RFR: 8265029: Preserve SIZED characteristics on slice operations (skip, limit) [v7]
Paul Sandoz
psandoz at openjdk.java.net
Mon May 24 21:57:08 UTC 2021
On Mon, 24 May 2021 04:35:42 GMT, Tagir F. Valeev <tvaleev at openjdk.org> wrote:
>> With the introduction of `toList()`, preserving the SIZED characteristics in more cases becomes more important. This patch preserves SIZED on `skip()` and `limit()` operations, so now every combination of `map/mapToX/boxed/asXyzStream/skip/limit/sorted` preserves size, and `toList()`, `toArray()` and `count()` may benefit from this. E. g., `LongStream.range(0, 10_000_000_000L).skip(1).count()` returns result instantly with this patch.
>>
>> Some microbenchmarks added that confirm the reduced memory allocation in `toList()` and `toArray()` cases. Before patch:
>>
>> ref.SliceToList.seq_baseline:·gc.alloc.rate.norm 10000 thrpt 10 40235,534 ± 0,984 B/op
>> ref.SliceToList.seq_limit:·gc.alloc.rate.norm 10000 thrpt 10 106431,101 ± 0,198 B/op
>> ref.SliceToList.seq_skipLimit:·gc.alloc.rate.norm 10000 thrpt 10 106544,977 ± 1,983 B/op
>> value.SliceToArray.seq_baseline:·gc.alloc.rate.norm 10000 thrpt 10 40121,878 ± 0,247 B/op
>> value.SliceToArray.seq_limit:·gc.alloc.rate.norm 10000 thrpt 10 106317,693 ± 1,083 B/op
>> value.SliceToArray.seq_skipLimit:·gc.alloc.rate.norm 10000 thrpt 10 106430,954 ± 0,136 B/op
>>
>>
>> After patch:
>>
>> ref.SliceToList.seq_baseline:·gc.alloc.rate.norm 10000 thrpt 10 40235,648 ± 1,354 B/op
>> ref.SliceToList.seq_limit:·gc.alloc.rate.norm 10000 thrpt 10 40355,784 ± 1,288 B/op
>> ref.SliceToList.seq_skipLimit:·gc.alloc.rate.norm 10000 thrpt 10 40476,032 ± 2,855 B/op
>> value.SliceToArray.seq_baseline:·gc.alloc.rate.norm 10000 thrpt 10 40121,830 ± 0,308 B/op
>> value.SliceToArray.seq_limit:·gc.alloc.rate.norm 10000 thrpt 10 40242,554 ± 0,443 B/op
>> value.SliceToArray.seq_skipLimit:·gc.alloc.rate.norm 10000 thrpt 10 40363,674 ± 1,576 B/op
>>
>>
>> Time improvements are less exciting. It's likely that inlining and vectorizing dominate in these tests over array allocations and unnecessary copying. Still, I notice a significant improvement in SliceToArray.seq_limit case (2x) and mild improvement (+12..16%) in other slice tests. No significant change in parallel execution time, though its performance is much less stable and I didn't run enough tests.
>>
>> Before patch:
>>
>> Benchmark (size) Mode Cnt Score Error Units
>> ref.SliceToList.par_baseline 10000 thrpt 30 14876,723 ± 99,770 ops/s
>> ref.SliceToList.par_limit 10000 thrpt 30 14856,841 ± 215,089 ops/s
>> ref.SliceToList.par_skipLimit 10000 thrpt 30 9555,818 ± 991,335 ops/s
>> ref.SliceToList.seq_baseline 10000 thrpt 30 23732,290 ± 444,162 ops/s
>> ref.SliceToList.seq_limit 10000 thrpt 30 14894,040 ± 176,496 ops/s
>> ref.SliceToList.seq_skipLimit 10000 thrpt 30 10646,929 ± 36,469 ops/s
>> value.SliceToArray.par_baseline 10000 thrpt 30 25093,141 ± 376,402 ops/s
>> value.SliceToArray.par_limit 10000 thrpt 30 24798,889 ± 760,762 ops/s
>> value.SliceToArray.par_skipLimit 10000 thrpt 30 16456,310 ± 926,882 ops/s
>> value.SliceToArray.seq_baseline 10000 thrpt 30 69669,787 ± 494,562 ops/s
>> value.SliceToArray.seq_limit 10000 thrpt 30 21097,081 ± 117,338 ops/s
>> value.SliceToArray.seq_skipLimit 10000 thrpt 30 15522,871 ± 112,557 ops/s
>>
>>
>> After patch:
>>
>> Benchmark (size) Mode Cnt Score Error Units
>> ref.SliceToList.par_baseline 10000 thrpt 30 14793,373 ± 64,905 ops/s
>> ref.SliceToList.par_limit 10000 thrpt 30 13301,024 ± 1300,431 ops/s
>> ref.SliceToList.par_skipLimit 10000 thrpt 30 11131,698 ± 1769,932 ops/s
>> ref.SliceToList.seq_baseline 10000 thrpt 30 24101,048 ± 263,528 ops/s
>> ref.SliceToList.seq_limit 10000 thrpt 30 16872,168 ± 76,696 ops/s
>> ref.SliceToList.seq_skipLimit 10000 thrpt 30 11953,253 ± 105,231 ops/s
>> value.SliceToArray.par_baseline 10000 thrpt 30 25442,442 ± 455,554 ops/s
>> value.SliceToArray.par_limit 10000 thrpt 30 23111,730 ± 2246,086 ops/s
>> value.SliceToArray.par_skipLimit 10000 thrpt 30 17980,750 ± 2329,077 ops/s
>> value.SliceToArray.seq_baseline 10000 thrpt 30 66512,898 ± 1001,042 ops/s
>> value.SliceToArray.seq_limit 10000 thrpt 30 41792,549 ± 1085,547 ops/s
>> value.SliceToArray.seq_skipLimit 10000 thrpt 30 18007,613 ± 141,716 ops/s
>>
>>
>> I also modernized SliceOps a little bit, using switch expression (with no explicit default!) and diamonds on anonymous classes.
>
> Tagir F. Valeev has updated the pull request incrementally with one additional commit since the last revision:
>
> Trailing whitespace removed
Very good. Thanks making the adjustments. Architecturally, i think we are in a better place. Just have some comments, mostly around code comments.
src/java.base/share/classes/java/util/stream/AbstractPipeline.java line 471:
> 469: int flags = getStreamAndOpFlags();
> 470: long size = StreamOpFlag.SIZED.isKnown(flags) ? spliterator.getExactSizeIfKnown() : -1;
> 471: if (size != -1 && StreamOpFlag.SIZE_ADJUSTING.isKnown(flags) && !isParallel()) {
Very nice. It's a good compromise to support only for sequential streams, since we have no size adjusting intermediate stateless op. If that was the case we would need to step back through the pipeline until the depth was zero, then step forward. I think it worth a comment here to inform our future selves if we ever add such an operation.
Strictly speaking we only need to call `exactOutputSize` if the stage is size adjusting. Not sure it really matters perf-wise. If we leave as is maybe add a comment.
src/java.base/share/classes/java/util/stream/StreamOpFlag.java line 331:
> 329:
> 330: /**
> 331: * Characteristic value signifying that an operation may adjust the
I think we need to add two additional constraints to the documentation:
1. The flag, if present, is only valid when SIZED is present; and
2. The flag is only valid for sequential streams.
The latter is a good compromise given we currently have no size adjusting stateless intermediate op.
-------------
PR: https://git.openjdk.java.net/jdk/pull/3427
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