When to Garbage Collect

Question

I have a piece of code that load a very big image in memory. So it seemed like a reasonable thing to call

System.gc();

before loading the image. From what I can tell it works with no issues.

Yesterday i decided to use a pretty useful piece of software called FindBugs that scans your code and reports back issues that might cause bugs or generally not advised strategies. The problem is that this piece of code i mentioned gets reported. The description is this:

... forces garbage collection; extremely dubious except in benchmarking code

And it goes on to elaborate :

Code explicitly invokes garbage collection. Except for specific use in benchmarking, this is very dubious.

In the past, situations where people have explicitly invoked the garbage collector in routines such as close or finalize methods has led to huge performance black holes. Garbage collection can be expensive. Any situation that forces hundreds or thousands of garbage collections will bring the machine to a crawl.

So my question is : Is it NOT OK to programmatically call the garbage collector in such a case? My code only calls it once and the method that it is in gets used rarely. And if it is not OK to call it then what should you do in a case where you need as much memory as possible before doing a very memory intensive operation and you need to free as much memory as posible prior to it?

Answer 1

Did you get any performance improvements with the System.gc()? I don't think so, since you probably dont have a lot of objects that needs to be collected before you load the image.

Usually modern garbage collectors know best when to run, so you shouldnt force a collection, unless you have a really really good reason to. (for example a benchmarking application as suggested by that plugin)

btw: Calling System.gc() recommends the VM to perform a "full" or "large" collection, which means that all threads are stopped shortly. Otherwise it will probably only make "small" garbage collections, which don't stop all threads.

Run your program with -verbose:gc to see how many bytes are collected.

There is also lots of technical information on garbage collection here: http://java.sun.com/developer/technicalArticles/Programming/GCPortal/

Answer 2

Typically the GC is smarter than you, so it's better to let it run whenever the runtime decides. If the runtime needs memory, it'll run the GC itself

Answer 3

It's fine to call the garbage collector, you don't get any "problems" from it. However, I doubt it will significently boost performance, unless that call also deals with defragging the allocated data. I don't know that.

What you should do in this case is profile the code. Run it several times, see what sort of results you get.