Re: Hash table performance
On Sat, 21 Nov 2009, Jon Harrop wrote:
I'm having trouble getting Java's hash tables
I note you're using java.util.Hashtable. That's an ancient class which has
a few things wrong with it: i'd strongly suggest using java.util.HashMap
instead. I wouldn't expect it to be significantly faster in this test,
though.
to run as fast as .NET's. [...] 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 can't comment on what the CLR is doing in a comparable situation, but
that is certainly a reasonable description of what java is doing.
Could you try changing the put line to:
hashtable.put(Double.toString(x), Double.toString(1.0 / x));
And making the corresponding change in the C# or whatever version, and
making the comparison again? That eliminates boxing in java, so if the
difference is due to boxing, it will be significantly reduced, which will
give you some clues as to what's going on.
If you need optimal performance for a double->double map in java, you
would need to write one specifically for that case. Or rather, someone
would: you could have a google to see if someone already has. Also, since
converting doubles to longs is cheap and reversible, you could look for a
long->long hashmap, which strikes me as more likely to exist. I didn't
find one in two minutes of googling, but i did find some long->Object
hashmaps, which get you halfway there.
tom
--
Oh, well of course *everything* looks bad if you remember it