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LAB-1205: Java Application Performance Analysis

Exercise 6: Old generation too big

Learning goals of this exercise

In this exercise you will learn:

Background for this exercise

Please see Exercise 1 for an overview of running the javaperf GUI launcher.

Setting the maximum heap size is important for optimal performance... especially if you want multiple instances of the JVM to co-exist on the same system without swapping memory.

Generally speaking it is not a performance problem to have the maximum heap size set too big, but if it is too high then the application may not even start (you could see errors like Error occurred during initialization of VM, Could not reserve enough space for object heap).  Setting the max heap size too big may waste memory (that could be used for other system resources or applications) and it may lead to long GC times once the heap does fill up.

Steps to follow

  1. For this exercise we increase maximum heap size to the oversized 256 MB.  We keep the young gen at our optimal 4 MB setting.
    compiler=
    heap=-XX:NewSize=4m -XX:MaxNewSize=4m -Xms256m -Xmx256m
    gc=
    other=

  2. Click [Start]. You will see the Java 2D Demo application start....  Immediately click on the "Transforms" tab. Then click into the "Shear" area so it fills the entire window.

  3. Notice how it seems like there is hardly any memory used in the old gen.

  4. When you see that the application runtime goes just over one minute click [Stop]

  5. This is the best time in GC so far, only 1.87%.

Summary

How do you know the old gen is too big (the maximum heap size is set to big)?  One way the JVM doesn't even start.  Another is if you see that it seems hardly any of the old gen space is used.  Another may be that the old space is used, but when there is a full GC it has a very long pause time (stay tuned for an exercise on the concurrent collector!).

Why is an oversized heap a problem?  It generally isn't a performance problem, but you may not be able to start the application (if there isn't enough RAM) or you are wasting memory.  In can be useful to test your application with a deliberately oversized heap to get a feeling for what the "right" heap size is.

Why shouldn't I just let the JVM figure out with the best old gen size is?  In practice that is often what you want to do... By setting the initial (Xms) and maximum values (Xmx) for the heap you allow the JVM to reserve the maximum amount, but only the memory that is needed is committed.  For the purpose of the hands-on lab, however, we've opted instead to set these values the same to lead to more predictable results.  For more about reserved vs. committed memory and resizing please see the Total Heap section of the Tuning Garbage Collection with the 5.0 Java™ Virtual Machine document.

Next Steps

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