> output labeled as StringBuffer but the jmh creates StringBuilder.
Ugh - sorry about that. But yes - this is about StringBuilder vs String.
> I would not be surprised that C2 has more optimizations for String than
for StringBuilder.
If that were true, it would not surprise me. However, these tests show the
opposite. String is /slower/ than StringBuilder.
On Mon, Jul 21, 2025 at 12:34 PM Roger Riggs <roger.ri...@oracle.com> wrote:
> Hi Brett,
>
> The labeling of the output is confusing, the test output labeled as
> StringBuffer but the jmh creates StringBuilder.
> (StringBuffer methods are all synchronized and could explain why they are
> slower).
>
> Also, I would not be surprised that C2 has more optimizations for String
> than for StringBuilder.
>
> Regards, Roger
>
> On 7/19/25 6:09 PM, Brett Okken wrote:
>
> Making sequence a local variable does improve things (especially for
> ascii), but a substantial difference remains. It appears that the
> performance difference for ascii goes all the way back to jdk 11. The
> difference for non-ascii showed up in jdk 21. I wonder if this is related
> to the index checks?
>
> jdk 11
>
> Benchmark (data) (source) Mode Cnt Score Error Units
> test ascii String avgt 3 1137.348 ± 12.835 ns/op
> test ascii StringBuffer avgt 3 712.874 ± 509.320 ns/op
> test non-ascii String avgt 3 668.657 ± 246.550 ns/op
> test non-ascii StringBuffer avgt 3 897.344 ± 4353.414 ns/op
>
>
> jdk 17
> Benchmark (data) (source) Mode Cnt Score Error Units
> test ascii String avgt 3 1321.497 ± 2107.466 ns/op
> test ascii StringBuffer avgt 3 715.936 ± 412.189 ns/op
> test non-ascii String avgt 3 722.986 ± 443.389 ns/op
> test non-ascii StringBuffer avgt 3 722.787 ± 771.816 ns/op
>
>
> jdk 21
> Benchmark (data) (source) Mode Cnt Score Error Units
> test ascii String avgt 3 1150.301 ┬▒ 918.549 ns/op
> test ascii StringBuffer avgt 3 713.183 ┬▒ 543.850 ns/op
> test non-ascii String avgt 3 4642.667 ┬▒ 11481.029 ns/op
> test non-ascii StringBuffer avgt 3 728.027 ┬▒ 936.521 ns/op
>
>
> jdk 25
> Benchmark (data) (source) Mode Cnt Score Error Units
> test ascii String avgt 3 1184.513 ┬▒ 2057.498 ns/op
> test ascii StringBuffer avgt 3 786.611 ┬▒ 411.657 ns/op
> test non-ascii String avgt 3 4197.585 ┬▒ 2761.388 ns/op
> test non-ascii StringBuffer avgt 3 716.375 ┬▒ 815.349 ns/op
>
>
> jdk 26
> Benchmark (data) (source) Mode Cnt Score Error Units
> test ascii String avgt 3 1107.207 ┬▒ 423.072 ns/op
> test ascii StringBuffer avgt 3 742.780 ┬▒ 178.890 ns/op
> test non-ascii String avgt 3 4043.914 ┬▒ 498.439 ns/op
> test non-ascii StringBuffer avgt 3 712.535 ┬▒ 583.255 ns/op
>
>
> On Sat, Jul 19, 2025 at 4:17 PM Chen Liang <liangchenb...@gmail.com>
> wrote:
>
>> Without looking at C2 IRs, I think there are a few potential culprits we
>> can look into:
>> 1. JDK-8351000 and JDK-8351443 updated StringBuilder
>> 2. Sequence field is read in the loop; I wonder if making it an explicit
>> immutable local variable changes anything here.
>>
>> On Sat, Jul 19, 2025 at 2:34 PM Brett Okken <brett.okken...@gmail.com>
>> wrote:
>>
>>> I was looking at the performance of StringCharBuffer for various
>>> backing CharSequence types and was surprised to see a significant
>>> performance difference between String and StringBuffer. I wrote a
>>> small jmh which shows that the String implementation of charAt is
>>> significantly slower than StringBuilder. Is this expected?
>>>
>>> Benchmark (data) (source) Mode Cnt
>>> Score Error Units
>>> CharSequenceCharAtBenchmark.test ascii String avgt 3
>>> 2537.311 ┬▒ 8952.197 ns/op
>>> CharSequenceCharAtBenchmark.test ascii StringBuffer avgt 3
>>> 852.004 ┬▒ 2532.958 ns/op
>>> CharSequenceCharAtBenchmark.test non-ascii String avgt 3
>>> 5115.381 ┬▒ 13822.592 ns/op
>>> CharSequenceCharAtBenchmark.test non-ascii StringBuffer avgt 3
>>> 836.230 ┬▒ 1154.191 ns/op
>>>
>>>
>>>
>>> @Measurement(iterations = 3, time = 5, timeUnit = TimeUnit.SECONDS)
>>> @Warmup(iterations = 2, time = 7, timeUnit = TimeUnit.SECONDS)
>>> @BenchmarkMode(Mode.AverageTime)
>>> @OutputTimeUnit(TimeUnit.NANOSECONDS)
>>> @State(Scope.Benchmark)
>>> @Fork(value = 1, jvmArgsPrepend = {"-Xms512M", "-Xmx512M"})
>>> public class CharSequenceCharAtBenchmark {
>>>
>>> @Param(value = {"ascii", "non-ascii"})
>>> public String data;
>>>
>>> @Param(value = {"String", "StringBuffer"})
>>> public String source;
>>>
>>> private CharSequence sequence;
>>>
>>> @Setup(Level.Trial)
>>> public void setup() throws Exception {
>>> StringBuilder sb = new StringBuilder(3152);
>>> for (int i=0; i<3152; ++i) {
>>> char c = (char) i;
>>> if ("ascii".equals(data)) {
>>> c = (char) (i & 0x7f);
>>> }
>>> sb.append(c);
>>> }
>>>
>>> switch(source) {
>>> case "String":
>>> sequence = sb.toString();
>>> break;
>>> case "StringBuffer":
>>> sequence = sb;
>>> break;
>>> default:
>>> throw new IllegalArgumentException(source);
>>> }
>>> }
>>>
>>> @Benchmark
>>> public int test() {
>>> int sum = 0;
>>> for (int i=0, j=sequence.length(); i<j; ++i) {
>>> sum += sequence.charAt(i);
>>> }
>>> return sum;
>>> }
>>> }
>>>
>>
>
