JVM heap parameters

java jvm-arguments
Manuel Selva picture Manuel Selva · Jul 8, 2009 · Viewed 191.8k times · Source

After reading already asked question on the subject and a lot of googling I am still not able to have a clear view of -Xms option

My question is: what's the difference between java -Xms=512m -Xmx=512m and java -Xms=64m -Xmx=512m?

For now I have the following answer:

The only difference is in the number of garbage collections that will be run during my application's run and the number of memory allocations. Am I right ?

Here are my reasons for this answer:

Setting the -Xms option to 512m doesn't result in my application using really 512M of physical memory after startup. I guess this is related to modern OS virtual memory management and lazy pages allocations. (I noticed that setting -Xms to 512M or to 64M doesn't change at all the initial used memory reported either by top on Linux or by the task manager on windows)

Can someone help me to understand the impact of this Xms option or point me to links that will help me to understand it?

Thanks in advance

Manu

Answer

Steve B. picture Steve B. · Jul 8, 2009

The JVM will start with memory useage at the initial heap level. If the maxheap is higher, it will grow to the maxheap size as memory requirements exceed it's current memory.

So,

  • -Xms512m -Xmx512m

JVM starts with 512 M, never resizes.

  • -Xms64m -Xmx512m

JVM starts with 64M, grows (up to max ceiling of 512) if mem. requirements exceed 64.

Related questions

“Error occurred during initialization of VM; Could not reserve enough space for object heap” using -Xmx3G

First of all, I have a box with 8gb of ram, so I doubt total memory is the issue. This application is running fine on machines with 6gb or less. I am trying to reserve 3GB of space using -Xmx3…

Read More
Increasing the JVM maximum heap size for memory intensive applications

I need to run a Java memory intensive application that uses more than 2GB, but I am having problems to increase the heap maximum size. So far, I have tried the following approaches: Setting the -Xmx parameter, e.g. -Xmx3000…

Read More
What does -XX:MaxPermSize do?

Specifically, why would it help to fix a PermGen OutOfMemoryError issue? Also, bonus points for an answer that points me to the documentation on JVM arguments...

Read More