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https://issues.apache.org/jira/browse/LOG4J2-1120?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14742443#comment-14742443
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Remko Popma commented on LOG4J2-1120:
-------------------------------------

Benchmark results: (iterMapXElement iterates over a ConcurrentHashMap 
containing X elements, iterArraySet... iterates over a CopyOnWriteArraySet)

*1 thread*
{code}
Benchmark                                                        Mode  Samples  
  Score   Error  Units
o.a.l.l.p.j.LoggerConfigBenchmark.iterArraySet1Element         sample   115366  
 34.429 ± 1.386  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.iterArraySet3Element         sample   193159  
 31.368 ± 1.041  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.iterMap1Element              sample   108869  
 79.615 ± 2.133  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.iterMap3Elements             sample   184664  
 67.984 ± 1.660  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.logWithCountersAndLock       sample   103840  
139.756 ± 2.643  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.logWithCountersNoLocks       sample   143068  
102.707 ± 2.006  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.logWithoutCountersOrLocks    sample   172408  
 84.748 ± 1.723  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.testBaseline                 sample   101878  
 19.656 ± 1.136  ns/op
{code}

*4 threads*
{code}
Benchmark                                                        Mode  Samples  
  Score    Error  Units
o.a.l.l.p.j.LoggerConfigBenchmark.iterArraySet1Element         sample   487222  
 74.218 ±  0.938  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.iterArraySet3Element         sample   391496  
 52.295 ±  0.915  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.iterMap1Element              sample   456270  
130.438 ±  1.790  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.iterMap3Elements             sample   393511  
137.019 ±  4.921  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.logWithCountersAndLock       sample   466650  
387.175 ±  6.982  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.logWithCountersNoLocks       sample   583383  
314.528 ±  2.917  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.logWithoutCountersOrLocks    sample   425858  
236.417 ± 26.062  ns/op
o.a.l.l.p.j.LoggerConfigBenchmark.testBaseline                 sample   708875  
 55.932 ±  0.732  ns/op
{code}

Iterating over a CopyOnWriteArraySet is roughly twice as fast as over 
ConcurrentHashMap.values(), both in single-threaded and multi-threaded 
scenarios.

Avoiding lock acquisition is faster, but not reading/updating the counters can 
make it even faster.

> LoggerConfig performance improvements
> -------------------------------------
>
>                 Key: LOG4J2-1120
>                 URL: https://issues.apache.org/jira/browse/LOG4J2-1120
>             Project: Log4j 2
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 2.3
>            Reporter: Remko Popma
>            Assignee: Remko Popma
>             Fix For: 2.4
>
>
> As discussed on the mailing list, pulling up the the {{shutdown.get()}} check 
> into {{afterLogEvent()}} will avoid acquiring the shutdownLock unnecessarily.
> Another performance improvement can be made by changing the data structure 
> that holds the AppenderControl objects from ConcurrentHashMap to 
> CopyOnWriteArraySet. Modifications on this data structure are extremely rare, 
> and the array-based collection has much better iteration performance.



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