Speed tradeoff of Java's -Xms and -Xmx options

The -Xmx argument defines the max memory size that the heap can reach for the JVM. You must know your program well and see how it performs under load and set this parameter accordingly. A low value can cause OutOfMemoryExceptions or a very poor performance if your program's heap memory is reaching the maximum heap size. If your program is running in dedicated server you can set this parameter higher because it wont affect other programs.

The -Xms argument sets the initial heap memory size for the JVM. This means that when you start your program the JVM will allocate this amount of memory instantly. This is useful if your program will consume a large amount of heap memory right from the start. This avoids the JVM to be constantly increasing the heap and can gain some performance there. If you don't know if this parameter is going to help you, don't use it.

In summary, this is a compromise that you have to decide based only in the memory behavior of your program.

It depends on the GC your java is using. Parallel GCs might work better on larger memory settings - I'm no expert on that though.

In general, if you have larger memory the less frequent it needs to be GC-ed - there is lots of room for garbage. However, when it comes to a GC, the GC has to work on more memory - which in turn might be slower.

I have found that in some cases too much memory can slow the program down.

For example I had a hibernate based transform engine that started running slowly as the load increased. It turned out that each time we got an object from the db, hibernate was checking memory for objects that would never be used again.

The solution was to evict the old objects from the session.

Stuart