The following program demonstrates that accessing elements of a direct (i.e.,
native-memory) byte buffer is significantly slower with C2 than accessing
elements of a heap-allocated byte buffer or of a simple byte array:

    % /usr/local/java/jdk1.4/linux-i386/bin/java -server -showversion BBSpeed
    java version "1.4.0-beta"
    Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.0-beta-b50)
    Java HotSpot(TM) Server VM (build 1.4beta-B50, mixed mode)

    array 37
    array rev 80
    heap 38
    heap rev 55
    direct 565
    direct rev 580
    % 

This is on Linux; I get similar numbers on Solaris (times are in ms).

Making both direct and heap buffers perform as well as arrays (or nearly so) is
critical to the performance of the java.nio subsystem.

-- mr@eng 2001/2/4


---- BBSpeed.java

import java.nio.*;


public class BBSpeed {

    static final int SIZE = 4096;
    static final int TRIALS = 4096;

    static int sum(ByteBuffer bb) {
	int n = bb.capacity();
	int a = 0;
	bb.rewind();
	for (int j = 0; j < n; j++)
	    a += bb.get(j);
	return a;
    }

    static int sumReverse(ByteBuffer bb) {
	int n = bb.capacity();
	int a = 0;
	bb.rewind();
	for (int j = n - 1; j >= 0; j--)
	    a += bb.get(j);
	return a;
    }

    static int time(String name, ByteBuffer bb) {
	int n = bb.capacity();
	bb.limit(bb.capacity());
	int a = 0;

	for (int i = 0; i < TRIALS; i++) sum(bb);
	long start = System.currentTimeMillis();
	for (int i = 0; i < TRIALS; i++) sum(bb);
	System.out.println(name + " " + (System.currentTimeMillis() - start));

	for (int i = 0; i < TRIALS; i++) sumReverse(bb);
	start = System.currentTimeMillis();
	for (int i = 0; i < TRIALS; i++) sumReverse(bb);
	System.out.println(name + " rev " + (System.currentTimeMillis() - start));

	return a;
    }

    static int sum(byte[] ba) {
	int n = ba.length;
	int a = 0;
	for (int j = 0; j < n; j++)
	    a += ba[j];
	return a;
    }

    static int sumReverse(byte[] ba) {
	int n = ba.length;
	int a = 0;
	for (int j = n - 1; j >= 0; j--)
	    a += ba[j];
	return a;
    }

    static int timeArray() {
	byte[] ba = new byte[SIZE];
	int a = 0;
	
	for (int i = 0; i < TRIALS; i++) a += sum(ba);
	long start = System.currentTimeMillis();
	for (int i = 0; i < TRIALS; i++) a += sum(ba);
	System.out.println("array " + (System.currentTimeMillis() - start));

	for (int i = 0; i < TRIALS; i++) a += sumReverse(ba);
	start = System.currentTimeMillis();
	for (int i = 0; i < TRIALS; i++) a += sumReverse(ba);
	System.out.println("array rev " + (System.currentTimeMillis() - start));

	return a;
    }

    public static void main(String[] args) {
	String key = "hda";
	if (args.length == 1)
	    key = args[0];
	int a = 0;
	if (key.indexOf('a') >= 0)
	    a += timeArray();
	if (key.indexOf('h') >= 0)
	    a += time("heap", ByteBuffer.allocate(SIZE));
	if (key.indexOf('d') >= 0)
	    a += time("direct", ByteBuffer.allocateDirect(SIZE));
	System.exit(a);
    }

}