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copied!<p>From IBM's <a href="http://publib.boulder.ibm.com/infocenter/wasinfo/v6r1/index.jsp?topic=/com.ibm.websphere.nd.doc/info/ae/ae/tprf_tunejvm_v61.html" rel="nofollow noreferrer">performance tuning tips</a> (so may not be directly applicable to Sun's VMs)</p> <blockquote> <p>The Java heap parameters influence the behavior of garbage collection. Increasing the heap size supports more object creation. Because a large heap takes longer to fill, the application runs longer before a garbage collection occurs. However, a larger heap also takes longer to compact and causes garbage collection to take longer.</p> <p>The JVM has thresholds it uses to manage the JVM's storage. When the thresholds are reached, the garbage collector gets invoked to free up unused storage. Therefore, garbage collection can cause significant degradation of Java performance. Before changing the initial and maximum heap sizes, you should consider the following information: In the majority of cases you should set the maximum JVM heap size to value higher than the initial JVM heap size. This allows for the JVM to operate efficiently during normal, steady state periods within the confines of the initial heap but also to operate effectively during periods of high transaction volume by expanding the heap up to the maximum JVM heap size. In some rare cases where absolute optimal performance is required you might want to specify the same value for both the initial and maximum heap size. This will eliminate some overhead that occurs when the JVM needs to expand or contract the size of the JVM heap. Make sure the region is large enough to hold the specified JVM heap. Beware of making the Initial Heap Size too large. While a large heap size initially improves performance by delaying garbage collection, a large heap size ultimately affects response time when garbage collection eventually kicks in because the collection process takes more time.</p> </blockquote> <p>So, I guess the reason that you can't change the value at runtime is because it may not help: either you have enough space in your heap or you don't. Once you run out, a GC cycle will be triggered. If that doesn't free up the space, you're stuffed anyway. You'd need to catch the OutOfMemoryException, increase the heap size, and then retry you calculation, hoping that this time you have enough memory.</p> <p>In general the VM won't use the maximum heap size unless you need it, so if you think you might need to expand the memory at runtime, you could just specify a large maximum heap size.</p> <p>I admit that's all a bit unsatisfying, and seems a bit lazy, since I can imagine a reasonable garbage collection strategy which would increase the heap size when GC fails to free enough space. Whether my imagination translates to a high performance GC implementation is another matter though ;)</p>

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