Re: Hash table performance

From:

=?UTF-8?Q?Marcin_Rze=C5=BAnicki?= <marcin.rzeznicki@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 22 Nov 2009 09:35:26 -0800 (PST)

Message-ID:

<9fa65931-40e9-4073-80e0-7cd8930c778c@m26g2000yqb.googlegroups.com>

On 21 Lis, 20:07, Arne Vajh=C3=B8j <a...@vajhoej.dk> wrote:

Marcin Rze=C5=BAnicki wrote:

On 21 Lis, 19:33, Jon Harrop <j...@ffconsultancy.com> 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) = " + hash=

table.get(100.0));

}
}

My guess is that this is because the JVM is boxing every floating poin=

number individually in the hash table due to type erasure whereas .NET
creates a specialized data structure specifically for a float->float h=

ash

table with the floats unboxed. Consequently, the JVM is doing enormous=

numbers of allocations whereas .NET is not.

Is that correct?

You are using Hashtable instead of HashMap - probably the performance
loss you've observed is due to synchronization (though "fat"
synchronization might be optimized away in case of single thread you
still pay the price, though lower). If you took a look at JavaDoc,
you'd notice that HashTable methods are synchronized As of boxing, you
are correct (though there is no type erasure in your example because
you did not specify type parameters at all) but I suspect that these
costs are not the most contributing factor to overall poor
performance. I'd blame synchronization in the first place.

That does not match measurements.

Arne

Yeah, I see. Well, what VM did Jon use? That might be crucial to know
what happened.