Tomcat PermGen – a fix?

While decent folk were asleep, I was profiling Tomcat. (YourKit rocks, funky UI notwithstanding.)

I found that our PermGen woes of old were indeed caused by classes not unloading. I was inspired by this how-to and yes, we do leak an instance of WebappClassLoader which has a reference to every class the app ever uses. Why?, I hear you ask.

There's a java.util.TimerThread named "MySQL Statement Cancellation Timer" that does not terminate when the app is unloaded and so it hangs on to the class loader, which in turn hangs on to every class under the sun. For a rigorous explanation, read the MySQL bug report.

According to that bug report, the MySQL developers capitulated and as of not that long ago, this is the status:

[18 Dec 2009 23:24] Mark Matthews

Fixed for 5.1.11. Unfortunately no great fix exists that lets us keep the cancellation timer shared amongst connection instances, so instead it's lazily created if need be per-instance, and torn down when the connection is closed.

So in theory, we can get rid of this problem by upgrading our MySQL Connector (JDBC driver) from 5.1.10 to 5.1.11.

If I have time over the weekend, I'll be testing the new JDBC driver with GH and monitoring it closely with YourKit to see if the new driver solves the problem.

Labels: mysql, tomcat

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