That is why communication performance for short messages is vital for scalable systems. This way, the ratio between computation and communication is expected to be roughly equal for the two systems. The definition of three different working set sizes for the two systems and their corresponding single CPU execution time is listed in table 2 . The scalability of Cray T3E is shown in table 3 . As expected, the large working set yields better scalability.
Scalability is less than linear for all cases.
For the small working set, the speedup seems to level out in excess of 2.1. We observe no super linear speedup, although the medium working set fits into the L2 cache of the Alpha processor for 2 CPUs and more. The scalability of Scali Pentium III cluster is shown in table 4 . We see very good scalability for the large working set, with super linear speedup running on 8 and 16 CPUs with one MPI process per node. Overall, utilizing two CPUs per node yields slightly less scalability compared to the one CPU per node case, except for the small working set case running on 8 or fewer CPUs, as discussed above .