On Wed, 23 Mar 2022 00:35:14 GMT, Xin Liu <x...@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
>> ...
>
> Xin Liu has updated the pull request incrementally with one additional commit
> since the last revision:
>
> rewording the comment of 'maybeLatin1'.
This introduced a bug where the following now incorrectly returns `false` with
`‑XX:+CompactStrings` as it produces an inflated `String` containing only
Latin1 characters:
new StringBuilder()
// insert Latin1 sub-slice "foo" of UTF-16 string:
.insert(0, "foo \uF000", 0, 3)
.toString()
.equals("foo");
Such a `String` can cause many weird bugs.
-------------
PR Comment: https://git.openjdk.org/jdk/pull/7671#issuecomment-2919134074
