Robinson, Eric wrote:
What are the possible downsides of setting a low initial memory pool and a high max pool? If a tomcat app usually needs approximately 64MB of heap space, but sometimes as much as 300-400MB, would it cause any problems to set the initial pool to 16M and the max pool to 512M?
It depends... ;-)
If you set different values to -Xms (at start) and -Xmx (maximum), then the JVM will spend some amount of time allocating and de-allocating Heap memory dynamically as needed, to stay within these boundaries. If they are equal, then the JVM allocates it at start, and never has to worry about it later. If your application needs 64MB of Heap space and you allocate only -Xms16M, then right at the start the JVM will have to increase the Heap to 64MB (minimum); so why would you do that ?
If you are talking about a workstation, and for testing, then you can do whatever pleases you and play around. If you are talking about a server, then the logical thing to do would be to estimate how big a Heap you need in a maximum expected load situation, round that up somewhat, and set -Xms and -Xmx both to that same value. There are other command-line parameters to the JVM, which allow to ask the Garbage Collector to write a log, which can help in determining what the optimal Heap size would be, in your situation. There is no "one size fits all".
Note that you are talking of "memory pool", which is a bit vague. The -Xms and -Xmx parameters relate to how big the Heap is, which is only one part of the memory space needed by the JVM.
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