Yesterday I deployed my first Grails (2.3.6) app to a dev server and began monitoring it. I just got an automated monitor stating that CPU was pinned on this machine, and so I SSHed into it. I ran top
and discovered that it was my Java app's PID that was pinning the server. I also noticed memory was at 40%. After a few seconds, the CPU stopped pinning, went down to a normal level, and memory went back down into the ~20% range. Classic major GC.
While it was collecting, I did a heap dump. After the GC, I then opened the dump in JVisualVM and saw that most of the memory was being allocated for an org.codehaus.groovy.runtime.metaclass.MetaMethodIndex.Entry
class. There were almost 250,000 instances of these in total, eating up about 25 MB of memory.
I googled this class and took a look at it's ultra helpful Javadocs. So I still have no idea what this class does.
But googling it also brought up about a dozen or so related articles (some of them SO questions) involving this class and a PermGen/classloader leak with Grails/Groovy apps. And while it seems that my app did in fact clean up these 250K instance with a GC, it still is troubling that there were so many instances of it, and that the GC pinned CPU for over 5 minutes.
My questions:
- What is this class and what is Groovy doing with it?
- Can someone explain this answer to me? Why would
-XX:+CMSClassUnloadingEnabled -XX:+CMSPermGenSweepingEnabled
help this particular problem? - Why is this class particularly troublesome for the PermGen?