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Backpressure: parallel #1573

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Aug 12, 2014
Merged

Backpressure: parallel #1573

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128 changes: 72 additions & 56 deletions rxjava-core/src/main/java/rx/internal/operators/OperatorParallel.java
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Original file line number Diff line number Diff line change
Expand Up @@ -15,14 +15,16 @@
*/
package rx.internal.operators;

import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicLong;

import rx.Observable;
import rx.Observable.OnSubscribe;
import rx.Observable.Operator;
import rx.Producer;
import rx.Scheduler;
import rx.Subscriber;
import rx.functions.Func1;
import rx.subjects.Subject;

/**
* Identifies unit of work that can be executed in parallel on a given Scheduler.
Expand All @@ -41,85 +43,99 @@ public OperatorParallel(Func1<Observable<T>, Observable<R>> f, Scheduler schedul

@Override
public Subscriber<? super T> call(final Subscriber<? super R> child) {

@SuppressWarnings("unchecked")
final UnicastPassThruSubject<T>[] subjects = new UnicastPassThruSubject[degreeOfParallelism];
@SuppressWarnings("unchecked")
final Observable<R>[] os = new Observable[degreeOfParallelism];
for (int i = 0; i < subjects.length; i++) {
subjects[i] = UnicastPassThruSubject.<T> create();
os[i] = f.call(subjects[i].observeOn(scheduler));
}

// subscribe BEFORE receiving data so everything is hooked up
Observable.merge(os).unsafeSubscribe(child);

return new Subscriber<T>(child) {

int index = 0; // trust that we receive data synchronously

@Override
public void onCompleted() {
for (UnicastPassThruSubject<T> s : subjects) {
s.onCompleted();
}
}

@Override
public void onError(Throwable e) {
// bypass the subjects and immediately terminate
child.onError(e);
}
@SuppressWarnings("unchecked")
final Subscriber<? super T>[] ss = new Subscriber[degreeOfParallelism];
final ParentSubscriber subscriber = new ParentSubscriber(child, ss);
for (int i = 0; i < os.length; i++) {
final int index = i;
Observable<T> o = Observable.create(new OnSubscribe<T>() {

@Override
public void onNext(T t) {
// round-robin subjects
subjects[index++].onNext(t);
if (index >= degreeOfParallelism) {
index = 0;
@Override
public void call(Subscriber<? super T> inner) {
ss[index] = inner;
child.add(inner); // unsubscribe chain
inner.setProducer(new Producer() {

@Override
public void request(long n) {
// as we receive requests from observeOn propagate upstream to the parent Subscriber
subscriber.requestMore(n);
}

});
}
}

};
});
os[i] = f.call(o.observeOn(scheduler));
}

// subscribe BEFORE receiving data so everything is hooked up
Observable.merge(os).unsafeSubscribe(child);
return subscriber;
}

private static class UnicastPassThruSubject<T> extends Subject<T, T> {

private static <T> UnicastPassThruSubject<T> create() {
final AtomicReference<Subscriber<? super T>> subscriber = new AtomicReference<Subscriber<? super T>>();
return new UnicastPassThruSubject<T>(subscriber, new OnSubscribe<T>() {
private class ParentSubscriber extends Subscriber<T> {

@Override
public void call(Subscriber<? super T> s) {
subscriber.set(s);
}

});
final Subscriber<? super R> child;
final Subscriber<? super T>[] ss;
int index = 0;
final AtomicLong initialRequest = new AtomicLong();
final AtomicBoolean started = new AtomicBoolean();

private ParentSubscriber(Subscriber<? super R> child, Subscriber<? super T>[] ss) {
super(child);
this.child = child;
this.ss = ss;
}

private final AtomicReference<Subscriber<? super T>> subscriber;
public void requestMore(long n) {
if (started.get()) {
request(n);
} else {
initialRequest.addAndGet(n);
}
}

protected UnicastPassThruSubject(AtomicReference<Subscriber<? super T>> subscriber, OnSubscribe<T> onSubscribe) {
super(onSubscribe);
this.subscriber = subscriber;
@Override
public void onStart() {
if (started.compareAndSet(false, true)) {
// if no request via requestMore has been sent yet, we start with 0 (rather than default Long.MAX_VALUE).
request(initialRequest.get());
}
}

@Override
public void onCompleted() {
subscriber.get().onCompleted();
for (Subscriber<? super T> s : ss) {
s.onCompleted();
}
}

@Override
public void onError(Throwable e) {
subscriber.get().onError(e);
child.onError(e);
}

@Override
public void onNext(T t) {
subscriber.get().onNext(t);
/*
* There is a possible bug here ... we could get a MissingBackpressureException
* if the processing on each of the threads is unbalanced. In other words, if 1 of the
* observeOn queues has space, but another is full, this could try emitting to one that
* is full and get a MissingBackpressureException.
*
* To solve that we'd need to check the outstanding request per Subscriber, which will
* need a more complicated mechanism to expose a type that has both the requested + the
* Subscriber to emit to.
*/
ss[index++].onNext(t);
if (index >= degreeOfParallelism) {
index = 0;
}
}

}
};

}
18 changes: 7 additions & 11 deletions rxjava-core/src/test/java/rx/internal/operators/OperatorParallelTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -21,7 +21,6 @@
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;

import org.junit.Ignore;
import org.junit.Test;

import rx.Observable;
Expand Down Expand Up @@ -57,7 +56,7 @@ public Integer[] call(Integer t) {
// TODO why is this exception not being thrown?
throw new RuntimeException(e);
}
// System.out.println("V: " + t + " Thread: " + Thread.currentThread());
// System.out.println("V: " + t + " Thread: " + Thread.currentThread());
innerCount.incrementAndGet();
return new Integer[] { t, t * 99 };
}
Expand Down Expand Up @@ -112,8 +111,6 @@ public void call(String v) {
assertEquals(NUM, count.get());
}

// parallel does not support backpressure right now
@Ignore
@Test
public void testBackpressureViaOuterObserveOn() {
final AtomicInteger emitted = new AtomicInteger();
Expand Down Expand Up @@ -148,12 +145,10 @@ public String call(Integer t) {
ts.awaitTerminalEvent();
ts.assertNoErrors();
System.out.println("testBackpressureViaObserveOn emitted => " + emitted.get());
assertTrue(emitted.get() < 2000 + RxRingBuffer.SIZE); // should have no more than the buffer size beyond the 2000 in take
assertEquals(2000, ts.getOnNextEvents().size());
assertTrue(emitted.get() < 20000 + (RxRingBuffer.SIZE * Schedulers.computation().parallelism())); // should have no more than the buffer size beyond the 20000 in take
assertEquals(20000, ts.getOnNextEvents().size());
}

// parallel does not support backpressure right now
@Ignore
@Test
public void testBackpressureOnInnerObserveOn() {
final AtomicInteger emitted = new AtomicInteger();
Expand Down Expand Up @@ -188,8 +183,8 @@ public String call(Integer t) {
ts.awaitTerminalEvent();
ts.assertNoErrors();
System.out.println("testBackpressureViaObserveOn emitted => " + emitted.get());
assertTrue(emitted.get() < 20000 + RxRingBuffer.SIZE); // should have no more than the buffer size beyond the 2000 in take
assertEquals(2000, ts.getOnNextEvents().size());
assertTrue(emitted.get() < 20000 + (RxRingBuffer.SIZE * Schedulers.computation().parallelism())); // should have no more than the buffer size beyond the 20000 in take
assertEquals(20000, ts.getOnNextEvents().size());
}

@Test(timeout = 10000)
Expand Down Expand Up @@ -226,7 +221,8 @@ public String call(Integer t) {
ts.awaitTerminalEvent();
ts.assertNoErrors();
System.out.println("emitted: " + emitted.get());
assertEquals(2000, emitted.get()); // no async, so should be perfect
// we allow buffering inside each parallel Observable
assertEquals(RxRingBuffer.SIZE * Schedulers.computation().parallelism(), emitted.get()); // no async, so should be perfect
assertEquals(2000, ts.getOnNextEvents().size());
}
}