large size hash-tables and CPU time

We are running ACL 6.2 on Sun OS and wonder what we are wrong..

Our implementation relies on storing data in hash-tables.  We do not
know a priori the number of data items to be stored (could vary from
20 data items to 60,000 even more).

We noticed (with profiler) that, with a fixed hash-table size, much
effort is spent on (re-)hashing.  So, we tried to declare the size of
the hashtable as some function of the input parameters.  We use the
following rehash arguments :rehash-threshold 0.9 :rehash-size 1.5.

When we run our experiments, the CPU time performance (measured with
the function time, w/o accounting for the effort spent on gc) is very
erratic: sometimes the fixed size hash-table shows a better
performance sometimes the opposite is true... 

This is a serious problem as it is preventing us from drawing any
conclusions WRT to the `algorithmic' benefits of our research.

Are we doing anything wrong?  how can we improve our implementation in
order to have a more `stable' results?

Thank you for any pointer.


Lin XU






                                                                                 
               .'*                                                              
            :`...' `.,'  '                                                      
          `.  ' .**.  ; ; ':                                                    
          ` ``:`****,'  .' :                                                    
        ..::.  ``**":.''   `.                                           
      .:    `: ; `,'        :    For everyone you love and love you!                                          
        `:    `   :         ;                                  
          :   :   :        ;                             
          :    :   :     .:                             
           :    :   :..,'  ``::.          Lin Xu                                      
            `....:..'  ..:;''                                                   
            .:   . ...::::                                      
           ,'''''``:::::::                                                      
                     `::::                                                      
                       `::.                                                     
                         `::