Hockney and Jesshope [13] introduced
two parameters to describe the performance of vector processors.
The first parameter is the theoretical peak performance, or
* asymptotic performance*, denoted . It is the
maximum possible rate
of computation, expressed as a number of floating point
operations per second. The parameter may be applied to a single
vector pipeline or to an entire system. Thus for a
single pipe
of the Cray Y-MP is 167 MFLOPS (6ns cycle, one result per cycle),
and approximately 2.6 GFLOPS for an 8-processor system with the
add and multiply units in operation simultaneously. The other
parameter, designated and known as the half
performance length,
is the length of the vector for which a system attains half of
its peak performance, i.e. . is
a function of vector startup
time and pipeline depth. As these values increase it becomes
harder and harder to achieve near peak performance for the system
because it requires algorithms with longer and longer vectors. As
we saw in section 2.2, the startup times for the CDC
vector computers were an order of magnitude greater than those
from Cray Research. This is reflected in for the two systems
differing also by an order of magnitude. Even though
was higher
for the CDC machines, the systems from Cray proved more popular
than those from CDC, so users seem to find lower more
important.