real time systems

Question 1

what is a real time process

Answer 1

a process that delivers results of processing in a given time span

Question 2

what is a real time system

Answer 2

a system where its correctness is based on the correct result but also on time

Question 3

deadline

Answer 3

the latest acceptable time for a process to complete

Question 4

soft real time

Answer 4

deadlines can be broken

Question 5

hard real time

Answer 5

deadlines are critical
e.g. embedded/control systems

Question 6

what is real time scheduling driven by

Answer 6

events and deadlines

Question 7

periodic events

Answer 7

predictable e.g. clock

Question 8

aperiodic events

Answer 8

unpredictable; expected but not sure when

Question 9

why do we require admission control

Answer 9

to ensure that we dont take on too much work so everything can be done before their deadline
ensures that resources are never being used more that 100% (including overhead)

Question 10

what is the rate of periodic tasks

Answer 10

1/period

Question 11

what two things do we need to check when carrying out periodic tasks

Answer 11

the processing time must be < the process deadline
the total processing time per cycle must be < the period

Question 12

what are the two types of preemptive real time schedulers

Answer 12

rate monotonic (rm)
earliest deadline first (edf)

Question 13

rate monotonic

Answer 13

assumes the processes have the same amount of work each cycle
fixed repeating schedule for worst case
the priority is inversely proportional to work required: more frequent = higher priority

Question 14

how does rate monotonic have a fixed repeating schedule

Answer 14

fixed priority: priority assigned once based on its period and never changes
tasks with shorter periods and higher frequency always have a higher priority than those with longer periods

repeating pattern: since tasks are periodic they always execute at fixed time intervals therefore the schedule is predictable