Page 130 - Anatomy of a Robot

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COMPUTER HARDWARE 115
Compute Crunch
It is quite difficult to get a sense of the amount of work the processor must perform in
real time. The processor will be executing instructions at full speed unless it is asked to
rest as part of saving power. During the time it is running instructions, it can only per-
form a discrete number of calculations per second. The amount of work the processor
can do is therefore limited. It is a fine art to be able to estimate how much work any one
processor can accomplish. The following should be considered during this process.

Algorithms Perform an analysis of the control algorithms and determine how many
instructions per second must be executed to accomplish the work required. If, for
instance, we know that the algorithm requires the processor to perform 5,000 instruc-
tions every 20 milliseconds, we have a measure of how busy it will be. It will be exe-
cuting at least 250,000 instructions per second (5000/.020). If the processor is capable
of executing 1 million instructions per second, then about 25 percent of its time will be
devoted to the algorithm in question.
Next, we can take a look at all the other algorithms the processor must execute and
determine how much of the processor time they will consume. Simply add up the per-
centages to get a gauge of how busy the processor will be. As a general rule of thumb,
don’t count on being able to load the processor down more than 50 percent. All proces-
sors will have housekeeping tasks to perform that cannot be easily accounted for.
Further, the more often any one algorithm is executed, the more overhead it will require.
Simplifying matters somewhat, it can be stated that a fixed amount of processor time is
required to execute any one algorithm once. If we make the algorithm shorter but have to
execute it more often, the processor will lose efficiency. Let’s look at an extreme example.
Suppose, for example, that the processor must execute 100,000 addition instructions
once a second. Suppose further that it requires 10 instructions just to kick off an algorithm.
Algorithm A executes 100,000 additions at the same time. To accomplish the required
100,000 additions per second using algorithm A, the processor must execute 100,010
instructions per second (1 (100,000 10)).
Algorithm B executes 100 additions every millisecond. To accomplish the required
100,000 additions per second using algorithm B, the processor must execute 110,000
instructions per second (1000 (100 10)).
Algorithm A will be much more efficient of the processor’s time, about 10 percent
more efficient than algorithm B. We’ll study the various methods of setting up control
algorithms in a later chapter.

Operations per Second Most computers are capable of executing many instructions
every second. Often, this number is given in millions of instructions per second (MIPS).