Manual garbage collection

Question

Im running a huge combinatorial generation algorithm and i'm starting to run into memory allocation errors on larger problem instances. I am using the -Xmx256m run time option on the JVM to give it more memory but im generating huge masses off stuff here.
The question I have is, if I know an object isnt going to be used again is it worth destroying it in memory yourself; i assume if you have some undesired object you can do something like this;
myObject = null;
and then call the garbage collector to free up the memory..firstly will this work, secondly what is there performance tradeoff for calling the garbage collector more often and lastly is the garbage collector allready efficient enough that doing things like this is probably not worth it.
Regards,
Dave

Answer 1

Setting references to null MAY help get immediate relief, but its not guarunteed to help. Same with the System.gc() call.
Doing both those thing will not enable the system to operate with less memory than before, as variables that are out of scope will be discovered during garbage collection, and garbage collection WILL run when the heap is fully utilized.
Doing the above may make the GC pause lower at the expense of worse performance.
Instead of doing the above, I'd recommand using the Incremental GC by specifying -Xincgc on the command line. This will run GC more often in the background, leading to shorter GC pauses, but about 10% performance hit in general.
If OTOH you are running into OutOfMemoryErrors and don't think you should be, you probably have a memory leak. Most likely you are allocating objects and storing them somewhere then forgetting about them. For example, if you stick an object in a HashMap, and then forget about it, that object will not be GC'd, unless the HashMap can be GC'd.