jengebr opened a new issue, #3935:
URL: https://github.com/apache/logging-log4j2/issues/3935
## Description
`HashMap.<init>(Map)` is slow because it contains polymorphic calls to
`Map.entrySet(`), `Set.iterator()`, `Iterator.hasNext()`, and
`Iterator.next()`. These calls are megamorphic and the JIT cannot optimize
them away, so the virtual method calls are quite expensive. I have attached a
JMH test that demonstrates this general problem.
Profiling of multiple large, efficiency-critical applications shows that
log4j contains one instance of this that is a hotspot in *all* of the
applications: `DefaultThreadContextMap.getCopy()`. The call path is
`CloseableThreadContext$Instance.putAll()` ->
`org.apache.logging.log4j.ThreadContext.getContext()` ->
`org.apache.logging.log4j.spi.DefaultThreadContextMap.getCopy()`. The line of
code is `return new HashMap<>(getMap(state));`.
This can be trivially improved by unrolling the loop:
```
Map<String, String> map = getMap(state);
HashMap<String, String> copy = new HashMap<>((int)(map.size() *
1.35));
for (Map.Entry<String, String> entry : map.entrySet()) {
copy.put(entry.getKey(), entry.getValue());
}
return copy;
```
Benchmark before:
```
Benchmark (mapSize) Mode Cnt Score
Error Units
DefaultThreadContextMapCopyBenchmark.copyMap 5 avgt 5 271.915
± 3.054 ns/op
```
Benchmark after:
```
Benchmark (mapSize) Mode Cnt Score
Error Units
DefaultThreadContextMapCopyBenchmark.copyMap 5 avgt 5 129.784
± 3.412 ns/op
```
## Configuration
## Logs
## Reproduction
Two JMH tests attached, one showing the general effect and one demonstrating
this opportunity.
[DefaultThreadContextMapCopyBenchmark.java](https://github.com/user-attachments/files/22432669/DefaultThreadContextMapCopyBenchmark.java)
[HashMapConstructorBenchmark.java](https://github.com/user-attachments/files/22432672/HashMapConstructorBenchmark.java)
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