how to choose the jvm heap size?

My question is how to choose the min
and max values, and the difference
between the two (should max-min be
small or big?)

Short answer: don’t guess, profile your application.

jconsole can give you useful high-level data such as a feeling for the main resident set vs. the transient data that we normally allocate and garbage collect. What you’ll see if you look at the memory tab of that display is usually something like a sawtooth. The lower corner of the sawteeth is about where I would normally set the heap minimum whereas I would use the peak or slope of the sawteeth to experiment with a heap maximum. If your teeth are very steep, you might consider a big heap just to delay the garbage collection. However, if they aren’t, you could try a smaller heap maximum to see if that might leave more resources for other processes on your machine (for example).

You should also consider the server VM as that will cause different garbage collection behavior.

All that said, you should also use a more detailed tool such as jvisualvm to profile the memory usage of your process. It’s possible that you have a memory leak or greedy allocator that you could tune or eliminate. That would completely change your heap needs.

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