On Mon, 23 Mar 2026 15:39:23 GMT, Roman Kennke <[email protected]> wrote:
> I analyzed the performance of Thread.setName() in response to a customer
> workload running Cassandra, where Thread.setName() showed up (mostly because
> of rather pathetic use of setName() from Cassanda *sigh*).
>
> Profiling showed that most time (around 75%) is spent in the actual syscall,
> so there are limits on what we can do. There is some fixed overheads like the
> cost for synchronized, and also some costs that scale with the length of the
> name, most importantly the UTF8 conversion.
>
> I implemented the following improvements:
> - Removed synchronized from setName(), as suggested by some folks in the JBS
> issue. This saves ~15 nanoseconds. Not sure if the method could be called
> contended by Cassandra, if so, the savings might be much larger.
> - Almost all thread names are Latin1/ASCII, and there is no need to convert
> to UTF8 in that case. Also, the various OS APIs to set the thread name don't
> even seem to specify the character encoding. Avoiding the UTF8 conversion
> brings down the length-dependent costs. In many cases we can also pass down
> the backing array of the string and avoid copying.
> - When the name doesn't change, we can skip updating the native name, which
> makes setName() almost a no-op.
> - For truncating the name on Linux to 16 chars, instead of using snprintf
> with a pattern, we can simply stitch together the name directly (first 7
> chars, last 6 chars, 2 dots in between), this saves ~100ns.
>
> In the end, we bring down performance for the small cases by ~7%, longer
> names by ~20% and completely removed the conversion overhead that primarily
> affected longer names.
>
> | Benchmark | (length) | Baseline (ns/op) | Optimized (ns/op) | Change
> |
>
> |---------------|----------|------------------:|-------------------:|--------:|
> | setName | 1 | 602.3 ± 2.0 | 561.9 ± 1.5 |
> -6.7% |
> | setName | 4 | 605.9 ± 2.1 | 570.2 ± 1.2 |
> -5.9% |
> | setName | 15 | 617.1 ± 2.7 | 570.4 ± 2.8 |
> -7.6% |
> | setName | 16 | 712.1 ± 6.0 | 569.4 ± 2.7 |
> -20.0% |
> | setName | 50 | 757.9 ± 5.2 | 566.3 ± 4.6 |
> -25.3% |
> | setName | 200 | 986.2 ± 2.7 | 569.9 ± 4.9 |
> -42.2% |
> | setNameSame | 1 | — | 7.4 ± 0.0 | —
> |
> | setNameSame | 4 | — | 7.4 ± 0.0 | —
> |
> | setNameSame | 15 | — | 7.4 ± 0.0 | —
> |
> | setN...
Some drive-by comments.
src/hotspot/os/bsd/os_bsd.cpp line 2261:
> 2259: // Add a "Java: " prefix to the name
> 2260: char buf[MAXTHREADNAMESIZE];
> 2261: (void) os::snprintf(buf, sizeof(buf), "Java: %.*s", (int)len, name);
You can get rid of the snprintf completely. Cap copy length to
min2(MAXTHREADNAMESIZE, len) and memcpy. With some adjustments for the leading
"Java" of course.
src/java.base/share/classes/java/lang/Thread.java line 1800:
> 1798: this.name = name;
> 1799: if (!isVirtual() && Thread.currentThread() == this &&
> !name.equals(oldName)) {
> 1800: setNativeName(name);
For Linux, you only need to compare the first 16 chars.
For MacOS, just the first 64 (MAXTHREADNAMESIZE).
-------------
PR Review: https://git.openjdk.org/jdk/pull/30374#pullrequestreview-3998117529
PR Review Comment: https://git.openjdk.org/jdk/pull/30374#discussion_r2980669159
PR Review Comment: https://git.openjdk.org/jdk/pull/30374#discussion_r2980632008
