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##

Vectorization

Designing vectorizable algorithms in linear algebra is usually
straightforward.
Indeed, for many computations there are several variants, all vectorizable,
but
with different characteristics in performance (see, for example,
[45]).
Linear algebra algorithms can come close to the peak performance of
many machines -- principally because peak performance depends on some form
of
chaining of vector addition and multiplication operations, and this is just
what the algorithms require.

However, when the algorithms are realized in straightforward Fortran 77
code,
the performance may fall well short of the expected level, usually because
vectorizing Fortran compilers fail to minimize the number of memory
references
-- that is, the number of vector load and store operations. This brings us
to
the next factor.

*Susan Blackford*

*1999-10-01*