Minimum Oracle Java VM heap memory setting?

Question

Is there a minimum -Xmx setting for Oracle's JVM? It looks like -Xmx2M does provide the application with more than 2 MB of heap size, as the Java Memory MX bean tells me it allocates like 10 MB...

Is there a minimum under which the JVM silently ignores the Xmx setting?

When the heap is small, it is a very small portion of the size of the JVM. I am not sure why this matters at that point. — Peter Lawrey
– Peter Lawrey, Commented Aug 13, 2013 at 18:20
I'm developing a framework with lazy loading (and unloading) in low-memory conditions, I want to test what the minimum Xmx settings are for my framework. — D.R.
– D.R., Commented Aug 13, 2013 at 18:21
I know about this page but cannot find any documented minimum Xmx setting. — D.R.
– D.R., Commented Aug 13, 2013 at 18:24
I would assume that a real application would do more than your framework does. If you can run with the minimum heap you can set I am not sure what the concern is. — Peter Lawrey
– Peter Lawrey, Commented Aug 13, 2013 at 18:25

Peter Lawrey · Accepted Answer · 2013-08-13 18:33:20Z

2

If I run

public static void main(String[] args) {
    System.out.println("Heap size is " + Runtime.getRuntime().totalMemory() / 100000 / 10.0 + " MB");
}

with -mx8m on Java 7 update 25 64-bit I get

Heap size is 8.0 MB

but if I run with -mx2m I get

Heap size is 3.2 MB

So it does appear you get slightly more heap than you asked for for very small sizes. However, even for a small mobile device I wouldn't be worrying about every last MB because the JVM itself is much larger (can be over 100 MB of shared memory, thread etc)

i.e. if you can't spare a few MB, you won't be able to run the JVM anyway.

answered Aug 13, 2013 at 18:33

Peter Lawrey

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2 Comments

D.R. Over a year ago

It is a little bit weird: minimum heap size seems to be 2M, otherwise the JVM won't start at all. However, if 2M is specified we get 3.2M instead. Is there n definitive answer to the question: Why is that so?

Peter Lawrey Over a year ago

I can only guess it adds some to s you JVM can start, but I don't know for sure. Perhaps reading the source of OpenJDK would have a comment as to why this is done.

Ashwani · Accepted Answer · 2013-08-13 18:42:48Z

0

For jvm to start minimum 2mb of heapsize is required. You can set heap size to 0 bytes, but jvm will not start. And maximum heap size recommended is 1/4 of total ram.

answered Aug 13, 2013 at 18:42

Ashwani

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