Class average is Undergraduates are performing better. Working with low-level microcontroller timers

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1 Student feedback Low grades of the midterm exam Class average is Undergraduates are performing better Cheat sheet on the final exam? You will be allowed to bring one page of cheat sheet to the final exam Working with low-level microcontroller timers You will have the chance in lab 3 Introducing more recent technologies in class? Consider to take Embedded System 2 in the spring! Self-learned knowledge is required for course project Chance to broaden your horizon and hands-on skills ECE 1160/2160 Embedded Systems Design 1

2 ECE 1160/2160 Embedded Systems Design Raspberry Pi Linux OS Scheduling Wei Gao ECE 1160/2160 Embedded Systems Design 2

3 Process What is it? It is created by operating system to execute your code (normally.c file). It is also a program. Several processes may be associated with the same program when executing. ECE 1160/2160 Embedded Systems Design 3

4 Process Attribute of a process in Linux Major attributes of a process could be classified as three main categories. Process identification data Process state data Process control data Process ID includes unique identifier for process. Process states are ready, running or block. Process control information is used to manage process by OS. ECE 1160/2160 Embedded Systems Design 4

5 Process Process states There are three states of a process: ready, running and block the transitions between each state are as follow: ECE 1160/2160 Embedded Systems Design 5

6 Process Process control data Process control information is used by OS to manage processes Process scheduling status: e.g. in terms of ready, block.. Process structuring information: process s child ID or other processes related to current one. Processes communication information: flags, signals and messages associated among independent processes. Priority: in terms of allowed access to system resources. Progress counter: pointer to next instruction Process ID, process status, CPU scheduling information, I/O status information, memory management information, CPU registers. ECE 1160/2160 Embedded Systems Design 6

7 Process Process control block (PCB) PCB is a data structure in the operating system kernel containing the information needed to manage a particular process. PCB contains ID data, status data and control data of a process Each process has its own PCB. ECE 1160/2160 Embedded Systems Design 7

8 Process Process control block (PCB) Architecture of PCB is dependent of operating system and may contain different information in different operating system. Process ID Unique identification for each of the process in the operating system State Pointer Priority CPU register I/O information Accounting information Program counter Current state of the process A pointer to parent process Priority of the process Various CPU registers where process need to be stored for execution for running state This includes a list of I/O devices allocated to the process. This includes the amount of CPU used for process execution, time limits, execution ID etc. Program Counter is a pointer to the address of the next instruction to be executed for this process. ECE 1160/2160 Embedded Systems Design 8

9 Process lifetime of a process Process creation: process is created through system call fork() or spawn(). Process which does the creating is parent process and the created one is termed child process ECE 1160/2160 Embedded Systems Design 9

10 Process lifetime of a process Two options for parent process after creating child processes Wait for child process to terminate before proceeding. It is done by system call wait(), for either a specific child or for any child. The parent process proceeds to block until the wait() returns. Run concurrently with the child process, continue to process without waiting. ECE 1160/2160 Embedded Systems Design 10

11 Process Parent and child processes share the same binary program Create a process id variable for childe process System call to implement child process ECE 1160/2160 Embedded Systems Design 11

12 Process Parent and child processes share the same binary program pid_t pid = fork(); // fork #1 pid = fork(); // fork #2 pid = fork(); // fork #3 if (pid == 0) // child process ID is 0 { fork(); // fork #4 } fork(); // fork #5 How many processes you have after each fork? After fork #1, 2; fork #2, 4; fork #3, 8; fork #4, 12; fork #5, 24 ECE 1160/2160 Embedded Systems Design 12

13 Process Process termination: process may request their own termination by making the exit() system call, typically returning an int. int = 0: successful completion; int =!0: problems occur. childe code: parent code: int exitcode; exit( exitcode); // return exitcode pid_t pid; int status pid = wait(&status) // pit indicates which child exited. ECE 1160/2160 Embedded Systems Design 13

14 Process Process termination Process terminated by other reasons: The inability of the system to deliver the necessary system resources. In response to a KILL command (ctr+c) or other unhandled process interrupts. A parent may kill its children if the task assigned to them is no longer needed. If the parent exits, the system may or may not allow the child to continue. ECE 1160/2160 Embedded Systems Design 14

15 Process Multiprocesses in parallel Multiple processes can be run simultaneously. Switch between processes to give appearance of many processes executing simultaneously. Processes are scheduled by priority to access to CPU and other resources. ECE 1160/2160 Embedded Systems Design 15

16 Priority Priority A priority is associated with each process Run highest priority ready job (some may be blocked) Round-robin among processes of equal priority Can be preemptive or nonpreemptive Priority can be statically assigned Some always have higher priority than others (starvation issue) Priority can be dynamically changed by operating system Aging: increase the priority processes that wait in ready queue of a long time ECE 1160/2160 Embedded Systems Design 16

17 Priority Two classes of process Soft real-time processes: always schedule highest priority process Priority ranges from 0 to 99(highest), but only 1 99 can be set by user First Come, First Serve (SCHED_FIFO) or Round Robin (SCHED_RR) for processes with same priority Normal processes: priority with aging Priority ranges from -20 to 19 with -20 as the highest priority RR for processes with same priority (SCHED_NORMAL) Both process has default priority 0 and real-time process always has higher priority than normal process ECE 1160/2160 Embedded Systems Design 17

18 Priority Priority assignment Internal assignment: operating system assigns priority to process depending on measurable and technical quantities. memory usage File/IO operations External assignment: priority is assigned by criteria beyond operating system User preference Significance of the process Sum of resources ECE 1160/2160 Embedded Systems Design 18

19 Priority Priority setting Priority can be set/got by system call setpriority() and getpriority() setpriority(): #include <sys/resource.h> int which = PRIO_PROCESS; // PRIO_PROCESS for priority of process id_t pid; int priority = -20; int ret; pit = getpid(); // get process id ret = setpriority(which, pid, priority); // set process priority ECE 1160/2160 Embedded Systems Design 19

20 Priority Priority setting Priority can be set/got by system call setpriority() and getpriority() getpriority() #include <sys/resource.h> int which = PRIO_PROCESS; // PRIO_PROCESS for priority of process id_t pid; int ret; pit = getpid(); // get process id ret = getpriority(which, pid); // get process priority ECE 1160/2160 Embedded Systems Design 20

21 Scheduling Algorithm First-Come, First-Serve (FCFS) Simplest CPU scheduling algorithm First jobs that requests the CPU gets the CPU Non-preemptive Implementation FIFO queue ECE 1160/2160 Embedded Systems Design 21

22 Scheduling Algorithm Example of FCFS Process table Gantt chart Average waiting time: ( )/4 = 7.5 ECE 1160/2160 Embedded Systems Design 22

23 Scheduling Algorithm Example of FCFS: different arrival order Arriving order Gantt chart Average waiting time: ( )/4 = 3.75 ECE 1160/2160 Embedded Systems Design 23

24 Scheduling Algorithm FCFS advantages and disadvantages Advantages: Simple Fair Disadvantages: Waiting time depends on arrival order Short process stuck waiting for long process Also called head of line blocking ECE 1160/2160 Embedded Systems Design 24

25 Scheduling Algorithm Round Robin (RR) Practical solution to support time-sharing Run process for a time slice, then move to back of FIFO queue Preempted if still running at the end of time slice ECE 1160/2160 Embedded Systems Design 25

26 Scheduling Algorithm Example of RR: time slice = 3 Gantt chart with time slice = 3 Arrive: Queue: Average waiting time: ( )/4 = 7 Average response time: ( )/4 = 2.75 Number of context switches: 7 ECE 1160/2160 Embedded Systems Design 26

27 Scheduling Algorithm Example of RR: smaller time slice Gantt chart with time slice = 1 Arrive: Queue: Average waiting time: ( )/4 = 5.5 Average response time: ( )/4 = 0.75 Number of context switches: 14 ECE 1160/2160 Embedded Systems Design 27

28 Scheduling Algorithm Example of RR: larger time slice Gantt chart with time slice = 10 Arrive: Queue: Average waiting time: ( )/4 = 4.75 Average response time: same Number of context switches: 3 (minimum) ECE 1160/2160 Embedded Systems Design 28

29 Scheduling Algorithm RR advantages and disadvantages Advantages Low response time, good interactivity Fair allocation of CPU across processes Low average waiting time when job lengths vary widely Disadvantages Poor average waiting time when jobs have similar lengths Average waiting time is even worse than FCFS Performance depends on length of time slice Too high degenerate to FCFS Too low too many context switches, costly ECE 1160/2160 Embedded Systems Design 29

30 Lab 3 Lab 3 is due on 11/27 and 11/29 6% in final grade You will work on it on your own No collaboration is allowed! Need to let the TA check you off Practicing Linux real-time scheduling over Raspberry Pi Built on the sample code provided RMS, EDF and priority inheritance ECE 1160/2160 Embedded Systems Design 30