Repost: How fragmented is my Java heap?

Originally posted in December 2007, reposted on request. The problem described here is generic and applies to all JVMs using mark-n-sweep or similar GC algorithms.

JRockit Mission Control is now free for development and evaluation, you can download it here.

One major cause for long GC pause times is heap fragmentation. How problematic this for an application depends on its allocation pattern. The worst possible case is an application that allocates a mix of objects with very different sizes and lifetimes. After the application has been running for a while, the Java heap will be fragmented by lots of long lived Java objects spread out across the heap. There may be plenty of free space available, but no large block of contiguous free memory. When the application then attempts to allocate a large object such as an array, it is unable to find room to store it. The result will be a long GC pause while the heap is compacted.

If you suspect you have this issue with your application, the first step is to try to find out exactly how fragmented the heap is. One simple way of doing this is by using the JRockit Runtime Analyzer. Here's an example:

1. Start a Java application on your workstation
2. Start up JRockit Mission Control using the Start menu icon in Windows or the $JR_HOME/bin/jrmc executable
3. Locate the process you want to analyze, right-click and select to start a JRA recording.

start-jra.PNG

4. Select recording time (here 2 minutes) and start the recording

jra-wizard.PNG

5. After the recording has finished, it will be opened in the JRMC GUI. Select the Heap tab. Heap fragmentation is displayed in black in the Heap Contents pie chart.

jra-heap.PNG

The application in this example is well behaved and shows only 11% fragmentation - well within acceptable limits. I would start getting concerned if it went above 30%, and more if it continued to increase. Another warning sign is if the free memory distribution (pie chart on the right) contains a very large proportion of smaller free blocks.

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