java increase xmx dynamically at runtime

Question

Hi,

I have a jvm server in my machine, now I want to have 2 apservers of mine sitting in same machine, however I want the standby one to have a really low amount of memory allocated with xmx because its passive, one the main server (active) goes down I want to allocate more memory to my passive server which is already up without restarting it (I have have them both having too much xmx - note they would consume memory at startup and I cant allow possibility of outOfMemory).

So I want passive - low xmx once active goes down I want my passive to receive much more xmx.

is there a way for me to achieve that. Thanks

Answer 1

The short answer is unless your particular JVM allows for these values to be changed after initialization, you cannot (I believe this is the case for HotSpot).

However, you may be able to accomplish your goals without changing Xmx on the fly. For example, you could use a small -Xms setting, but keep -Xmx relatively high. If the passive server is not using much memory / generating garbage while still serving as the backup, then memory will stay near the Xms value. However, once the backup server takes over it would be allowed to expand allocated memory up to the Xmx value on an as-needed basis.

See java (windows) or java (*nix) as appropriate (though -Xms and -Xmx have the same general meaning on all platforms).

Answer 2

You don't need to adjust Xmx on the standby instance as long as it's not doing anything (or much of anything) because it should stay close to the value you set with Xms until it starts doing real work.

The Xmx switch governs the maximum amount of heap size the Java instance may consume. Xms governs the startup amount.

If you set Xms small on your standby instance and Xmx to whatever maximum your program needs, and then switch over to the Standby instance (killing the regular instance) it should work out fine.

It may be necessary to actually stop/kill the regular Java process depending on your available memory in order for the standby process to allocate all of the heap it needs as it moves from the initial lower heap size to toward it's maximum.

Answer 3

It would be nice, but as far as I know it's not an option with the Sun provided JVMs.

The Xmx option is to specify maximum memory, it's there to prevent the JVM from consuming the entire machine's free memory. If you want to set it higher, it won't require the JVM allocate all of that memory. Why not just set it to a very high number and let the JVM grow into it over time?

To make sure your JVM doesn't start off with too little memory (creating lots of pauses as it grows the memory to the required size), adjust Xms to the size you want to allocate for the JVM at startup.