Add fast Semaphore.

In addition, the `SegmentQueue` data structure, which emulates an infinite array with fast removing from the middle, is introduced for storing suspended acquirers in semaphore/mutex/channel algorithms.

Fixes Kotlin#1088

benchmarks/src/jmh/kotlin/benchmarks/SemaphoreBenchmark.kt

@@ -0,0 +1,97 @@
+package benchmarks
+
+import kotlinx.coroutines.*
+import kotlinx.coroutines.channels.Channel
+import kotlinx.coroutines.scheduling.ExperimentalCoroutineDispatcher
+import kotlinx.coroutines.sync.Semaphore
+import kotlinx.coroutines.sync.withPermit
+import org.openjdk.jmh.annotations.*
+import java.util.concurrent.ForkJoinPool
+import java.util.concurrent.ThreadLocalRandom
+import java.util.concurrent.TimeUnit
+
+@Warmup(iterations = 3, time = 500, timeUnit = TimeUnit.MICROSECONDS)
+@Measurement(iterations = 10, time = 500, timeUnit = TimeUnit.MICROSECONDS)
+@Fork(value = 1)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
+@State(Scope.Benchmark)
+open class SemaphoreBenchmark {
+ @Param
+ private var _1_dispatcher: SemaphoreBenchDispatcherCreator = SemaphoreBenchDispatcherCreator.FORK_JOIN
+
+ @Param("0", "1000")
+ private var _2_coroutines: Int = 0
+
+ @Param("1", "2", "4", "8", "32", "128", "100000")
+ private var _3_maxPermits: Int = 0
+
+ @Param("1", "2", "4") // local machine
+// @Param("1", "2", "4", "8", "16", "32", "64", "128", "144") // dasquad
+// @Param("1", "2", "4", "8", "16", "32", "64", "96") // Google Cloud
+ private var _4_parallelism: Int = 0
+
+ private lateinit var dispatcher: CoroutineDispatcher
+ private var coroutines = 0
+
+ @InternalCoroutinesApi
+ @Setup
+ fun setup() {
+ dispatcher = _1_dispatcher.create(_4_parallelism)
+ coroutines = if (_2_coroutines == 0) _4_parallelism else _2_coroutines
+ }
+
+ @Benchmark
+ fun semaphore() = runBlocking {
+ val n = BATCH_SIZE / coroutines
+ val semaphore = Semaphore(_3_maxPermits)
+ val jobs = ArrayList<Job>(coroutines)
+ repeat(coroutines) {
+ jobs += GlobalScope.launch {
+ repeat(n) {
+ semaphore.withPermit {
+ doWork(WORK_INSIDE)
+ }
+ doWork(WORK_OUTSIDE)
+ }
+ }
+ }
+ jobs.forEach { it.join() }
+ }
+
+ @Benchmark
+ fun channelAsSemaphore() = runBlocking {
+ val n = BATCH_SIZE / coroutines
+ val semaphore = Channel<Unit>(_3_maxPermits)
+ val jobs = ArrayList<Job>(coroutines)
+ repeat(coroutines) {
+ jobs += GlobalScope.launch {
+ repeat(n) {
+ semaphore.send(Unit) // acquire
+ doWork(WORK_INSIDE)
+ semaphore.receive() // release
+ doWork(WORK_OUTSIDE)
+ }
+ }
+ }
+ jobs.forEach { it.join() }
+ }
+}
+
+enum class SemaphoreBenchDispatcherCreator(val create: (parallelism: Int) -> CoroutineDispatcher) {
+ FORK_JOIN({ parallelism -> ForkJoinPool(parallelism).asCoroutineDispatcher() }),
+ EXPERIMENTAL({ parallelism -> ExperimentalCoroutineDispatcher(corePoolSize = parallelism, maxPoolSize = parallelism) })
+}
+
+private fun doWork(work: Int) {
+ // We use geometric distribution here
+ val p = 1.0 / work
+ val r = ThreadLocalRandom.current()
+ while (true) {
+ if (r.nextDouble() < p) break
+ }
+}
+
+private const val WORK_INSIDE = 80
+private const val WORK_OUTSIDE = 40
+private const val BATCH_SIZE = 1000000

binary-compatibility-validator/reference-public-api/kotlinx-coroutines-core.txt

@@ -1008,6 +1008,19 @@ public final class kotlinx/coroutines/sync/MutexKt {
  public static synthetic fun withLock$default (Lkotlinx/coroutines/sync/Mutex;Ljava/lang/Object;Lkotlin/jvm/functions/Function0;Lkotlin/coroutines/Continuation;ILjava/lang/Object;)Ljava/lang/Object;
 }
 
+public abstract interface class kotlinx/coroutines/sync/Semaphore {
+ public abstract fun acquire (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
+ public abstract fun getAvailablePermits ()I
+ public abstract fun release ()V
+ public abstract fun tryAcquire ()Z
+}
+
+public final class kotlinx/coroutines/sync/SemaphoreKt {
+ public static final fun Semaphore (II)Lkotlinx/coroutines/sync/Semaphore;
+ public static synthetic fun Semaphore$default (IIILjava/lang/Object;)Lkotlinx/coroutines/sync/Semaphore;
+ public static final fun withPermit (Lkotlinx/coroutines/sync/Semaphore;Lkotlin/jvm/functions/Function0;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
+}
+
 public final class kotlinx/coroutines/test/TestCoroutineContext : kotlin/coroutines/CoroutineContext {
  public fun <init> ()V
  public fun <init> (Ljava/lang/String;)V

kotlinx-coroutines-core/common/src/internal/SegmentQueue.kt

@@ -0,0 +1,176 @@
+package kotlinx.coroutines.internal
+
+import kotlinx.atomicfu.AtomicRef
+import kotlinx.atomicfu.atomic
+import kotlinx.atomicfu.loop
+
+/**
+ * Essentially, this segment queue is an infinite array of segments, which is represented as
+ * a Michael-Scott queue of them. All segments are instances of [Segment] class and
+ * follow in natural order (see [Segment.id]) in the queue.
+ */
+internal abstract class SegmentQueue<S: Segment<S>>() {
+ private val _head: AtomicRef<S>
+ /**
+ * Returns the first segment in the queue.
+ */
+ protected val head: S get() = _head.value
+
+ private val _tail: AtomicRef<S>
+ /**
+ * Returns the last segment in the queue.
+ */
+ protected val tail: S get() = _tail.value
+
+ init {
+ val initialSegment = newSegment(0)
+ _head = atomic(initialSegment)
+ _tail = atomic(initialSegment)
+ }
+
+ /**
+ * The implementation should create an instance of segment [S] with the specified id
+ * and initial reference to the previous one.
+ */
+ abstract fun newSegment(id: Long, prev: S? = null): S
+
+ /**
+ * Finds a segment with the specified [id] following by next references from the
+ * [startFrom] segment. The typical use-case is reading [tail] or [head], doing some
+ * synchronization, and invoking [getSegment] or [getSegmentAndMoveHead] correspondingly
+ * to find the required segment.
+ */
+ protected fun getSegment(startFrom: S, id: Long): S? {
+ // Go through `next` references and add new segments if needed,
+ // similarly to the `push` in the Michael-Scott queue algorithm.
+ // The only difference is that `CAS failure` means that the
+ // required segment has already been added, so the algorithm just
+ // uses it. This way, only one segment with each id can be in the queue.
+ var cur: S = startFrom
+ while (cur.id < id) {
+ var curNext = cur.next
+ if (curNext == null) {
+ // Add a new segment.
+ val newTail = newSegment(cur.id + 1, cur)
+ curNext = if (cur.casNext(null, newTail)) {
+ if (cur.removed) {
+ cur.remove()
+ }
+ moveTailForward(newTail)
+ newTail
+ } else {
+ cur.next!!
+ }
+ }
+ cur = curNext
+ }
+ if (cur.id != id) return null
+ return cur
+ }
+
+ /**
+ * Invokes [getSegment] and replaces [head] with the result if its [id] is greater.
+ */
+ protected fun getSegmentAndMoveHead(startFrom: S, id: Long): S? {
+ @Suppress("LeakingThis")
+ if (startFrom.id == id) return startFrom
+ val s = getSegment(startFrom, id) ?: return null
+ moveHeadForward(s)
+ return s
+ }
+
+ /**
+ * Updates [head] to the specified segment
+ * if its `id` is greater.
+ */
+ private fun moveHeadForward(new: S) {
+ _head.loop { curHead ->
+ if (curHead.id > new.id) return
+ if (_head.compareAndSet(curHead, new)) {
+ new.prev.value = null
+ return
+ }
+ }
+ }
+
+ /**
+ * Updates [tail] to the specified segment
+ * if its `id` is greater.
+ */
+ private fun moveTailForward(new: S) {
+ _tail.loop { curTail ->
+ if (curTail.id > new.id) return
+ if (_tail.compareAndSet(curTail, new)) return
+ }
+ }
+}
+
+/**
+ * Each segment in [SegmentQueue] has a unique id and is created by [SegmentQueue.newSegment].
+ * Essentially, this is a node in the Michael-Scott queue algorithm, but with
+ * maintaining [prev] pointer for efficient [remove] implementation.
+ */
+internal abstract class Segment<S: Segment<S>>(val id: Long, prev: S?) {
+ // Pointer to the next segment, updates similarly to the Michael-Scott queue algorithm.
+ private val _next = atomic<S?>(null)
+ val next: S? get() = _next.value
+ fun casNext(expected: S?, value: S?): Boolean = _next.compareAndSet(expected, value)
+ // Pointer to the previous segment, updates in [remove] function.
+ val prev = atomic<S?>(null)
+
+ /**
+ * Returns `true` if this segment is logically removed from the queue.
+ * The [remove] function should be called right after it becomes logically removed.
+ */
+ abstract val removed: Boolean
+
+ init {
+ this.prev.value = prev
+ }
+
+ /**
+ * Removes this segment physically from the segment queue. The segment should be
+ * logically removed (so [removed] returns `true`) at the point of invocation.
+ */
+ fun remove() {
+ check(removed) { " The segment should be logically removed at first "}
+ // Read `next` and `prev` pointers.
+ var next = this._next.value ?: return // tail cannot be removed
+ var prev = prev.value ?: return // head cannot be removed
+ // Link `next` and `prev`.
+ prev.moveNextToRight(next)
+ while (prev.removed) {
+ prev = prev.prev.value ?: break
+ prev.moveNextToRight(next)
+ }
+ next.movePrevToLeft(prev)
+ while (next.removed) {
+ next = next.next ?: break
+ next.movePrevToLeft(prev)
+ }
+ }
+
+ /**
+ * Updates [next] pointer to the specified segment if
+ * the [id] of the specified segment is greater.
+ */
+ private fun moveNextToRight(next: S) {
+ while (true) {
+ val curNext = this._next.value as S
+ if (next.id <= curNext.id) return
+ if (this._next.compareAndSet(curNext, next)) return
+ }
+ }
+
+ /**
+ * Updates [prev] pointer to the specified segment if
+ * the [id] of the specified segment is lower.
+ */
+ private fun movePrevToLeft(prev: S) {
+ while (true) {
+ val curPrev = this.prev.value ?: return
+ if (curPrev.id <= prev.id) return
+ if (this.prev.compareAndSet(curPrev, prev)) return
+ }
+ }
+}