Friday, December 5, 2008

Is is a memory leak or a performance problem?

We are about to start user acceptance testing for the next release of our project. The application passed the performance tests in our environment, but failed at our client site. Time to panic?

It doesn't have to be. This is how I analyzed our problem to determine if it was a memory leak or performance problem.

First some context:
The application is a Java J2EE web application running on Weblogic and integrating with about a dozen external systems through a variety of middleware platforms. The most dramatic change for this release involves rewriting one of those integrations to support a new and dramatically different version of an external system. This integration is very complicated and heavy on the xml.

So now I know there is a problem, but I don't know how to reproduce it in our environment. This kind of problem could easily be caused by a memory leak. The easiest way to tell if there is a leak is to look at the garbage collection logs.

The gc.log looks something like this:

[INFO ][memory ] 20.407: parallel nursery GC 378204K->202844K (2097152K), 220.715 ms
[INFO ][memory ] 24.512: parallel nursery GC 463356K->266172K (2097152K), 108.514 ms
[INFO ][memory ] 158.352: parallel nursery GC 594933K->397813K (2097152K), 223.864 ms
[INFO ][memory ] 354.614: parallel nursery GC 710277K->554373K (2097152K), 297.761 ms
[INFO ][memory ] 356.632: parallel nursery GC 918935K->772951K (2097152K), 246.615 ms
[INFO ][memory ] 359.976: parallel nursery GC 1121408K->1000192K (2097152K), 254.630 ms
[INFO ][memory ] 362.469: parallel nursery GC 1372915K->1239347K (2097152K), 245.692 ms
[INFO ][memory ] 366.508: parallel nursery GC 1595123K->1468403K (2097152K), 249.064 ms
[INFO ][memory ] 368.933: parallel nursery GC 1824064K->1698240K (2097152K), 240.599 ms
[INFO ][memory ] 371.724: parallel nursery GC 2047811K->2097152K (2097152K), 229.808 ms

While you could look through this file and try to analyze it in your head, I wouldn't recommend it. This is where a visualization tool comes in really handy. I suggest GCViewer (Thanks Andy)

  • First delete any lines off the top of the file that don't look like the lines I pasted above. They are interesting to read but will confuse GCViewer.
  • Then open the edited gc.log using GCViewer and find a section of the log where the system performing badly. Timestamps are useful for this.
The log will look something like:

The important things to look at in the graph are:
  • The saw tooth lines up. Each "tooth" where the memory usage drops a little represents a partial garbage collection. These partial garbage collections only grab the free memory that is easily available. These collections are fast, but not very effective.
  • The dramatic drops represent a full garbage collection. When you run out of memory, your system will do a full garbage collection, which should free up all the space that is available. This garbage collection is much slower, but is much more effective.
  • The gray box indicates a system pause for garbage collection. If you see a large gray box like the one on the right of the graph, this is a noticeable pause.
  • The amount of memory that is available after a full garbage collection remains approximately constant. This indicates that we are using memory quickly, but releasing it when we are done. If the amount of used memory was increasing after a full collection, that would indicate a memory leak. A leak happens when you hold on to a memory reference when you are done processing instead of releasing it.
Based on the analysis of this graph, I knew that there was not a memory leak, but instead that the system was using too much memory, too quickly. This was causing the JVM to do a full garbage collection every 15-20 seconds, and taking up to 2 seconds for a full collection.

In my next post, I'll talk about how I figured out what was using too much memory and how we fixed it.


Bernd Eckenfels said...

Still curious how you solved and debugged the memory pig.

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