When I run my timing test program in Java Hotspot client, I get consistent behavior. However, when I run it in Hotspot server, I get unexpected result. Essentially, the cost of polymorphism is unacceptably high in certain situations that I've tried to duplicate bellow.
Is this a known issue/bug with Hotspot server, or am I doing something wrong?
Test program and timing are given bellow:
Intel i7, Windows 8
Java HotSpot(TM) 64-Bit Server VM (build 24.45-b08, mixed mode)
Mine2: 0.387028831 <--- polymorphic call with expected timing
Trivial: 1.545411765 <--- some more polymorphic calls
Mine: 0.727726371 <--- polymorphic call with unexpected timing. Should be about 0.38
Mine: 0.383132698 <--- direct call with expected timing
The situation gets worse as I add additional tests. Timing of the tests near the end of the list are completely off.
interface canDoIsSquare {
boolean isSquare(long x);
}
final class Trivial implements canDoIsSquare {
@Override final public boolean isSquare(long x) {
if (x > 0) {
long t = (long) Math.sqrt(x);
return t * t == x;
}
return x == 0;
}
@Override public String toString() {return "Trivial";}
}
final class Mine implements canDoIsSquare {
@Override final public boolean isSquare(long x) {
if (x > 0) {
while ((x & 3) == 0)
x >>= 2;
if ((x & 2) != 0 || (x & 7) == 5)
return false;
final long t = (long) Math.sqrt(x);
return (t * t == x);
}
return x == 0;
}
@Override public String toString() {return "Mine";}
}
final class Mine2 implements canDoIsSquare {
@Override final public boolean isSquare(long x) {
// just duplicated code for this test
if (x > 0) {
while ((x & 3) == 0)
x >>= 2;
if ((x & 2) != 0 || (x & 7) == 5)
return false;
final long t = (long) Math.sqrt(x);
return (t * t == x);
}
return x == 0;
}
@Override final public String toString() {return "Mine2";}
}
public class IsSquared {
static final long init = (long) (Integer.MAX_VALUE / 8)
* (Integer.MAX_VALUE / 2) + 1L;
static long test1(final canDoIsSquare fun) {
long r = init;
long startTimeNano = System.nanoTime();
while (!fun.isSquare(r))
++r;
long taskTimeNano = System.nanoTime() - startTimeNano;
System.out.println(fun + ": " + taskTimeNano / 1e9);
return r;
}
static public void main(String[] args) {
Mine mine = new Mine();
Trivial trivial = new Trivial();
Mine2 mine2 = new Mine2();
test1(mine2);
test1(trivial);
test1(mine);
long r = init;
long startTimeNano = System.nanoTime();
while (!mine.isSquare(r))
++r;
long taskTimeNano = System.nanoTime() - startTimeNano;
System.out.println(mine + ": " + taskTimeNano / 1e9);
System.out.println(r);
}
}
javaccompiler optimises almost nothing, it is the JIT which does this. You can't expect the JVM to perform magic. – PilferhashCode()behaves exactly the same way. You are passing a reference to acanDoIsSquarenot a concrete type. Create two copies of test1 i.e. test2 and you as long as you always pass the same implementation class you will get the best performance. – Pilfer