We recently pushed a couple of new Java programs to our client’s application server (HPUX) and soon thereafter they started to receive alerts about the swap running out of space. Of course, being the “new kid on the block” our programs were suspect. While these programs were not the only ones taking up a good deal of swap space, we were a bit perplexed why they were taking much at all.
These programs were fairly small in terms of functionality, but the amount of swap space it was using was around 600-700MB. Here is a summary of our findings (note that some of these details might be different on a Windows system due to differences in memory management):
- A common theme is that most java processes have a big difference between the residual memory and the total virtual memory.
- Java will allocate at a minimum about 200 MB of swap even for the simplest of programs. We tested using a simple program on both a HPUX and a linux server. The best and only documentation I could find about this was at the URL below. The bottom line is that this is probably the behavior of the JVM and out of our control so we just have to deal with it.
http://groups.google.com/group/comp.lang.java.programmer/browse_thread/thread/778089cbe0c9814c/5b0dfcea715d9055?lnk=st&q=java+uses+virtual+memory&rnum=20&hl=en#5b0dfcea715d9055It
It is reserving space for example for threads that you may or may not create. It is awkward to rearrange memory on the fly, so the JVM parcels out the virtual ram for various tables when it first loads.
- The -Xms and -Xmx parameters do affect the java heap but it does not control the total memory footprint. It is only a piece of the total size.
- -Xmx alone can affect the total virtual memory size. Currently the processes have -Xms24m and -Xmx512m. Immediately after starting these programs, we saw the swap allocation jump to 600-700MB. When we changed -Xmx to 24m, it dropped to a little over 200MB.
- In relation to the above, the normal behavior of the garbage collector is to garbage collect only when it needs to free up memory. Having a too high of a -Xmx value could mean that it will never garbage collect, which means it won’t reuse memory. We see this in the Weblogic environment too but in that environment all of the memory usually gets used at some point.
- There is a new tool built into Java 5 and up called jmap that will give you heap information about a java process. This tool would make tuning the heap parms really simple. Unfortunately the process also has to be running using Java 5 so we cannot use it at this time. There is a version for 1.4 but it only works on Solaris.