jengebr commented on issue #3106:
URL:
https://github.com/apache/logging-log4j2/issues/3106#issuecomment-2438548710
## Problem
Data shows 2.24.x has nearly zero object allocation during the benchmark,
but 2.25.0 has far more than that. The allocation-heavy code in 2.25.x also
comes with significant cpu load.
**Caveat**: I'm not 100% certain I reproduced the specific issue reported.
## Data
1. Async-profiler data shows that the overall hot path
is`org.apache.logging.log4j.core.pattern.ThrowableExtendedStackTraceRenderer$ExtendedContext.lambda$createClassResourceInfoByName$0`.
Put another way,
`ThrowableExtendedStackTraceRenderer.createClassResourceInfoByName()`.
2. The benchmark (attached below) shows the old code allocates almost zero
objects but the new code allocates rather more than that.
3. The benchmark shows the old code is roughly 2x faster than the new,
although that number may vary by system.
## Analysis
Examination of `createClassResourceInfoByName` shows a very large amount of
object allocation right at the beginning:
```
final Map<String, ClassResourceInfo> classResourceInfoByName =
StackLocatorUtil.getCurrentStackTrace().stream()
.collect(Collectors.toMap(
Class::getName,
clazz -> new
ClassResourceInfo(clazz, true),
(classResourceInfo1,
classResourceInfo2) -> classResourceInfo1));
final Set<Throwable> visitedThrowables = new HashSet<>();
final Queue<Throwable> pendingThrowables = new
ArrayDeque<>(Collections.singleton(rootThrowable));
```
... but what doesn't jump out is that there is a `new ClassResourceInfo()`
for each item in the stack trace. Diving into that constructor shows an
indirect call to `String.format` - which is a very expensive and object-heavy
convenience method.
## Recommendations
1. Do whatever it takes to eliminate `String.format` and any per-element
object allocations.
2. Reduce the allocations from the `HashSet` and `ArrayDeque`.
## Benchmark
My benchmark is attached, and should be run with `-Xmx1g -Xms1g
-XX:+UseParallelGC`. It definitely reproduces *a* problem but I'm not certain
it's exactly the one that was reported.... please let me know if I'm off.
Config file:
[log4j2.xml.txt](https://github.com/user-attachments/files/17525924/log4j2.xml.txt)
Benchmark:
[Log4jExceptionSpeedTest.java.txt](https://github.com/user-attachments/files/17525920/Log4jExceptionSpeedTest.java.txt)
Support class for the benchmark:
[TestRunner.java.txt](https://github.com/user-attachments/files/17525921/TestRunner.java.txt)
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