Java app that uses a lot of memory. Use -Xmx?

Question

I have a java app that uses about 15G on a machine with 16G. I don't know if I should set the max heap size.

If set will the jvm eat all the ram up to the limit and then start garbage collecting and stop everything while it churns through 15G of heap objects?

If not will the jvm hurt performance by not using all of the available ram on the machine.

My specific vm is: Java HotSpot(TM) 64-Bit Server VM (build 1.6.0_03-b05, mixed mode).

Thanks

Answer 1

-Xmx15G will set the maximum heap size to 15 gig. Java will only allocate what it needs as it runs. If you don't set it, it will only use the default. For info on the default, see this post.

-Xms15G sets the minimum heap to 15 gig. This forces java to allocate 15 gig of heap space before it starts executing, whether it needs it or not.

Usually you can set them both to appropriate values depending on how you're tuning the JVM.

Answer 2

In Java 6, the default maximum heap size is determined by the amount of system memory present.

According to the Garbage Collector Ergonomics page, the maximum heap size is:

Smaller of 1/4th of the physical memory or 1GB. Before J2SE 5.0, the default maximum heap size was 64MB.

By using the -Xmx switch can be used to change the maximum heap size. See the java - the Java application launcher documentation for usage details.