On Thu, 28 Jul 2022 00:48:32 GMT, Carter Kozak <d...@openjdk.org> wrote:
>> I would like to contribute an optimized version of
>> `StackTraceElement#toString()` that uses a single StringBuilder throughout
>> creation to avoid intermediate `String` allocations.
>> `StackTraceElement#toString()` is used in a number of JDK code paths
>> including `Throwable#printStackTrace()`, as well as many JDK consumers may
>> transform `StackTraceElement` `toString()` in logging frameworks capturing
>> throwables and exceptions, and diagnostics performing dumps.
>>
>> Given this usage and some observed JFR profiles from production services,
>> I'd like to reduce the intermediate allocations to reduce CPU pressure in
>> these circumstances. I have added a couple benchmarks for a sample
>> `Throwable#printStackTrace()` converted to String via `StringWriter` and
>> individual `StackTraceElement` `toString`. The former shows ~15%
>> improvement, while the latter shows ~40% improvement.
>>
>> Before
>>
>> Benchmark Mode Cnt Score Error
>> Units
>> StackTraceElementBench.printStackTrace avgt 15 167147.066 ± 4260.521
>> ns/op
>> StackTraceElementBench.toString avgt 15 132.781 ± 2.095
>> ns/op
>>
>>
>> After
>>
>> Benchmark Mode Cnt Score Error
>> Units
>> StackTraceElementBench.printStackTrace avgt 15 142909.133 ± 2290.720
>> ns/op
>> StackTraceElementBench.toString avgt 15 78.939 ± 0.469
>> ns/op
>
> src/java.base/share/classes/java/lang/StackTraceElement.java line 374:
>
>> 372: * @throws IOException If an I/O error occurs
>> 373: */
>> 374: private void appendTo(Appendable dest) throws IOException {
>
> Perhaps this could be package-private for reuse by
> `Throwable.printStackTrace`, avoiding the intermediate toString.
Yes, I think the `PrintStreamOrWriter` could implement `Appendable` as both
`PrintStream` and `PrintWriter` implement `Appendable`.
https://github.com/openjdk/jdk/blob/348a0521e1cd602c4093955310f838cf4ce4daae/src/java.base/share/classes/java/lang/Throwable.java#L756
This would also potentially allow optimizing away the additional intermediate
strings from prefixing the lines with `"\tat "` concatenated with the
`StackTraceElement.toString()`
https://github.com/openjdk/jdk/blob/348a0521e1cd602c4093955310f838cf4ce4daae/src/java.base/share/classes/java/lang/Throwable.java#L686-L687
https://github.com/openjdk/jdk/blob/348a0521e1cd602c4093955310f838cf4ce4daae/src/java.base/share/classes/java/lang/Throwable.java#L724-L725
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PR: https://git.openjdk.org/jdk/pull/9665
