RFR: 8282429: StringBuilder/StringBuffer.toString() skip compressing for UTF16 strings [v3]
Claes Redestad
redestad at openjdk.java.net
Tue Mar 15 23:51:47 UTC 2022
On Wed, 9 Mar 2022 08:33:36 GMT, Xin Liu <xliu at openjdk.org> wrote:
>> If AbstractStringBuilder only grow, the inflated value which has been encoded in UTF16 can't be compressed.
>> toString() can skip compression in this case. This can save an ArrayAllocation in StringUTF16::compress().
>>
>> java.io.BufferedRead::readLine() is a case that StringBuilder grows only.
>>
>> In microbench, we expect to see that allocation/op reduces 20%. The initial capacity of StringBuilder is S in bytes. When it encounters the 1st character that can't be encoded in LATIN1, it inflates and allocate a new array of 2*S. `toString()` will try to compress that value so it need to allocate S bytes. The last step allocates 2*S bytes because it has to copy the string. so it requires to allocate 5 * S bytes in total. By skipping the failed compression, it only allocates 4 * S bytes. that's 20%. In real execution, we observe 16% allocation reduction, tracked by JMH GC profiler `gc.alloc.rate.norm `. I think it's because HotSpot can't track all allocations.
>>
>> Not only allocation drops, the runtime performance(ns/op) also increases from 3.34% to 18.91%.
>>
>> Before:
>>
>> $$make test TEST="micro:org.openjdk.bench.java.lang.StringBuilders.toStringWithMixedChars" MICRO="OPTIONS=-prof gc -gc true -o before.log -jvm $HOME/Devel/jdk_baseline/bin/java"
>>
>> Benchmark (MIXED_SIZE) Mode Cnt Score Error Units
>> StringBuilders.toStringWithMixedChars 128 avgt 15 649.846 ± 76.291 ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate 128 avgt 15 872.855 ± 128.259 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm 128 avgt 15 880.121 ± 0.050 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space 128 avgt 15 707.730 ± 194.421 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm 128 avgt 15 706.602 ± 94.504 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space 128 avgt 15 0.001 ± 0.002 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm 128 avgt 15 0.001 ± 0.001 B/op
>> StringBuilders.toStringWithMixedChars:·gc.count 128 avgt 15 113.000 counts
>> StringBuilders.toStringWithMixedChars:·gc.time 128 avgt 15 85.000 ms
>> StringBuilders.toStringWithMixedChars 256 avgt 15 1316.652 ± 112.771 ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate 256 avgt 15 800.864 ± 76.869 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm 256 avgt 15 1648.288 ± 0.162 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space 256 avgt 15 599.736 ± 174.001 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm 256 avgt 15 1229.669 ± 318.518 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space 256 avgt 15 0.001 ± 0.001 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm 256 avgt 15 0.001 ± 0.002 B/op
>> StringBuilders.toStringWithMixedChars:·gc.count 256 avgt 15 133.000 counts
>> StringBuilders.toStringWithMixedChars:·gc.time 256 avgt 15 92.000 ms
>> StringBuilders.toStringWithMixedChars 1024 avgt 15 5204.303 ± 418.115 ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate 1024 avgt 15 768.730 ± 72.945 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm 1024 avgt 15 6256.844 ± 0.358 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space 1024 avgt 15 655.852 ± 121.602 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm 1024 avgt 15 5315.265 ± 578.878 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space 1024 avgt 15 0.002 ± 0.002 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm 1024 avgt 15 0.014 ± 0.011 B/op
>> StringBuilders.toStringWithMixedChars:·gc.count 1024 avgt 15 96.000 counts
>> StringBuilders.toStringWithMixedChars:·gc.time 1024 avgt 15 86.000 ms
>>
>>
>> After
>>
>> $make test TEST="micro:org.openjdk.bench.java.lang.StringBuilders.toStringWithMixedChars" MICRO="OPTIONS=-prof gc -gc true -o after.log"
>>
>> Benchmark (MIXED_SIZE) Mode Cnt Score Error Units
>> StringBuilders.toStringWithMixedChars 128 avgt 15 627.522 ± 54.804 ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate 128 avgt 15 751.353 ± 83.478 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm 128 avgt 15 736.120 ± 0.062 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space 128 avgt 15 589.102 ± 164.083 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm 128 avgt 15 575.425 ± 127.021 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space 128 avgt 15 ≈ 10⁻³ MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm 128 avgt 15 ≈ 10⁻³ B/op
>> StringBuilders.toStringWithMixedChars:·gc.count 128 avgt 15 116.000 counts
>> StringBuilders.toStringWithMixedChars:·gc.time 128 avgt 15 86.000 ms
>> StringBuilders.toStringWithMixedChars 256 avgt 15 1185.884 ± 137.364 ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate 256 avgt 15 746.179 ± 92.534 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm 256 avgt 15 1376.244 ± 0.125 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space 256 avgt 15 579.137 ± 208.636 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm 256 avgt 15 1055.150 ± 307.010 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space 256 avgt 15 0.002 ± 0.002 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm 256 avgt 15 0.003 ± 0.003 B/op
>> StringBuilders.toStringWithMixedChars:·gc.count 256 avgt 15 126.000 counts
>> StringBuilders.toStringWithMixedChars:·gc.time 256 avgt 15 97.000 ms
>> StringBuilders.toStringWithMixedChars 1024 avgt 15 4220.415 ± 440.169 ns/op
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate 1024 avgt 15 791.945 ± 75.231 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.alloc.rate.norm 1024 avgt 15 5217.435 ± 0.543 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space 1024 avgt 15 657.270 ± 235.803 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Eden_Space.norm 1024 avgt 15 4232.470 ± 1267.388 B/op
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space 1024 avgt 15 0.005 ± 0.002 MB/sec
>> StringBuilders.toStringWithMixedChars:·gc.churn.G1_Survivor_Space.norm 1024 avgt 15 0.033 ± 0.014 B/op
>> StringBuilders.toStringWithMixedChars:·gc.count 1024 avgt 15 202.000 counts
>> StringBuilders.toStringWithMixedChars:·gc.time 1024 avgt 15 121.000 ms
>
> Xin Liu has updated the pull request incrementally with one additional commit since the last revision:
>
> Change growOnly to maybeLatin.
>
> This patch also copys over the attribute from the other AbstractStringBuilder.
> Add a unit test to cover methods which cause maybeLatin1 becomes true.
Code changes look ok overall. We might inflate instance size of `StringBuilder`s a bit (could be examined via https://openjdk.java.net/projects/code-tools/jol/), but since SBs are typically short-lived entities I'm not too concerned about a few extra bytes here.
The microbenchmark needs a small refactoring to avoid running micros that don't care about the newly added `MIXED_SIZE` `@Param` multiple times.
src/java.base/share/classes/java/lang/AbstractStringBuilder.java line 540:
> 538: }
> 539: StringUTF16.putCharSB(value, index, ch);
> 540: maybeLatin1 = true;
Only possible if `StringLatin1.canEncode(ch) == true` (and strictly speaking if the char at `index` is not latin1-encodable). Considering the overheads of adding a test it might be preferable to be conservative and just always mark it as a "maybe" like you do, though.
src/java.base/share/classes/java/lang/AbstractStringBuilder.java line 1008:
> 1006: this.count = newCount;
> 1007: putStringAt(start, str);
> 1008: if (end - start > 0) {
regardless of value of `end - start` you could also skip setting `maybeLatin1 = true` if:
- `str.coder() == UTF16`
- `this.coder == LATIN1`
test/micro/org/openjdk/bench/java/lang/StringBuilders.java line 208:
> 206: }
> 207:
> 208: @Param({"128", "256", "1024"})
I think adding this parameter at this level means all other benchmarks will now be redundantly run once per different value for MIXED_SIZE. You can avoid this by refactor this parameter and the associated Benchmark into subclass.
-------------
Changes requested by redestad (Reviewer).
PR: https://git.openjdk.java.net/jdk/pull/7671
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