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;
}
}
