Java heap space : pros/cons of size in performance terms

Question

What are the limitations on Java heap space? What are the advantages/disadvantages of not increasing the size to a large value?

This question stems from a program that I am running sometimes running out of space. I am aware that we can change the size of the heap to a larger value in order to reduce the chance of this. However, I am looking for pros and cons of keeping the heap size small or large. Does decreasing it increase speed?

Answer 1

Increasing the heap size may, under some circumstances, delay the activities of the garbage collector. If you are lucky, it may be delayed until your program has finished its work and terminated.

Depending on the data structures that you are moving around in your program, a larger heap size may also improve performance because the GC can work more effectively. But to tune a program into that direction is … tricky, at best.

Using in-memory caches in your program will definitely improve performance (if you have a decent cache-hit ratio), and of course, this will need heap space, perhaps a lot.

But if your program reports OutOfMemoryError: heap space because of the data to process, you do not have much alternatives other than increasing the heap space; performance is your least problem in this case. Or you change your algorithm that it will not load all data into memory, instead processing it on disk. But then again, performance is out of the door.

If you run a server of some kind, having about 80% to 85% heap utilisation is a good value, if you do not have heavy spikes. If you know for sure that incoming request do not cause significant additional memory load, you may even go up to 95%. You want value for money, and you paid for the memory, one or the other way – so you want to use it!

You can even set Xms and Xmx to different values; then the JVM can increase the heap space when needed, and today it can even release that additional memory if no longer needed. But this increase costs performance – on the other side, a slow system is always better than one that crashes.

A too small heap size may affect performance if your system also does not have enough cores, so that the garbage collectors do compete over the CPU with the business threads. At some point, the CPU spends a significant time on garbage collection. Again, if you run a server, this behaviour is nearly unavoidable, but for a tool-like program, increasing the heap can prevent it (because the program may come to an end before the GC needs to get active). This is already said in the beginning …