Seeking Insight on Java GC Intracacies

Question

Hi, I'm a technical QA, and I'm trying to help identify a memory leak in one of our applications. In order to better understand what the memory usage numbers on our system mean, I've been refreshing my knowledge about how the GC and Java memory management work. My question is, when the young generation fills up, will the JVM prefer to expand young memory in to virtual space, or will it prefer to run a minor GC and promote things in to tenured space if there isn't enough space in young memory before expanding in to virtual space?
Any help would be greatly appreciated. Thanks.

Answer 1

Look at the GC whitepaper at http://java.sun.com/j2se/reference/whitepapers/memorymanagement_whitepaper.pdf for a lot of background. That's for JDK-1.6.0, not JDK-1.4.1 but a lot of the principles apply.
In answer to your specific question: in JDK-1.4.1 the young generation is essentially fixed in size. When it fills up, a young generation collection copies all the live objects to either the survivor space or the old generation. In JDK-1.6.0 we can change the size of the eden and the survivor spaces to try to control the pause time and maximize throughput. It's the old generation that starts out small and commits additional virtual address space as needed, up to your maximum (-Xmx). As the old generation grows the young may grow to keep the ratio of young to old approximately constant, but I forget if that was in JDK-1.4.1 or was added later.