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Posted to log4j-dev@logging.apache.org by "Tim Gokcen (JIRA)" <ji...@apache.org> on 2016/10/20 17:34:59 UTC
[jira] [Updated] (LOG4J2-1644) Inefficient locking in
{{AbstractLoggerAdapter}}
[ https://issues.apache.org/jira/browse/LOG4J2-1644?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Tim Gokcen updated LOG4J2-1644:
-------------------------------
Attachment: AbstractLoggerAdapter.patch
Attached is the diff of our patch.
> Inefficient locking in {{AbstractLoggerAdapter}}
> ------------------------------------------------
>
> Key: LOG4J2-1644
> URL: https://issues.apache.org/jira/browse/LOG4J2-1644
> Project: Log4j 2
> Issue Type: Improvement
> Components: API
> Affects Versions: 2.6.1
> Reporter: Tim Gokcen
> Labels: patch
> Attachments: AbstractLoggerAdapter.patch
>
>
> In {{org.apache.logging.log4j.spi.AbstractLoggerAdapter}}, the {{getLoggersInContext}} method has a {{synchronize}} block to prevent concurrent destructive access to the {{registry}}, a {{java.util.WeakHashMap}}:
> {code} public ConcurrentMap<String, L> getLoggersInContext(final LoggerContext context) {
> synchronized (registry) {
> ConcurrentMap<String, L> loggers = registry.get(context);
> if (loggers == null) {
> loggers = new ConcurrentHashMap<>();
> registry.put(context, loggers);
> }
> return loggers;
> }
> }{code}
> However, in the case when loggers are already in the map, the large {{synchronize}} block means that two threads cannot simultaneously read from the map, which hurts performance in highly multi-threaded applications that constantly re-instantiate loggers.
> In our application, we have modified this method to use a ReadWriteLock instead, providing unlimited concurrent {{get()}} operations but blocking {{put()}} operations by using a double-checked locking idiom:
> {code}import java.util.concurrent.locks.ReadWriteLock;
> import java.util.concurrent.locks.ReentrantReadWriteLock;
> (...)
> private final ReadWriteLock lock = new ReentrantReadWriteLock (true);
> (...)
> public ConcurrentMap<String, L> getLoggersInContext(final LoggerContext context) {
> ConcurrentMap<String, L> loggers;
> lock.readLock ().lock ();
> try {
> loggers = registry.get (context);
> } finally {
> lock.readLock ().unlock ();
> }
> if (loggers != null) {
> return loggers;
> } else {
> lock.writeLock ().lock ();
> try {
> loggers = registry.get (context);
> if (loggers == null) {
> loggers = new ConcurrentHashMap<> ();
> registry.put (context, loggers);
> }
> return loggers;
> } finally {
> lock.writeLock ().unlock ();
> }
> }
> }{code}
> The {{ReadWriteLock}} interface and the {{ReentrantReadWriteLock}} implementation have been available since Java 5. The performance gains from using read locks have so far been considerable.
> Why are we constantly re-instantiating loggers instead of, for example, keeping a {{static final Logger}} in our classes? In many cases it's because the class which holds the logger is a base class, and it can't use a static logger in case a different outlet has been specified for the actual derived class which has been instantiated. Some of these objects, for example {{AbstractMediaTypeExpression}} in the Spring framework, are constantly being destroyed and reconstructed. Where reasonable for our application, we've also patched those other classes to just use {{static final}} Loggers, but there are a lot of them and it is ultimately better to solve this problem at the source.
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