readme.carefully

openmp版的banchmark

Please note:  The IS code in this directory known as is.c is the most
compact serial version of the NPB2.3 parallel IS that it is possible
to devise.  As such, it is completely unnecessary to have any notion 
of buckets at all in order to correctly solve the specified NPB1 IS 
benchmark problem.  

Nevertheless, it is possible to turn on bucketing via #ifdef'ed code.
Then, the sort first rearranges the keys into buckets by range (the
bucket's ranges evenly subdivide the total key range), and then
ranks the contents of each bucket.  This results in key transfers
first into contiguous elements of buckets.  This is relatively
cache efficient, since there are a relatively small number of buckets.
Then the key counting that occurs accesses contiguous array elements.
Once again, accesses reuse cache lines efficiently.  Finally, the 
accumulation of key multiplicities (the key count) which gives the key
ranks also reuses cache line efficiently.

But using the buckets more than doubles the amount of computational
work that must be performed.  On machines with very large caches, the 
aforementioned benefits may not exist, and the extra processing looks
expensive. These examples apply to both CLASS A and B problems:

    SP2-66MhzWN:  50% speedup with buckets                          
    SGI Indy5000: 50% slowdown with buckets             
    SGI O2000:   400% slowdown with buckets (Wow!)                


Default setting is 

    /* #define USE_BUCKETS */

i.e., buckets turned off!  

It is a conjecture that cache access is the underlying mechanism 
causing these variations.

Note: If reporting timing results, either of these modes may be used 
      without penalty.