Proposal to enhance Stream.collect

Tagir Valeev amaembo at gmail.com
Thu Feb 28 06:55:52 UTC 2019


Hello!

I wouldn't use presized HashSet, because you never know how many duplicates
are expected. What if the input stream has a million of elements, but only
two of them are distinct? Do you really want to allocate a hash table for
million elements in advance?

For toMap() without custom supplier and merger preallocation sounds
reasonable as in this case duplicating key is an exceptional case, so we
may expect a specific number of elements.

чт, 28 февр. 2019 г., 11:38 August Nagro augustnagro at gmail.com:

> 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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