Re: java.util.HashMap issues -- SimpleORM Benchmarking

After much stuffing around the bug turned out to be under my nose.

SDataSet.contains used to call map.containsValue instead of 
map.containsKey.  So was doing an n^2 linear search.

Certainly the hash function is poor, with 3991 .equals calls for just 
2001 contains(), but that is not enough to kill the whole performance.

Anthony

At 05:58 PM 8/08/2008, Anthony Berglas wrote:

>I have been tidying up the benchmarks, and I have installed and
>tested against Oracle 11g.
>
>The Netbeans profiling suggests that our main performance issue is
>the HashMap in SDataSet. We are getting *Many* collisions, with our
>Equals method sometimes being called dozens of times per hash lookup
>(SDataSet.contains).
>
>I checked the hash code, and the values returned look fine -- no
>duplicates. I tried making the hash map huge, still lots of collisions.
>
>Then I looked at the strange hash code in Java, which is well described in
><http://hioctane.blogspot.com/2005/09/some-insight-into-hashmap-and-hashcode.html>http://hioctane.blogspot.com/2005/09/some-insight-into-hashmap-and-hashcode.html
>
>And sure enough, after passing our hash code through their secondary
>hasher we get lots of collisions.
>
>So a better hash map would be good. Does anyone have one or want to build one?
>
>Set SDataSet.TRACE_HASH to true to see the traces.
>
>For our (relatively) slow querySequential test the hash takes 140 ms
>out of 203 ms.
>(This is actually the fastest test, at 203 micro seconds/record,
>which makes the hash problem visible.)
>
>Anthony
>
>Dr Anthony Berglas, 
><mailto:anthony%40berglas.org>[email protected] Mobile: +61 4 4838 8874
>Just because it is possible to push twigs along the ground with ones nose
>does not necessarily mean that is the best way to collect firewood.
>
>

Dr Anthony Berglas, [email protected]       Mobile: +61 4 4838 8874
Just because it is possible to push twigs along the ground with ones nose
does not necessarily mean that is the best way to collect firewood.

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