What is causing the OS to become slow after running a memory intensive Java application?

Question

I am currently running a small java class for scientific calculations on graphs (which internally creates lots of huge collections) from within Eclipse on MacOSX Snow Leopard. I have a Macbook with 2GB of RAM and to successfully run the app without OutOfMemory Error I need to run it from eclipse with -Xmx1200m (I know this is very much).

It takes about 500seconds to do all the computations I need to run and it also uses up all the memory I have given it (I monitored it with macosx progress monitor and it goes to 1,2GB for Real Mem)

After this application is done my MacOSX is almost unusable. Even if I close Eclipse everything becomes sluggish and slow. I also notice that whatever I do afterwards (even after closing Eclipse) that e.g. Finder navigation is very slow. It takes ages to render some folders or to change directories. Sometimes I restart the system to be able to work again efficiently.

I would like to understand what is happening in the background when this happens? I was under the impression, that when a java class / app has finished running, that the Garbage Collector at least at the end will cleanup everything and I would have expected that other programs are free to use this memory again. But it "feels" like a memory leak which affects the whole system, even if the Java app is not running anymore.

I would appreciate any hints or readings in this direction.

Answer 1

Make sure that java process ends properly after you exit your application. I had similar issue with JBoss AS

Answer 2

Your Mac is running slow because it has 2GB, you have Eclipse running AND you have a 1.2GB app. Its probably swapping like mad.

Your Java program probably has a memory leak. You are adding to collections and not removing from them. You could use a profiler such as TPTP, NetBeans or VisualVM to search for it.

Java programs can leak as much as any other language if you do the wrong thing.

If you genuinely need that much memory and its not a leak then you may need to buy more if you can't split the program up or save intermediate results to file or a database.

Answer 3

What you're experiencing is called page-swapping. The OS has evicted (or paged-out) a bunch of virtual memory pages to disk in order to accommodate the requirements of your Java code. When your program is done, and you try to switch back to some other program you have running, the OS has to page-in from disk before it can resume.

You can see all of this happening by using tools such as top or Activity Monitor.

Answer 4

Do yourself a favour and buy another 2Gb RAM. I'm running the 32 bit kernel (i.e. the smaller one) right now on my MBP and the kernel itself has 800Mb RAM wired. For you that would leave 1.2Gb in total for everything else. That's not enough. Believe me, if you buy the extra RAM, not only will your Java process be better, but everything will seem much snappier.