Hardware Reference
In-Depth Information
ing challenging. The degree to which these hurdles are difficult or easy is determined both by
the application and by the architecture.
The first hurdle has to do with the limited parallelism available in programs, and the second
arises from the relatively high cost of communications. Limitations in available parallelism
make it difficult to achieve good speedups in any parallel processor, as our first example
shows.
Example
Suppose you want to achieve a speedup of 80 with 100 processors. What frac-
tion of the original computation can be sequential?
Answer
Recall from
Chapter 1
that Amdahl's law is
For simplicity in this example, assume that the program operates in only two
modes: parallel with all processors fully used, which is the enhanced mode, or
serial with only one processor in use. With this simplification, the speedup in
enhanced mode is simply the number of processors, while the fraction of en-
hanced mode is the time spent in parallel mode. Substituting into the previous
equation:
Simplifying this equation yields:
Search WWH ::
Custom Search