Question

I just encountered the following code (slightly simplified):

/* periodically requests garbagecollect to improve memory usage and 
   garbage collect performance under most JVMs */
static class GCThread implements Runnable {
    public void run() {
        while(true) {
            try {
              Thread.sleep(300000);
            } catch (InterruptedException e) {}
            System.gc();           
        }
    }
}

Thread gcThread = new Thread(new GCThread());
gcThread.setDaemon(true);
gcThread.start();

I respect the author of the code, but no longer has easy access to ask him to defend his assertion in the comment on top.

Is this true? It very much goes against my intuition that this little hack should improve anything. I would expect the JVM to be much better equipped to decide when to perform a collection.

The code is running in a web-application running inside a IBM WebSphere on Z/OS.

Answer 1

It depends.

1. The JVM can completely ignore System.gc() so this code could do absolutely nothing.

2. Secondly, a GC has a cost impact. If your program wouldn’t otherwise have done a GC (say, it doesn’t generate much garbage, or it has a huge heap and never needs to GC) then this code will be added overhead.

3. If the program would normally run with just minor GCs and this code causes a major GC, you will have a negative impact.

All in all, this kind of optimisation makes absolutely no sense whatsoever unless you have concrete evidence that it provides benefit and you would need to re-evaluate that evidence every time the program materially changed.