On Tue, 24 Mar 2026 04:34:21 GMT, David Holmes <[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 | —... > > 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); > > Surprised this would make any difference. In itself, doesn't. But it allows the raw backing char* to be used, instead of having to convert© the string to a NULL-terminated string, which helps. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/30374#discussion_r2980557392
