Re: Hash table performance
Patricia Shanahan wrote:
Jon Harrop wrote:
I'm having trouble getting Java's hash tables to run as fast as .NET's.
Specifically, the following program is 32x slower than the equivalent
on .NET:
import java.util.Hashtable;
public class Hashtbl {
public static void main(String args[]){
Hashtable hashtable = new Hashtable();
for(int i=1; i<=10000000; ++i) {
double x = i;
hashtable.put(x, 1.0 / x);
}
System.out.println("hashtable(100.0) = " + hashtable.get(100.0));
}
}
My guess is that this is because the JVM is boxing every floating point
number individually in the hash table due to type erasure whereas .NET
creates a specialized data structure specifically for a float->float hash
table with the floats unboxed. Consequently, the JVM is doing enormously
numbers of allocations whereas .NET is not.
Is that correct?
I think there has to be something more to it than just the autoboxing.
My reasoning is that you never reuse a key, so every put call creates a
new Entry instance. Creating a Double from a double is about as simple
as object creation can be, so I don't see how the boxing could to more
than triple the time spent in object creation during an average put
call. That cannot, by itself, account for a 32x performance ratio.
I think it can.
I did some test on my PC.
Java both HashMap and Hashtable are about 8 times slower
than .NET Dictionary<double,double>, but both .NET
Dictionary<object,object> and Hashtable are about the
same speed as Java.
Arne
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