Proposal to enhance Stream.collect
August Nagro
augustnagro at gmail.com
Thu Feb 28 04:38:40 UTC 2019
Tagir,
Great to see the validating benchmarks.
> I think it's the best solution given the fact that very few collectors may
benefit from the exact known size, and this benefit usually disappears when
collectors are composed (e.g. using groupingBy: downstream toList() would
not win anything if it provides a sizedSupplier).
I would like to see your benchmarks for that statement too. After all,
Hashmap, HashSet, etc all have presizing constructors, aren't those there
for a reason?
So I am still convinced that presizing is important for other collector
types, including custom collectors. Although I'm open to having my mind
changed.
I'm happy to contribute an implementation as well, I was hoping that this
this (smallish) patch would help me learn the OpenJdk process and make
future contributions easier!
- August
On Wed, Feb 27, 2019 at 1:06 AM Tagir Valeev <amaembo at gmail.com> wrote:
> Hello!
>
> > A less intrusive API direction might be a version of Collector whose
> > supplier function took a size-estimate argument; this might even help in
> > parallel since it allows for intermediate results to start with a better
> > initial size guess. (And this could be implemented as a default that
> > delegates to the existing supplier.) Still, not really sure this
> > carries its weight.
>
> It's interesting that such optimization is possible without API
> change, using the "hidden knowledge".
> While public API stays the same, the collect method implementation may
> check if custom
> non-public Collector implementation is supplied, and in this case may
> use the sized supplier.
> I think it's the best solution given the fact that very few collectors
> may benefit from the exact
> known size, and this benefit usually disappears when collectors are
> composed (e.g. using groupingBy:
> downstream toList() would not win anything if it provides a sizedSupplier).
>
> I created a quick patch and launched a quick benchmark like this:
> return new SplittableRandom(0).ints(length, 0,
> 100).boxed().collect(Collectors.toList());
> For length = 100, 10000, 1000000
>
> Here's results for vanilla 13-ea+8:
> length = 100, avg time = 1434,469 ± 27,147 ns/op, alloc rate = 1712 B/op
> length = 10000, avg time = 155032,390 ± 2657,927 ns/op, alloc rate =
> 169280 B/op
> length = 1000000, avg time = 27099621,763 ± 1979054,500 ns/op, alloc
> rate = 14586750 B/op
>
> Patched:
> length = 100, avg time = 1414,480 ± 30,040 ns/op, alloc rate = 768 B/op
> length = 10000, avg time = 137338,320 ± 1316,789 ns/op, alloc rate =
> 40368 B/op
> length = 1000000, avg time = 17654635,871 ± 210607,441 ns/op, alloc
> rate = 4000382 B/op
>
> As you can see, exact allocation really helps to reduce alloc pressure
> and produces better performance for big lists.
>
> If such kind of patch is acceptable for Stream API, I can file a
> ticket and submit a webrev.
>
> With best regards,
> Tagir Valeev
>
> The patch follows:
>
> --- src/java.base/share/classes/java/util/stream/Collectors.java
> (revision 53626:e2fc434b410a35b28ab433c29863c8a26e4e813a)
> +++ src/java.base/share/classes/java/util/stream/Collectors.java
> (revision 53626+:e2fc434b410a+)
> @@ -50,6 +50,7 @@
> import java.util.function.BinaryOperator;
> import java.util.function.Consumer;
> import java.util.function.Function;
> +import java.util.function.IntFunction;
> import java.util.function.Predicate;
> import java.util.function.Supplier;
> import java.util.function.ToDoubleFunction;
> @@ -194,23 +195,34 @@
> */
> static class CollectorImpl<T, A, R> implements Collector<T, A, R> {
> private final Supplier<A> supplier;
> + private final IntFunction<A> sizedSupplier;
> private final BiConsumer<A, T> accumulator;
> private final BinaryOperator<A> combiner;
> private final Function<A, R> finisher;
> private final Set<Characteristics> characteristics;
>
> CollectorImpl(Supplier<A> supplier,
> + IntFunction<A> sizedSupplier,
> BiConsumer<A, T> accumulator,
> BinaryOperator<A> combiner,
> Function<A,R> finisher,
> Set<Characteristics> characteristics) {
> this.supplier = supplier;
> + this.sizedSupplier = sizedSupplier;
> this.accumulator = accumulator;
> this.combiner = combiner;
> this.finisher = finisher;
> this.characteristics = characteristics;
> }
>
> + CollectorImpl(Supplier<A> supplier,
> + BiConsumer<A, T> accumulator,
> + BinaryOperator<A> combiner,
> + Function<A,R> finisher,
> + Set<Characteristics> characteristics) {
> + this(supplier, null, accumulator, combiner, finisher,
> characteristics);
> + }
> +
> CollectorImpl(Supplier<A> supplier,
> BiConsumer<A, T> accumulator,
> BinaryOperator<A> combiner,
> @@ -228,6 +240,10 @@
> return supplier;
> }
>
> + IntFunction<A> sizedSupplier() {
> + return sizedSupplier;
> + }
> +
> @Override
> public BinaryOperator<A> combiner() {
> return combiner;
> @@ -275,8 +291,11 @@
> */
> public static <T>
> Collector<T, ?, List<T>> toList() {
> - return new CollectorImpl<>((Supplier<List<T>>)
> ArrayList::new, List::add,
> + return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new,
> + ArrayList::new,
> + List::add,
> (left, right) -> {
> left.addAll(right); return left; },
> + castingIdentity(),
> CH_ID);
> }
>
> Index: src/java.base/share/classes/java/util/stream/ReduceOps.java
> IDEA additional info:
> Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
> <+>UTF-8
> ===================================================================
> --- src/java.base/share/classes/java/util/stream/ReduceOps.java
> (revision 53626:e2fc434b410a35b28ab433c29863c8a26e4e813a)
> +++ src/java.base/share/classes/java/util/stream/ReduceOps.java
> (revision 53626+:e2fc434b410a+)
> @@ -36,6 +36,7 @@
> import java.util.function.BinaryOperator;
> import java.util.function.DoubleBinaryOperator;
> import java.util.function.IntBinaryOperator;
> +import java.util.function.IntFunction;
> import java.util.function.LongBinaryOperator;
> import java.util.function.ObjDoubleConsumer;
> import java.util.function.ObjIntConsumer;
> @@ -155,13 +156,20 @@
> public static <T, I> TerminalOp<T, I>
> makeRef(Collector<? super T, I, ?> collector) {
> Supplier<I> supplier =
> Objects.requireNonNull(collector).supplier();
> + @SuppressWarnings("unchecked")
> + IntFunction<I> sizedSupplier = collector instanceof
> Collectors.CollectorImpl ?
> + ((Collectors.CollectorImpl<?, I, ?>)
> collector).sizedSupplier() : null;
> BiConsumer<I, ? super T> accumulator = collector.accumulator();
> BinaryOperator<I> combiner = collector.combiner();
> class ReducingSink extends Box<I>
> implements AccumulatingSink<T, I, ReducingSink> {
> @Override
> public void begin(long size) {
> - state = supplier.get();
> + if (sizedSupplier != null && size >= 0 && size <=
> Integer.MAX_VALUE) {
> + state = sizedSupplier.apply((int) size);
> + } else {
> + state = supplier.get();
> + }
> }
>
> @Override
>
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