<div dir="ltr"><div>>
output labeled as
StringBuffer but the jmh creates StringBuilder.<br></div><div><br></div><div>Ugh - sorry about that. But yes - this is about StringBuilder vs String.</div><div><br></div><div>> I would not be surprised that C2 has more optimizations for
String than for StringBuilder.</div><div><br></div><div>If that were true, it would not surprise me. However, these tests show the opposite. String is /slower/ than StringBuilder.</div></div><br><div class="gmail_quote gmail_quote_container"><div dir="ltr" class="gmail_attr">On Mon, Jul 21, 2025 at 12:34 PM Roger Riggs <<a href="mailto:roger.riggs@oracle.com">roger.riggs@oracle.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><u></u>
<div>
Hi Brett,<br>
<br>
The labeling of the output is confusing, the test output labeled as
StringBuffer but the jmh creates StringBuilder.<br>
(StringBuffer methods are all synchronized and could explain why
they are slower).<br>
<br>
Also, I would not be surprised that C2 has more optimizations for
String than for StringBuilder.<br>
<br>
Regards, Roger<br>
<br>
<div>On 7/19/25 6:09 PM, Brett Okken wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">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?<br>
<br>
<span style="font-family:monospace">jdk 11<br>
<br>
Benchmark (data) (source) Mode Cnt Score
Error Units<br>
test ascii String avgt 3 1137.348 ±
12.835 ns/op<br>
test ascii StringBuffer avgt 3 712.874 ±
509.320 ns/op<br>
test non-ascii String avgt 3 668.657 ±
246.550 ns/op<br>
test non-ascii StringBuffer avgt 3 897.344 ±
4353.414 ns/op<br>
<br>
<br>
jdk 17<br>
Benchmark (data) (source) Mode Cnt Score
Error Units<br>
test ascii String avgt 3 1321.497 ±
2107.466 ns/op<br>
test ascii StringBuffer avgt 3 715.936 ±
412.189 ns/op<br>
test non-ascii String avgt 3 722.986 ±
443.389 ns/op<br>
test non-ascii StringBuffer avgt 3 722.787 ±
771.816 ns/op</span><span style="font-family:monospace"><br>
<br>
<br>
jdk 21<br>
Benchmark (data) (source) Mode Cnt Score
Error Units<br>
test ascii String avgt 3 1150.301 ┬▒
918.549 ns/op<br>
test ascii StringBuffer avgt 3 713.183 ┬▒
543.850 ns/op<br>
test non-ascii String avgt 3 4642.667 ┬▒
11481.029 ns/op<br>
test non-ascii StringBuffer avgt 3 728.027 ┬▒
936.521 ns/op<br>
<br>
<br>
jdk 25<br>
Benchmark (data) (source) Mode Cnt Score
Error Units<br>
test ascii String avgt 3 1184.513 ┬▒
2057.498 ns/op<br>
test ascii StringBuffer avgt 3 786.611 ┬▒
411.657 ns/op<br>
test non-ascii String avgt 3 4197.585 ┬▒
2761.388 ns/op<br>
test non-ascii StringBuffer avgt 3 716.375 ┬▒
815.349 ns/op<br>
<br>
<br>
jdk 26<br>
Benchmark (data) (source) Mode Cnt Score Error
Units<br>
test ascii String avgt 3 1107.207 ┬▒
423.072 ns/op<br>
test ascii StringBuffer avgt 3 742.780 ┬▒
178.890 ns/op<br>
test non-ascii String avgt 3 4043.914 ┬▒
498.439 ns/op<br>
test non-ascii StringBuffer avgt 3 712.535 ┬▒
583.255 ns/op</span><br>
</div>
<br>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Sat, Jul 19, 2025 at
4:17 PM Chen Liang <<a href="mailto:liangchenblue@gmail.com" target="_blank">liangchenblue@gmail.com</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div dir="ltr">
<div dir="ltr">Without looking at C2 IRs, I think there are
a few potential culprits we can look into:</div>
<div>1. JDK-8351000 and JDK-8351443 updated StringBuilder</div>
<div>2. Sequence field is read in the loop; I wonder if
making it an explicit immutable local variable changes
anything here.</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Sat, Jul 19, 2025 at
2:34 PM Brett Okken <<a href="mailto:brett.okken.os@gmail.com" target="_blank">brett.okken.os@gmail.com</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">I
was looking at the performance of StringCharBuffer for
various<br>
backing CharSequence types and was surprised to see a
significant<br>
performance difference between String and StringBuffer.
I wrote a<br>
small jmh which shows that the String implementation of
charAt is<br>
significantly slower than StringBuilder. Is this
expected?<br>
<br>
Benchmark (data)
(source) Mode Cnt<br>
Score Error Units<br>
CharSequenceCharAtBenchmark.test ascii
String avgt 3<br>
2537.311 ┬▒ 8952.197 ns/op<br>
CharSequenceCharAtBenchmark.test ascii
StringBuffer avgt 3<br>
852.004 ┬▒ 2532.958 ns/op<br>
CharSequenceCharAtBenchmark.test non-ascii
String avgt 3<br>
5115.381 ┬▒ 13822.592 ns/op<br>
CharSequenceCharAtBenchmark.test non-ascii
StringBuffer avgt 3<br>
836.230 ┬▒ 1154.191 ns/op<br>
<br>
<br>
<br>
@Measurement(iterations = 3, time = 5, timeUnit =
TimeUnit.SECONDS)<br>
@Warmup(iterations = 2, time = 7, timeUnit =
TimeUnit.SECONDS)<br>
@BenchmarkMode(Mode.AverageTime)<br>
@OutputTimeUnit(TimeUnit.NANOSECONDS)<br>
@State(Scope.Benchmark)<br>
@Fork(value = 1, jvmArgsPrepend = {"-Xms512M",
"-Xmx512M"})<br>
public class CharSequenceCharAtBenchmark {<br>
<br>
@Param(value = {"ascii", "non-ascii"})<br>
public String data;<br>
<br>
@Param(value = {"String", "StringBuffer"})<br>
public String source;<br>
<br>
private CharSequence sequence;<br>
<br>
@Setup(Level.Trial)<br>
public void setup() throws Exception {<br>
StringBuilder sb = new StringBuilder(3152);<br>
for (int i=0; i<3152; ++i) {<br>
char c = (char) i;<br>
if ("ascii".equals(data)) {<br>
c = (char) (i & 0x7f);<br>
}<br>
sb.append(c);<br>
}<br>
<br>
switch(source) {<br>
case "String":<br>
sequence = sb.toString();<br>
break;<br>
case "StringBuffer":<br>
sequence = sb;<br>
break;<br>
default:<br>
throw new
IllegalArgumentException(source);<br>
}<br>
}<br>
<br>
@Benchmark<br>
public int test() {<br>
int sum = 0;<br>
for (int i=0, j=sequence.length(); i<j; ++i)
{<br>
sum += sequence.charAt(i);<br>
}<br>
return sum;<br>
}<br>
}<br>
</blockquote>
</div>
</div>
</blockquote>
</div>
</blockquote>
<br>
</div>
</blockquote></div>