On Thu, 3 Mar 2022 08:11:19 GMT, Daniel Jeliński <[email protected]> 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
>
> src/java.base/share/classes/java/lang/StringBuffer.java line 715:
>
>> 713: public synchronized String toString() {
>> 714: if (toStringCache == null) {
>> 715: return toStringCache = new String(this);
>
> When you modify a method labeled `IntrinsicCandidate`, you should probably
> modify the intrinsic as well
_StringBuilder_toString and _StringBuffer_toString are intrinsics indeed, but
they are only used for method recognition in c2/StringOpts. C2 doesn't expand
them. if StringOpts misses, C2 will inline these 2 methods.
-------------
PR: https://git.openjdk.java.net/jdk/pull/7671
