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https://issues.apache.org/jira/browse/LOG4J2-1434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15334828#comment-15334828
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Luke Butters edited comment on LOG4J2-1434 at 6/16/16 11:03 PM:
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-Could you elaborate on why the issue is only one if it occurs under tomcat? If
i don't use tomcat should I switch to a different logging framework?- No need
Ralph points out I have created confusion by referencing tickets which only
related to tomcat. To simplify replication I am not running a web server to
demonstrate the issue.
I think we are talking about different issues. Perhaps the term memory leak is
incorrect and a better wording would have been "StringBuffer in ThreadLocal
causes excessive memory usage after large log messages". I think in your case
you actually had a leaking where in this case the ThreadLocal was created
correctly however the object held by the thread local can only grow.
The example code demonstrates this. As the threads are kept around (e.g. from
being re-used) the large StringBuffer is never cleaned up. So we can and end up
in the case where 100 threads each have a StringBuffer which is large enough to
hold 10M chars. This results in the log framework taking up a significant
portion of memory.
I think it is worthwhile trying out the above code, if the logging framework
has no issues then you will not run out of memory (I only gave my JVM 640M but
we can increase the number of threads to make the problem occur) . Switching
the log line to something like {{largeString.length()}} resulted in no memory
issues, confirming the memory issue is in the log framework.
Having a look at your code base it seems you re-use getStringBuffer() which is
good, however I suspect it will make fixing this issue even harder as the
memory issue will be all over your application. Unless you know of a single
spot where the size of the StringBuffer can be checked and the StringBuffer be
shrunk or removed if it is too large.
was (Author: lukebutters7):
-Could you elaborate on why the issue is only one if it occurs under tomcat? If
i don't use tomcat should I switch to a different logging framework?- No need
Ralph points out I have created confusion by referencing tickets which only
related to tomcat. To simplify replication I am not running a web server to
demonstrate the issue.
I think we are talking about different issues. Perhaps the term memory leak is
incorrect and a better wording would have been "StringBuffer in ThreadLocal
causes excessive memory usage after large log messages". I think in your case
you actually had a leaking where in this case the ThreadLocal was created
correctly however the object held by the thread local can only grow.
The example code demonstrates this. As the threads are kept around (e.g. from
being re-used) the large StringBuffer is never cleaned up. So we can and end up
in the case where 100 threads each have a StringBuffer which is large enough to
hold 10M chars. This results in the log framework taking up a significant
portion of memory.
I think it is worthwhile trying out the above code, if the logging framework
has no issues then you will not run out of memory (I only gave my JVM 640M but
we can increase the number of threads to make the problem occur) . Switching
the log line to something like {{largeString.length()}} resulted in no memory
issues, confirming the memory issue is in the log framework.
Having a look at your code base it seems you re-use getStringBuffer() which is
good, however I suspect it will make fixing this issue even harder as the
memory issue will be all over your application. Unless you know of a spot where
the StringBuffer's internal array can be shrunk or the StringBuffer can be
removed if it gets too large.
> 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
>
> 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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