Friday, May 22, 2009


Java - memory, x64, performance

A long time ago, I wrote how bad x64 Java is. Well the good news is -- no more. With 6u14 just around the corner, sanity will be back. Starting update 14, the 64-bit version with Heap sizes below 32GB will use almost the same amount of memory as the 32-bit JVM. All this because of the use compressed pointers to save memory.

It gets even better, the compressed pointers are already available with the Java SE Performance Release since July 2008 (someone should have told me earlier). All that needs to be done is, use -XX:+UseCompressedOops. So I finally decided to give it a try and compare memory usages.

To compare, I used the following test program with 1.6.0_11i686 (32-bit), 1.6.0_11x64 (64-bit), and 1.6.0_06performace with -XX:+UseCompressedOops (64-bit performance) on my desktop with 8GB memory and Intel quad core processor. The minimum amount of memory required to run the program are reported below.

import java.util.HashMap;
import java.util.Map;
public class JavaMemoryProblems {
   public static void main(String[] args) {
       Map<String, Integer> map = new HashMap<String, Integer>();
       int max = 1000000;
       for (int i=0; i<max; i++) {
           map.put("key"+i, new Integer(i));
       System.out.println("Finished with a map of size "+map.size());

with -XX:+UseSerialGC with -XX:+UseParallelGC
32-bit 110 MB 110 MB
64-bit 180 MB 220 MB
performance 130 MB 160 MB

The test program is the same as my previous post, and the decrease in the memory is quite amazing. Time to throw the 32-bit JVM out of the window. I haven't done any tests around increase in CPU time, but I do not expect any penalty in run time based on few of my google searches


Update on 31st May 2009: After using the performance release in production for a few days I realize it is broken. The application is performing weirdly, giving incorrect output and throwing exceptions. I have now switched back to the regular release 6u13 release, and everything is fine again. I am not sure if it is really a Java bug or something else, but I am not taking any chances.

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If you need a HashMap with low memory footprint have a look at:
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