Java Top

I just announced the new Learn Spring course, focused on the fundamentals of Spring 5 and Spring Boot 2:

>> CHECK OUT THE COURSE

1. Overview

In this short article, we're going to see the difference between various memory size metrics in the JVM.

First, we'll talk about how adaptive sizing works, and then we'll evaluate the difference between max, used, and committed sizes.

2. Max Size and Adaptive Sizing

Two values control the size of the JVM heap: one initial value specified via the -Xms flag and another maximum value controlled by the -Xmx tuning flag.

If we don't specify these flags, then the JVM will choose default values for them. These default values depend on the underlying OS, amount of available RAM, and, of course, the JVM implementation itself:Intial Size

 

Regardless of the actual size and default values, the heap size starts with an initial size. As we allocate more objects, the heap size may grow to accommodate for that. The heap size, however, can't go beyond the maximum heap size.

Put simply, the max heap size is the size specified via the -Xmx flag. Also, when we don't explicitly specify the -Xmx, the JVM calculates a default max size.

3. Used Size

Now, let's suppose we allocated a few objects since the program started. The heap size may grow a bit to accommodate for new objects:

Used Space

The used space is the amount of memory that is currently occupied by Java objects. It's always less than or equal to the max size.

4. Committed Size

The committed size is the amount of memory guaranteed to be available for use by the Java virtual machine. The committed memory size is always greater than or equal to the used size.

5. Conclusion

In this short article, we saw the difference between max, used, and committed heap size.

Java bottom

I just announced the new Learn Spring course, focused on the fundamentals of Spring 5 and Spring Boot 2:

>> CHECK OUT THE COURSE
Comments are closed on this article!