes a
waiting goroutine; the implementation of this aspect is nearly identical to
that of sleep.Sleeper, except that it represents *runtime.g as unsafe.Pointer
rather than uintptr.)
- syncevent.Waiter.Wait (analogous to sleep.Sleeper.Fetch(block=true)) does not
automatically un-assert returned events. This is useful in cases where the
path for handling an event is not the same as the path that observes it, such
as for application signals (a la Linux's TIF_SIGPENDING).
- Unlike sleep.Sleeper, which Fetches Wakers in the order that they were
Asserted, the event bitmasks used by syncevent.Receiver have no way of
preserving event arrival order. (This is similar to select, which goes out of
its way to randomize event ordering.)
The disadvantage of the syncevent package is that, since events are represented
by bits in a uint64 bitmask, each syncevent.Receiver can "only" multiplex
between 64 distinct events; this does not affect any known use case.
Benchmarks:
BenchmarkBroadcasterSubscribeUnsubscribe
BenchmarkBroadcasterSubscribeUnsubscribe-12 45133884 26.3 ns/op
BenchmarkMapSubscribeUnsubscribe
BenchmarkMapSubscribeUnsubscribe-12 28504662 41.8 ns/op
BenchmarkQueueSubscribeUnsubscribe
BenchmarkQueueSubscribeUnsubscribe-12 22747668 45.6 ns/op
BenchmarkBroadcasterSubscribeUnsubscribeBatch
BenchmarkBroadcasterSubscribeUnsubscribeBatch-12 31609177 37.8 ns/op
BenchmarkMapSubscribeUnsubscribeBatch
BenchmarkMapSubscribeUnsubscribeBatch-12 17563906 62.1 ns/op
BenchmarkQueueSubscribeUnsubscribeBatch
BenchmarkQueueSubscribeUnsubscribeBatch-12 26248838 46.6 ns/op
BenchmarkBroadcasterBroadcastRedundant
BenchmarkBroadcasterBroadcastRedundant/0
BenchmarkBroadcasterBroadcastRedundant/0-12 100907563 11.8 ns/op
BenchmarkBroadcasterBroadcastRedundant/1
BenchmarkBroadcasterBroadcastRedundant/1-12 85103068 13.3 ns/op
BenchmarkBroadcasterBroadcastRedundant/4
BenchmarkBroadcasterBroadcastRedundant/4-12 52716502 22.3 ns/op
BenchmarkBroadcasterBroadcastRedundant/16
BenchmarkBroadcasterBroadcastRedundant/16-12 20278165 58.7 ns/op
BenchmarkBroadcasterBroadcastRedundant/64
BenchmarkBroadcasterBroadcastRedundant/64-12 5905428 205 ns/op
BenchmarkMapBroadcastRedundant
BenchmarkMapBroadcastRedundant/0
BenchmarkMapBroadcastRedundant/0-12 87532734 13.5 ns/op
BenchmarkMapBroadcastRedundant/1
BenchmarkMapBroadcastRedundant/1-12 28488411 36.3 ns/op
BenchmarkMapBroadcastRedundant/4
BenchmarkMapBroadcastRedundant/4-12 19628920 60.9 ns/op
BenchmarkMapBroadcastRedundant/16
BenchmarkMapBroadcastRedundant/16-12 6026980 192 ns/op
BenchmarkMapBroadcastRedundant/64
BenchmarkMapBroadcastRedundant/64-12 1640858 754 ns/op
BenchmarkQueueBroadcastRedundant
BenchmarkQueueBroadcastRedundant/0
BenchmarkQueueBroadcastRedundant/0-12 96904807 12.0 ns/op
BenchmarkQueueBroadcastRedundant/1
BenchmarkQueueBroadcastRedundant/1-12 73521873 16.3 ns/op
BenchmarkQueueBroadcastRedundant/4
BenchmarkQueueBroadcastRedundant/4-12 39209468 31.2 ns/op
BenchmarkQueueBroadcastRedundant/16
BenchmarkQueueBroadcastRedundant/16-12 10810058 105 ns/op
BenchmarkQueueBroadcastRedundant/64
BenchmarkQueueBroadcastRedundant/64-12 2998046 376 ns/op
BenchmarkBroadcasterBroadcastAck
BenchmarkBroadcasterBroadcastAck/1
BenchmarkBroadcasterBroadcastAck/1-12 44472397 26.4 ns/op
BenchmarkBroadcasterBroadcastAck/4
BenchmarkBroadcasterBroadcastAck/4-12 17653509 69.7 ns/op
BenchmarkBroadcasterBroadcastAck/16
BenchmarkBroadcasterBroadcastAck/16-12 4082617 260 ns/op
BenchmarkBroadcasterBroadcastAck/64
BenchmarkBroadcasterBroadcastAck/64-12 1220534 1027 ns/op
BenchmarkMapBroadcastAck
BenchmarkMapBroadcastAck/1
BenchmarkMapBroadcastAck/1-12 26760705 44.2 ns/op
BenchmarkMapBroadcastAck/4
BenchmarkMapBroadcastAck/4-12 11495636 100 ns/op
BenchmarkMapBroadcastAck/16
BenchmarkMapBroadcastAck/16-12 2937590 343 ns/op
BenchmarkMapBroadcastAck/64
BenchmarkMapBroadcastAck/64-12 861037 1344 ns/op
BenchmarkQueueBroadcastAck
BenchmarkQueueBroadcastAck/1
BenchmarkQueueBroadcastAck/1-12 19832679 55.0 ns/op
BenchmarkQueueBroadcastAck/4
BenchmarkQueueBroadcastAck/4-12 5618214 189 ns/op
BenchmarkQueueBroadcastAck/16
BenchmarkQueueBroadcastAck/16-12 1569980 713 ns/op
BenchmarkQueueBroadcastAck/64
BenchmarkQueueBroadcastAck/64-12 437672 2814 ns/op
BenchmarkWaiterNotifyRedundant
BenchmarkWaiterNotifyRedundant-12 650823090 1.96 ns/op
BenchmarkSleeperNotifyRedundant
BenchmarkSleeperNotifyRedundant-12 619871544 1.61 ns/op
BenchmarkChannelNotifyRedundant
BenchmarkChannelNotifyRedundant-12 298903778 3.67 ns/op
BenchmarkWaiterNotifyWaitAck
BenchmarkWaiterNotifyWaitAck-12 68358360 17.8 ns/op
BenchmarkSleeperNotifyWaitAck
BenchmarkSleeperNotifyWaitAck-12 25044883 41.2 ns/op
BenchmarkChannelNotifyWaitAck
BenchmarkChannelNotifyWaitAck-12 29572404 40.2 ns/op
BenchmarkSleeperMultiNotifyWaitAck
BenchmarkSleeperMultiNotifyWaitAck-12 16122969 73.8 ns/op
BenchmarkWaiterTempNotifyWaitAck
BenchmarkWaiterTempNotifyWaitAck-12 46111489 25.8 ns/op
BenchmarkSleeperTempNotifyWaitAck
BenchmarkSleeperTempNotifyWaitAck-12 15541882 73.6 ns/op
BenchmarkWaiterNotifyWaitMultiAck
BenchmarkWaiterNotifyWaitMultiAck-12 65878500 18.2 ns/op
BenchmarkSleeperNotifyWaitMultiAck
BenchmarkSleeperNotifyWaitMultiAck-12 28798623 41.5 ns/op
BenchmarkChannelNotifyWaitMultiAck
BenchmarkChannelNotifyWaitMultiAck-12 11308468 101 ns/op
BenchmarkWaiterNotifyAsyncWaitAck
BenchmarkWaiterNotifyAsyncWaitAck-12 2475387 492 ns/op
BenchmarkSleeperNotifyAsyncWaitAck
BenchmarkSleeperNotifyAsyncWaitAck-12 2184507 518 ns/op
BenchmarkChannelNotifyAsyncWaitAck
BenchmarkChannelNotifyAsyncWaitAck-12 2120365 562 ns/op
BenchmarkWaiterNotifyAsyncWaitMultiAck
BenchmarkWaiterNotifyAsyncWaitMultiAck-12 2351247 494 ns/op
BenchmarkSleeperNotifyAsyncWaitMultiAck
BenchmarkSleeperNotifyAsyncWaitMultiAck-12 2205799 522 ns/op
BenchmarkChannelNotifyAsyncWaitMultiAck
BenchmarkChannelNotifyAsyncWaitMultiAck-12 1238079 928 ns/op
Updates #1074
PiperOrigin-RevId: 295834087
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