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I'm developing a desktop java application and am trying to optimize memory usage at runtime, using Java visualVM.

The thing is that I've noticed that my used heap is growing about 1mb/s and I'm trying to find out what's causing it. This is what it looks like:

enter image description here

First of all, I've noticed that the JVM seems to be allocating quite a lot of memory at runtime by itself. I've tried to make a dummy program that just sleeps and the increase in used heap seemed to be about 0.2Mb/s.

However, My application obviously allocates more. And I know I'm creating quite a lot of objects at runtime, but it shouldn't be close to MBs/s, more like bytes/second.

So, I've made a few dumps and compared them. Here's the first, when a fresh GC has been invoked:

enter image description here

And here's the other, where the used heap has grown ~100MB:

enter image description here

Here's the comparison:

enter image description here

Now, what's confusing me is that I can't find any trace of the additional 100 mb in the two dumps.

Total bytes are the same. Classes are the same. Instances are about the same. And the comparison gives me no hints. The dumps, however, confirms that I'm only allocating a handful of classes/s. So what gives?

Update 1: Using the eclipse memory manager:

enter image description here

(The suspects aren't culprits). And I'd like to stress that visualVM showed me a 100MB difference in used heap at the time of the dumps. My in-application 

print("Used Mbs: " + (r.totalMemory() - r.freeMemory())/mb);

Also confirmed this. ('r' is a Runtime instance)

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  • could you show us code ? Commented Jun 1, 2016 at 17:11
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    Can you check for "unreferenced" objects within the heap dump? The objects may be allocated but not referenced and subject for GC. Commented Jun 1, 2016 at 17:11
  • Konrad, I can't seem to find out how to do this with Java visualVM. Do you know? Commented Jun 1, 2016 at 17:19
  • niceman. I've got 500 classes running in parallel on 5 threads. I wouldn't know what to show you... Commented Jun 1, 2016 at 17:20
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    Excellent, then I don't have a memory leak, because each time I call GC, I get the exact same amount of bytes. Commented Jun 1, 2016 at 18:32

2 Answers 2

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I don't think you have a problem.

You need to learn about the JVM memory model and generations.

The sawtooth pattern for young generation says so. That's the garbage collector doing its job.

The permanent stuff you see that grows is perm gen (or it was for JDK 7 and earlier). That's where Strings, .class byte code, etc. live.

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3 Comments

That's the answer I'm hoping for. So you would say this is JVM related and not due to shitty coding?
I haven't seen your code or that of any of the libraries you're using, so I can't comment on quality. (I'll bet the moderators wish you'd chosen a different adjective.) I can say that your simple test case isn't a good one for ferreting out memory leak problems. Better to use real code and subject it to loads that represent your real use cases over a long period of time. That's where memory leaks will be a problem. How large are the JARs you're loading? Those .class file bytes have to go someplace when they're loaded.
But would you agree that the results of my simple test are weird?
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I found this neat function in visualVM - monitoring.

So, I've been monitoring this:

    public static void main(String[] args) {

    while (true){
        try {
            Thread.sleep(10);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }


}

And it seems that the JVM is allocating loads of chars[], Object[], byte[], int[], TreeMap$Entry and such. I've been watching my application as well and it seems to follow the same pattern, only larger volumes of it. So, I'm going to rule this out as all Oracles' fault. That the JVM overhead grows along with the application. Unless someone can come up with a better idea?

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