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https://issues.apache.org/jira/browse/LOG4J2-1434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15338441#comment-15338441
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Mikael Ståldal commented on LOG4J2-1434:
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After a second thought, not turn off all ThreadLocals (not ThreadContext). But
rather to turn all ThreadLocals which are used to achieve garbage free logging.
And the reason is that they increase memory usage.
The garbage free logging comes with a cost of increased memory usage. Some
users who do not need garbage free logging may not want to pay that cost.
> StringBuffer in ThreadLocal can cause excessive memory usage after large log
> messages
> -------------------------------------------------------------------------------------
>
> Key: LOG4J2-1434
> URL: https://issues.apache.org/jira/browse/LOG4J2-1434
> Project: Log4j 2
> Issue Type: Bug
> Affects Versions: 2.6.1
> Reporter: Luke Butters
> Assignee: Remko Popma
> Fix For: 2.6.2
>
>
> In an effort to speed up logging ThreadLocals have been introduced see
> LOG4J2-1125 however this does causes memory issues.
> The problem of the ThreadLocal occurs when threads are re-used which is an
> absolutely valid way of using java. For example an executor service can
> re-use threads as well as Jetty.
> Below I demonstrate a contrived example of the memory leak:
> {code}
> int stringSize = 1024*1024*10; //~10MB maybe 20MB for UTF-16
> StringBuilder sb = new StringBuilder(stringSize);
> for(int i = 0; i < stringSize; i++) {
> sb.append('a' + i % 5);
> }
>
> String largeString = sb.toString();
>
> sb = null; //Let it be GC'ed
> ExecutorService es = Executors.newFixedThreadPool(100);
> final CountDownLatch countDownLatch = new CountDownLatch(100);
> for(int i = 0; i < 100; i++) {
> es.execute(()-> {
> //Log the big string to demonstrate the issue.
> log.fatal(largeString);
>
> //Ensure we use all 100 of our threads by not releasing this
> thread yet.
> countDownLatch.countDown();
> });
>
> //We sleep for 2s so we more easily watch memory growth
> Thread.sleep(2000);
> }
> {code}
> I recommend that log4j2 immediately remove the ThreadLocal as a small gain in
> performance does not outweigh the problems associated with memory leaks.
> Finally other options for caching the StringBuilder with a ThreadLocal could
> be considered for example we might re-use StringBuilders that are no larger
> than 3k while removing the ones which are larger than 3k.
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