Howdy, >It is so difficult to scale this. using vmstat, I can see my free memory >shrinking and shrinking . It is really a case of "suck and see".
The free memory will keep going down until the VM feels it has to garbage-collect. You can tune this behavior using other non-standard options, for example -Xminf, -Xmaxf, -XX:NewSize, -XX:MaxNewSize, and the various GC algorithms like -Xincgc and -Xconcgc. >The doco says m or M means MegaBytes >So does -Xmx256m mean 256 MegaBytes ??? Yes. >What do the others mean please k for KB, g for GB. But you can probably just make do with m, as that's the range for most applications. For example: -Xmx256m means the heap can grow up to 256MB. -Xmx1024m means the heap can grow up to 1024MB = 1GB. -Xmx2m means the heap can grow up to 2MB. (I wouldn't do this ;) Although an early release of J2ME had a 4MB default heap limit I think. ;)) Please keep in mind that OS-level tools, like top and vmstat, display the process space, which is more than the heap. The above numbers control just the heap. In practice, overhead like thread addressing, locks, etc, add more to the process space. So the numbers you see using vmstat could exceed the -Xmx number. How do you check that you setting took effect? Test it. Do stuff with your app that will take 200MB and make sure it works. Then do stuff with your app that will take 300MB and make sure it doesn't work, i.e. get an OutOfMemoryError. If your app can't do this, why do you need -Xmx? But believe me: -Xmx works. It's become one of the first and most important things people tune, because JVM sizes are ever-growing. Yoav Shapira Millennium ChemInformatics
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