On Tue, 31 Mar 2026 14:21:20 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.
>>
>> I implemented the following improvements:
>> - 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
>> (if possible) brings down the length-dependent costs. In many cases we can
>> also pass down the backing array of the string and avoid copying altogether.
>> - 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 | —
>> |
>> | setNameSame | 16 | — | 7.4 ± 0.0 | —
>> |
>> | setNameSame | 50 | — | 7.4 ± 0.0 | —
>> |
>> | setNameSame | 200 | — | 7.4 ± 0.0 | —
>> |
>>
>>
>> Testing:
>> - [x] tier1
>> - [ ] tier2
>>
>> The failing GHA test in langtools seems unrelated.
>
> Roman Kennke has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Improve truncation code on BSD and Windows, add comment to Linux
A couple of additional suggestions but tacit approval in any case.
Thanks
src/hotspot/os/bsd/os_bsd.cpp line 2265:
> 2263: size_t name_len = MIN2(len, sizeof(buf) - 7);
> 2264: memcpy(buf + 6, name, name_len);
> 2265: buf[6 + name_len] = '\0';
Suggestion:
const char* prefix = "Java: ";
const size_t prefix_len = strlen(prefix);
memcpy(buf, "Java: ", prefix_len);
size_t name_len = MIN2(len, sizeof(buf) - (prefix_len + 1));
memcpy(buf + prefix_len, name, name_len);
buf[prefix_len + name_len] = '\0';
src/hotspot/os/windows/os_windows.cpp line 1069:
> 1067: // Windows APIs require NUL-terminated strings; the name pointer
> 1068: // may not be NUL-terminated, so copy into a stringStream.
> 1069: stringStream ss;
Suggestion:
stringStream ss(len);
We can avoid the need to grow on the write call. (Not sure if we need a +1 ?)
-------------
Marked as reviewed by dholmes (Reviewer).
PR Review: https://git.openjdk.org/jdk/pull/30374#pullrequestreview-4041630537
PR Review Comment: https://git.openjdk.org/jdk/pull/30374#discussion_r3019732455
PR Review Comment: https://git.openjdk.org/jdk/pull/30374#discussion_r3019748062
