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https://issues.apache.org/jira/browse/LOG4J2-1434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15337380#comment-15337380
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Remko Popma commented on LOG4J2-1434:
-------------------------------------
Where possible I prefer not to store non-JDK classes in ThreadLocals, but I
think the try with resources is a good idea.
I really want to avoid creating temporary objects during steady state logging.
I wonder if this will work:
{code}
// AbstractStringLayout
private static final ThreadLocal<StringBuilder> threadLocal = new
ThreadLocal<>();
protected static final Closeable stringBuilderCleaner = new Closeable() {
public void close() {
StringBuilder sb = threadLocal.get();
if (sb.length() > MAX_LENGTH) {
sb.setLength(MAX_LENGTH);
sb.trimToSize();
}
}
}
// all code interested in getting the cached StringBuilder needs to go
through this accessor method
protected static StringBuilder getStringBuilder() {
// as current
....
{code}
Then in Layout code we do
{code}
// some layout
@Override
public byte[] toByteArray(final LogEvent event) {
try (this.stringBuilderCleaner) {
StringBuilder sb = this.getStringBuilder();
final StringBuilder text = toText(event, sb, false);
final byte[] bytes = getBytes(text.toString());
return compressionType != CompressionType.OFF && bytes.length >
compressionThreshold ? compress(bytes) : bytes;
}
}
{code}
Thoughts?
> 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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