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UNDERSTANDING RXJAVA SCHEDULER RX JA NIGHT #2 2017.06.12 HIROSHI KUROKAWA
 FABLIC, INC.

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SCHEDULERS ▸ Schedulers.computation() ▸ Schedulers.io() ▸ Schedulers.newThread() ▸ Schedulers.trampoline()

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What’s the difference?

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COMPUTATION SCHEDULER ▸ for CPU-bound work ▸ bounded number of workers ▸ could be blocked if the maximum number of threads are being used

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val time = measureTimeMillis { val observers = List(8) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.computation()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌}

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val time = measureTimeMillis { val observers = List(8) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.computation()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌}

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val time = measureTimeMillis { val observers = List(8) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.computation()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌}

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val time = measureTimeMillis { val observers = List(8) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.computation()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌}

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val time = measureTimeMillis { val observers = List(8) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.computation()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌}

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val time = measureTimeMillis { val observers = List(8) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.computation()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌}

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val time = measureTimeMillis { val observers = List(8) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.computation()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌}

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val time = measureTimeMillis { val observers = List(8) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.computation()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌} println("$time [msec]”) // -> ?

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val time = measureTimeMillis { val observers = List(8) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.computation()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌} println(“${Runtime.getRuntime().availableProcessors()}") // -> 4 println("$time [msec]”) // -> ?

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val time = measureTimeMillis { val observers = List(8) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.computation()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌} println(“${Runtime.getRuntime().availableProcessors()}") // -> 4 println("$time [msec]”) // -> 2156 [msec]

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TEXT CAVEAT ‣ Keep in mind how much concurrency you need when using computation scheduler

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val time = measureTimeMillis { val observer = TestObserver.create() Observable.range(1, 8) .flatMap { Observable.fromCallable { Thread.sleep(1_000L) println(Thread.currentThread()) it ‌}.subscribeOn(Schedulers.computation()) ‌} .subscribe(observer) observer.awaitTerminalEvent() ‌} println("Elapsed time: $time [msec]") // -> ?

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val time = measureTimeMillis { val observer = TestObserver.create() Observable.range(1, 8) .flatMap { Observable.fromCallable { Thread.sleep(1_000L) println(Thread.currentThread()) it ‌}.subscribeOn(Schedulers.computation()) ‌} .subscribe(observer) observer.awaitTerminalEvent() ‌} println("Elapsed time: $time [msec]") // -> ?

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val time = measureTimeMillis { val observer = TestObserver.create() Observable.range(1, 8) .flatMap { Observable.fromCallable { Thread.sleep(1_000L) println(Thread.currentThread()) it ‌}.subscribeOn(Schedulers.computation()) ‌} .subscribe(observer) observer.awaitTerminalEvent() ‌} println("Elapsed time: $time [msec]") // -> ?

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val time = measureTimeMillis { val observer = TestObserver.create() Observable.range(1, 8) .flatMap { Observable.fromCallable { Thread.sleep(1_000L) println(Thread.currentThread()) it ‌}.subscribeOn(Schedulers.computation()) ‌} .subscribe(observer) observer.awaitTerminalEvent() ‌} println("Elapsed time: $time [msec]") // -> ?

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val time = measureTimeMillis { val observer = TestObserver.create() Observable.range(1, 8) .flatMap { Observable.fromCallable { Thread.sleep(1_000L) println(Thread.currentThread()) it ‌}.subscribeOn(Schedulers.computation()) ‌} .subscribe(observer) observer.awaitTerminalEvent() ‌} println("Elapsed time: $time [msec]") // -> ?

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val time = measureTimeMillis { val observer = TestObserver.create() Observable.range(1, 8) .flatMap { Observable.fromCallable { Thread.sleep(1_000L) println(Thread.currentThread()) it ‌}.subscribeOn(Schedulers.computation()) ‌} .subscribe(observer) observer.awaitTerminalEvent() ‌} println("Elapsed time: $time [msec]") // -> 2314 [msec]

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val time = measureTimeMillis { val observer = TestObserver.create() Observable.range(1, 8) .flatMap { Observable.fromCallable { Thread.sleep(1_000L) println(Thread.currentThread()) it ‌}.subscribeOn(Schedulers.computation()) ‌} .subscribe(observer) observer.awaitTerminalEvent() ‌} println("Elapsed time: $time [msec]") // -> ?

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val time = measureTimeMillis { val observer = TestObserver.create() Observable.range(1, 8) .subscribeOn(Schedulers.computation()) .flatMap { Observable.fromCallable { Thread.sleep(1_000L) println(Thread.currentThread()) it ‌} ‌} .subscribe(observer) observer.awaitTerminalEvent() ‌} println("Elapsed time: $time [msec]") // -> ?

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val time = measureTimeMillis { val observer = TestObserver.create() Observable.range(1, 8) .subscribeOn(Schedulers.computation()) .flatMap { Observable.fromCallable { Thread.sleep(1_000L) println(Thread.currentThread()) it ‌} ‌} .subscribe(observer) observer.awaitTerminalEvent() ‌} println("Elapsed time: $time [msec]") // -> 8211 [msec]

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IO SCHEDULER ▸ for IO-bound work ▸ unbounded number of workers ▸ could cause OOM

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val time = measureTimeMillis { val observers = List(24) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.io()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌} println("$time [msec]”) // -> ? [msec]

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val time = measureTimeMillis { val observers = List(24) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.io()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌} println("$time [msec]”) // -> ? [msec]

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val time = measureTimeMillis { val observers = List(24) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.io()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌} println("$time [msec]”) // -> 1142 [msec]

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val time = measureTimeMillis { val observers = List(2026) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.io()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌} println("$time [msec]”) // -> 1142 [msec]

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val time = measureTimeMillis { val observers = List(2026) { i -> val observer = TestObserver.create() Completable.fromAction { Thread.sleep(1_000L) } .subscribeOn(Schedulers.io()) .doOnComplete { println("Done $i") } .subscribe(observer) observer ‌} observers.forEach { it.awaitTerminalEvent() } ‌} println("$time [msec]”) // -> 1142 [msec] java.lang.OutOfMemoryError: unable to create new native thread

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NEW SCHEDULER ▸ If io or computation scheduler does not fit ▸ Create a new thread for each worker ▸ Shut down at dispose()

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Scheduler under the hood "

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SCHEDULER AND WORKER ▸ Scheduler does not schedule (!) ▸ Scheduler just creates a worker ▸ Worker processes tasks in a FIFO manner

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WORKER CONTRACT ▸ All methods should be thread-safe. ▸ Execute tasks in FIFO order. ▸ Make best effort to cancel outstanding tasks when unsubscribed. ▸ Unsubscription of a Worker should not affect other Worker instances of the same Scheduler.

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SCHEDULER API public Worker createWorker() public long now(TimeUnit unit) public void start() public void shutdown() public Disposable scheduleDirect(Runnable run) public Disposable scheduleDirect(Runnable run, long delay, TimeUnit unit) public Disposable schedulePeriodicallyDirect(Runnable run, long initialDelay, long period, TimeUnit unit)

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SCHEDULER API public Worker createWorker() public long now(TimeUnit unit) public void start() public void shutdown() public Disposable scheduleDirect(Runnable run) public Disposable scheduleDirect(Runnable run, long delay, TimeUnit unit) public Disposable schedulePeriodicallyDirect(Runnable run, long initialDelay, long period, TimeUnit unit)

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WORKER API public Disposable schedule(Runnable run) public Disposable schedule(Runnable run, long delay, TimeUnit unit) public Disposable schedulePeriodically(Runnable run, long initialDelay, final long period, final TimeUnit unit) public long now(TimeUnit unit)

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TRAMPOLINE VS. IMMEDIATE ▸ Schedulers.immediate() was dismissed in RxJava 2 ▸ Instead, Schedulers.trampoline() is recommended ▸ Why?

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IMMEDIATE SCHEDULER (RXJAVA 1) val worker = Schedulers.immediate().createWorker() worker.schedule { println("outer task start") worker.schedule { println("inner task start") println("inner task end") } println("outer task end") }     $POUSBDU7JPMBUJPO ˑ  &YFDVUFUBTLTJO'*'0PSEFS˒

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val worker = Schedulers.trampoline().createWorker() worker.schedule { println("outer task start") worker.schedule { println("inner task start") println("inner task end") ‌} println("outer task end") ‌} TRAMPOLINE SCHEDULER     $POGPSNJOHUPUIFDPOUSBDU#

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WHEN WORKER IS BORN, WHEN A TASK IS ASSIGNED ▸ A worker is created when a scheduler-related operator is subscribed ▸ delay(), observeOn(), subscribeOn(), etc. ▸ A task is scheduled when an event is emitted to the operator ▸ Events through an operator are serialised (FIFO) when not delayed

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FURTHER READING ▸ Advanced Reactive Java: Schedulers (part 1~4)
 http://akarnokd.blogspot.jp/2015/05/schedulers- part-1.html ▸ Read source code
 e.g. SingleScheduler, TrampolineScheduler, ExecutorScheduler

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Thank you! Hiroshi Kurokawa (ࠇ઒ ༸) @hydrakecat https://github.com/hkurokawa https://speakerdeck.com/hkurokawa hydrakecat.hatenablog.jp