What in Java is using 400M in virtual memory and how do I lower that usage?

Simple program:

public class SleepTest {
    public static void main(String[] args) throws InterruptedException {
        Thread.sleep(60 * 1000);
    }
}

Then

$ javac SleepTest.java 
$ java -cp . SleepTest

For OpenJDK 1.6.0_20 this uses 600M of virtual memory on my machine! That is, "top" shows "VIRT" 600M and RES 10m. (I am on Ubuntu 10.04, 32-bit or 64-bit).

For Sun's Java 1.6.0_22 it uses 400M of virtual memory.

What is using all that virtual memory, and how do I lower that usage?

Full "java -version":

OpenJDK:

java version "1.6.0_20"
OpenJDK Runtime Environment (IcedTea6 1.9.4) (6b20-1.9.4-0ubuntu1~10.04.1)
OpenJDK Client VM (build 19.0-b09, mixed mode, sharing)

Sun:

$ /usr/lib/jvm/java-6-sun-1.6.0.22/jre/bin/jav -version
java version "1.6.0_22"
Java(TM) SE Runtime Environment (build 1.6.0_22-b04)
Java HotSpot(TM) Client VM (build 17.1-b03, mixed mode, sharing)

Edit:

Compiling with javac from either package doesn't seem to help.

Adding some code to print used memory is as follows:

private static String megabyteString(long bytes) {
    return String.format("%.1f", ((float)bytes) / 1024 / 1024); 
}
private static void printUsedMemory() {
    Runtime run = Runtime.getRuntime();
    long free = run.freeMemory();
    long total = run.totalMemory();
    long max = run.maxMemory();
    long used = total - free;
    System.out.println("Memory: used " + megabyteString(used) + "M"
            + " free " + megabyteString(free) + "M"
            + " total " + megabyteString(total) + "M"
            + " max " + megabyteString(max) + "M");
}

shows

Sun:

Memory: used 0.3M free 15.2M total 15.5M max 247.5M

OpenJDK:

Memory: used 0.2M free 15.3M total 15.5M max 494.9M

even with -Xmx5m, so it must have a minimum? I've read about defaults before (depends on jvm, virtual machine, a common strategy by default one quarter of physical memory), but is that causing the large virtual memory use and can I not decrease it?

Edit #2:

Adding -Xmx changes things:

$ java -Xmx5m -cp . SleepTest

OpenJDK:

$ java -Xmx5m SleepTest
Memory: used 0.2M free 4.7M total 4.9M max 5.8M

uses "only" 150M of virtual memory for either JVM.

Edit #3:

nos, bestsss, Mikaveli, and maybe others pointed out that virtual memory does not use swap. nos claims the OOM killer is smart enough to go by real memory usage. If those things are true, then I guess I don't care about virtual memory usage. RES (resident size) is small, so I'm good.

Edit #4:

Not sure which answer to accept. Either of these, if it shows up as an answer: "Don't worry about it because virtual memory is cheap" or some explanation of why Java reserves at least 150M in virtual memory no matter what -Xmx or -Xms I give it, even though real memory usa开发者_如何转开发ge is tiny.

Edit #5:

This is a dup. I voted to close.

"RES" is resident set size - essentially the physical memory you're using for that process.

From your example, top states that it's using 10 MiB - that's roughly what I'd expect for a -Xmx setting of 5m (the total physical memory used often seems to be double, from my experience of Java on *nix systems.

Are you actually getting any memory issues or are you just concerned about the misleading output from "top"?

Also, the *nix virtual memory includes of the available memory space - physical and swap. If the process isn't using any "swap", then it is only using physical "resident" memory.

Stack Overflow answer to why JVM uses more memory than just heap setting.

$ java -cp . SleepTest -Xmx5m

wow! the -Xmx5m is an argument you can find at public static void main(String[] a). It is parameter to the java program NOT to the VM

move it 1st $ java -Xmx5m -cp . SleepTest

Try using -Xms to specify minimum memory?

The JVM takes some defaults for heap space (that is all that memory usage) in count. This defaults are coming from the machine caracteristics (RAM, cores, etc) and the JVM implementation (Sun/Oracle, OpenJDK).

I think -Xmx5m is too low and the JVM ignores this parameter silently.

Here is a good reference with lots of links to read:

Java Memory explained (SUN JVM)