Re: Should -Xmx be a multiple of -Xms?

From:

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <arne@vajhoej.dk>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 01 Jun 2010 19:17:47 -0400

Message-ID:

<4c059507$0$286$14726298@news.sunsite.dk>

On 01-06-2010 14:06, Tom Anderson wrote:

A colleague mentioned that he'd heard (from this guy's cousin's
mechanic's guy who he met in a bar's grandfather's dealer's sysop) that
the JVM requests memory from the OS in chunks of the size of -Xms, and
that you should therefore always set -Xmx to be a whole multiple of
-Xms, otherwise it would never actually request its way up to it
(because you can't make a litre from any whole number of fluid ounces).

I think i'd heard something similar at some point, although from a less
reliable source.

Is there any truth to this? Was there ever?

No truth with SUN Java 1.6.

I very much doubt there was any truth with other implementations. But
one never knows.

To very try run this with various Xms and Xmx:

import java.util.ArrayList;
import java.util.List;

public class MemIncr {
     public static void main(String[] args) {
         Runtime rt = Runtime.getRuntime();
         List<byte[]> lst = new ArrayList<byte[]>();
         while(true) {
             System.out.println((rt.totalMemory() - rt.freeMemory()) + "
out of " + rt.totalMemory() + " used (max is " + rt.maxMemory() + ")");
             lst.add(new byte[1000000]);
         }
     }
}

Arne