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In-Depth Information
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FIGURE 17.2
Interrupt events.
It is important to understand the worst-case interrupt disabling sequence, as a
real-time system depends on the critical events in the system being executed within
the required time frame.
17.3.3
Time Loading
The CPU utilization or time-loading factor U is a measure of the percentage of
nonidle processing in a computer. A system is considered time overloaded if the CPU
utilization is more than 100%. Figure 17.3
17
is an illustration of the typical CPU
utilization zones and typical applications and recommendations.
A utilization of about 50% is common for new products; however, a CPU utilization
of up to about 80% may be acceptable for a system that does not anticipate growth
(LaPlante, 2005). A CPU utilization of about 70% is probably the most common
and most recommended CPU utilization for a real-time system. However, there are
several different opinions available. For example, one study indicates that system
designers should strive to keep CPU use below 50%, as a CPU with a high utilization
will lead to unpredictable real-time behavior. Also, it is possible that the high-priority
tasks in the system will starve the low-priority tasks of any CPU time. This can cause
the low-priority tasks to misbehave (Eventhelix.com, 2001).
CPU utilization,
U
, can be defined by the following:
U
=
100%
−
(time spent in a idle task)
where the idle task is the task with the absolute lowest priority in a multitasking
system.
18
This task also sometimes is called the background task or background
loop. This logic traditionally has a while(1) type of loop in which an infinite loop
spins the CPU waiting for an indication that critical work needs to be done.
17
www.cse.buffalo.edu/
bina/cse321/fall2007/IntroRTSAug30.ppt
18
http://www.design-reuse.com/articles/8289/how-to-calculate-cpu-utilization.html
∼
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