8/17/2019 Slides04_03 RTS
1/19
Scheduling
Basic Concepts
Ref: Hard Real-Time Computing Systems
Giorgio Buttazzo
Ref: Real-Time Systems & Software Alan Shaw
Processes - Tasks - Threads
8/17/2019 Slides04_03 RTS
2/19
Real-Time Systems
Periodic and periodic
Hard & Soft !eadlines
Scheduling of Tasks " Static & !ynamic
Sharing of Resources and Critical Sections
!omino #ffect
8/17/2019 Slides04_03 RTS
3/19
Best #ffort Scheduling lgorithms
8/17/2019 Slides04_03 RTS
4/19
#$ample Cost %unctionsWhat is a “good” system?
8/17/2019 Slides04_03 RTS
5/19
etrics for Performance #'aluationWhat is a “good” system?
8/17/2019 Slides04_03 RTS
6/19
ttempting to inimi(e )ateness
8/17/2019 Slides04_03 RTS
7/19
Scheduling nomalies
8/17/2019 Slides04_03 RTS
8/19
Precedence #$ample
8/17/2019 Slides04_03 RTS
9/19
dding a processor
8/17/2019 Slides04_03 RTS
10/19
Reducing Computation Times *
8/17/2019 Slides04_03 RTS
11/19
+eaken Constraints
8/17/2019 Slides04_03 RTS
12/19
Resource Constraints
8/17/2019 Slides04_03 RTS
13/19
Successive events are separated by at least p
The Sporadic process
Shaw’s notation:
8/17/2019 Slides04_03 RTS
14/19
Cyclic Executives for Scheduling Periodic Tasks:
Broken into ,locks:
aor Cycles .CT/ " shortest period where all task periods align
all tasks e$ecute in a maor cycle
inor Cycles .mct/ " 0frames1 used to allocate tasks or portions of tasks
#$ample: Say you ha'e two tasks:
T2 3 .c24 p24 d2/ 3 .24 54 5/
T5 3 .c54 p54 d5/ 3 .24 64 6/
7$$$$$$8888887$$$$$$8888887$$$$$$888888799
7888888$$$$$$8888887$$$$$$888888888888799
; 2 5 6 < = >
CT 3 >
mct 3 5 .or 2/
llocations are: ?T24T5@ ?T24T5@ ?T2@4 repeat
8/17/2019 Slides04_03 RTS
15/19
Can this ,e scheduled*
+hat is CT*
+hat is mct*
+hat is a schedule*
8/17/2019 Slides04_03 RTS
16/19
8/17/2019 Slides04_03 RTS
17/19
da-like implementation of this scheduling
Clock returns the current time
Delayuntil x causes the task to ,lock until at least time x
8/17/2019 Slides04_03 RTS
18/19
Some principles a,out periodic scheduling:
+e assume all processes are released at time t; +hy*
The CT is the least common multiple .lcm/ of the periods +hy*
The mct must di'ide the CT +hy*
The mct must ,e A3 the longest computation ,lock +hy*
mct 3 di for all 3 2494 n
mct D .mct - gcd.mct4 pi// 3 di
8/17/2019 Slides04_03 RTS
19/19
!o"e work #$: .)ecture