Question

What is the benefit of setting the -Xms parameter, and having the initial memory larger for example, then the default calculated one (64 MB in my case, according to Java GC tunning:
http://java.sun.com/javase/technologies/hotspot/gc/gc_tuning_6.html#par_gc.ergonomics.default_size)?

Also, is there any good to setting both the initial and maximum memories to same size?

Thanks.

Answer 1

The benefit is that there is a performance penalty when you use up enough of the heap that it has to be resized. If you set it initially to 64MB but it turns out your application under load needs 250MB, when you hit near 64MB the JVM will allocate more heap space and possibly move around some objects and do other book-keeping. This of course takes time.

When your application is under load, you want all resources dedicated to making it run, so this extra work can make the application slower to respond, or even in some instances it can crash if it runs out of memory before the heap is resized.

Sometimes when using a Java app, you’ll see instructions like “set Xms and Xmx to the same value”. This is done to avoid the resizing altogether, so that your application launches with its heap already as big as it will ever be.