does large value for -Xmx postpone Garbage Collection?

Question

I have a lot of JVMs running on a Linux Redhat with 32GB of physical memory and a 32GB of virtual memory. these JVMs are configured to have a total value for Xmx which is more than 32GB and might have the Linux used its virtual memory.
my question is that if I specify the Xmx more than required heap size would it delay the Garbage Collection and as a result allocate more heapsize than necessary? so that it will cause the OS to allocate memory from its virtual memory which leads to a dwindled performance.

Answer 1

The JVM reserves the maximum heap size (as well as other memory areas) on startup. This means if you have a maximum heap size of 32 GB it will use about 33-35 GB of virtual memory including shared libraries threads etc.

Its is worth nothing that if you make the maximum heap size 32 GB it has to use 64-bit references and you can end up with less usable memory than a JVM with a maximum heap of 24 GB and it will use 32-bit references. Some people have estimated that if you make the heap size 32 Gb or more you have to increase it to 48 GB to get more usable memory.

Given the size of your machine I suggest you limit the heap to 24 GB (or less) and where possible use off heap memory as this can have both performance advantages and greater scalability.

If you have a low GC program and you want to avoid collection you can create a massive eden size and GC once per day or once per week. To do this you have to keep your garbage discarded to a minimum and you could create an eden size of say 20 GB in which case provided you create less than 20 Gb in a day you can avoid triggering any GCs (even minor ones) and run a full GC as an over night maintainence task. e.g. at 2 am in the morning.

If you use a large heap, you want to avoid using the swap at all costs. This is because the GC needs random access to the heap and as soon as one triggers which is large enough to swap, your machine will thrash for quite a long time (possibly hours), and possibly lock up. You might even have to reboot to get you machine to behave normally. (it is hard to kill a process/machine which is in such a state)