Synergy User Manual and Tutorial. - THE CORE MEMORY

Synergy User Manual and Tutorial
Petri Net
Amdahl’s Law
Gene Amdahl, a computer architect, entrepreneur, former IBM employee and one of the
creators of the IBM System 360 architecture, devised this method in 1967 to determine
the maximum expected improvement to a system when only part of it has been improved.
He presented this as an argument against parallel processing. This law is similar to the
law of diminished returns, which states that as more input is applied, each additional
input unit will produce less additional output. Amdahl’s law states that a number of
functions or operations must be executed sequentially, decreasing a computer’s speed
when more processors are added. In other words, the number of tasks that must be
completed sequentially limits computational speedup. This causes a bottleneck in the
workflow, slowing the overall task. However as the size of a task increases the effect of
Amdahl’s law decreases. The speedup of a system is:
unimproved _ time
= speedup =
improved _ time
performance _ with _ improvement
performance _ without _ improvement
If you make an improvement that greatly increases performance (maybe 100 times or
more) in part of a computation but the overall improvement is only 25 percent, then the
upper limit for speedup S is:
S
unimproved _ time
=
improved _ time
1.00
= = 1.333
1.00 − 0.25
Note: The unimproved execution time is 1.00 = 100% because this example makes use of
the ratio between the two times, not the actual values. Assume that an unimproved
computation takes 4 seconds and the improved computation takes 3 seconds. The
equation is:
S
unimproved _ time
=
improved _ time
=
4sec
3sec
= 1.333
If the improved computation is taken to be 100 percent performance, then by the
relationship above the unimproved computation has 75 percent performance with respect
to the improved.
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