[jira] [Resolved] (LOG4J2-1434) StringBuffer in ThreadLocal can cause excessive memory usage after large log messages

["Remko Popma (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/LOG4J2-1434?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Remko Popma resolved LOG4J2-1434.
---------------------------------
       Resolution: Fixed
    Fix Version/s: 2.6.2

Fixed in master. Added system property {{log4j.layoutStringBuilder.maxSize}} to control the maximum size of the buffer.

This may not be a perfect solution, but should cover the vast majority. Please verify and close.

> 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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