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From "Luke Butters (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LOG4J2-1434) StringBuffer in ThreadLocal can cause excisive memory usage after large log messages
Date Thu, 16 Jun 2016 22:54:05 GMT

    [ https://issues.apache.org/jira/browse/LOG4J2-1434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15334883#comment-15334883
] 

Luke Butters commented on LOG4J2-1434:
--------------------------------------

I will remove the relation to that issue, I guess it is only related to LOG4J2-1125 as that
ticket introduced the ThreadLocal for the speed up.

> StringBuffer in ThreadLocal can cause excisive 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
>
> In an effort to speed up logging ThreadLocals have been introduced see LOG4J2-1125 however
this does causes memory issues reported http://stackoverflow.com/questions/32867327/log4j2-threadlocal-memory-leak-in-servletcontextlistener
and subsequently ignored in LOG4J2-1142
> 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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