Re: Hash table performance

From:
=?ISO-8859-1?Q?Arne_Vajh=F8j?= <arne@vajhoej.dk>
Newsgroups:
comp.lang.java.programmer
Date:
Sat, 21 Nov 2009 14:06:27 -0500
Message-ID:
<4b083a34$0$269$14726298@news.sunsite.dk>
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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