Does the Sun JVM slow down when more memory is allocated via -Xmx?

Question

Does the Sun JVM slow down when more memory is available and used via -Xmx? (Assumption: The machine has enough physical memory so that virtual memory swapping is not a problem.)

I ask because my production servers are to receive a memory upgrade. I'd like to bump up the -Xmx value to something decadent. The idea is to prevent any heap space exhaustion failures due to my own programming errors that occur from time to time. Rare events, but they could be avoided with my rapidly evolving webapp if I had an obscene -Xmx value, like 2048mb or higher. The application is heavily monitored, so unusual spikes in JVM memory consumption would be noticed and any flaws fixed.

Possible important details:

• Java 6 (runnign in 64-bit mode)
• 4-core Xeon
• RHEL4 64-bit
• Spring, Hibernate
• High disk and network IO

EDIT: I tried to avoid posting the configuration of my JVM, but clearly that makes the question ridiculously open ended. So, here we go with relevant configuration parameters:

-Xms256m 
-Xmx1024m 
-XX:+UseConcMarkSweepGC 
-XX:+AlwaysActAsServerClassMachine 
-XX:MaxGCPauseMillis=1000 
-XX:MaxGCMinorPauseMillis=1000 
-XX:+PrintGCTimeStamps 
-XX:+HeapDumpOnOutOfMemoryError 

Answer 1

By adding more memory, it will take longer for the heap to fill up. Consequently, it will reduce the frequency of garbage collections. However, depending on how mortal your objects are, you may find that how long it takes to do any single GC increases.

The primary factor for how long a GC takes is how many live objects there are. Thus, if virtually all of your objects die young and once you get established, none of them escape the young heap, you may not notice much of a change in how long it takes to do a GC. However, whenever you have to cycle the tenured heap, you may find everything halting for an unreasonable amount of time since most of these objects will still be around. Tune the sizes accordingly.

Answer 2

If you just throw more memory at the problem, you will have better throughput in your application, but your responsiveness can go down if you're not on a multi core system using the CMS garbage collector. This is because fewer GCs will occur, but they will have more work to do. The upside is that you will get more memory freed up with your GCs, so allocation will continue to be very cheap, hence the higher througput.

You seem to be confusing -Xmx and -Xms, by the way. -Xms just sets the initial heap size, whereas -Xmx is your max heap size.