Reminder. Final exam. Tentative time: December 6th, 4:30pm-5:45pm Location: G26. ECE 1160/2160 Embedded System Design 1

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1 Reminder Final exam Tentative time: December 6th, 4:30pm-5:45pm Location: G26 ECE 1160/2160 Embedded System Design 1

2 Recap from last class Hardware support CMOS, Clock gating, Supply shutdown, Dynamic voltage scaling Dynamic power management Adjusts power mode to adapt to workload variations Baseline: greedy policy Break-even time T BE Energy saving Predictive techniques Power manager Advanced Configuration and Power Interface (ACPI) Holistic approach ECE 1160/2160 Embedded System Design 2

3 ECE 1160/2160 Embedded System Design Operating Systems - I Wei Gao Fall

4 Operating Systems for Embedded Systems Processes Context switch Scheduling Inter-process communication Examples TinyOS POSIX Real-Time OS ECE 1160/2160 Embedded System Design 4

5 Basic Functions OS controls resources: who gets the CPU; when I/O takes place; how much memory is allocated. Application programs run on top of OS services Challenge: manage multiple, concurrent tasks. ECE 1160/2160 Embedded System Design 5

6 Example: Engine Control Concurrent tasks: spark plug firing crankshaft sensing fuel/air mixture oxygen sensor Tasks have different periods / rates Must finish within the period Rate of spark plug control relies on engine speed engine controller ECE 1160/2160 Embedded System Design 6

7 Example: Sensors Sensing Sampling (multiple) sensors in different rates Communication Send data Receive data Computation Data processing and aggregation Routing ECE 1160/2160 Embedded System Design 7

8 Life without Processes Code turns into a mess: Spaghetti code do all tasks in a single piece of code (with a loop) no separation of concern Control structure gets very complex Hard to guarantee different timing requirements Cannot handle varying execution time or rate Cannot handle dynamic task arrivals time A B C A C A_code(); B_code(); if (C) C_code(); A_code(); switch (x) { case C: C(); case D: D();... ECE 1160/2160 Embedded System Design 8

9 Co-Routines Methodology Idea to realize controlled concurrency Separate tasks into different code segments (co-routines) Co-routines voluntarily give up control to other co-routines. Pattern of control transfers and context-switch are embedded in the code. Co-routine 1 Co-routine 2 ADR r14,co2a co1a ADR r13,co1b MOV r15,r14 co1b ADR r13,co1c MOV r15,r14 co1c... co2a ADR r14,co2b MOV r15,r13 co2b ADR r14,co2c MOV r15,r13 co2c ECE 1160/2160 Embedded System Design 9

10 Process A process is a unique execution of a program. Several copies of a program may run simultaneously or at different times. A process has its own context: Data in registers, PC, status, memory. Stored in activation record OS manages processes. TinyOS: a task is similar to a process ECE 1160/2160 Embedded System Design 10

11 Processes and CPUs Activation record process context. Context switch: current CPU context goes out new CPU context goes in process 1 context process 2 context PC registers... CPU memory ECE 1160/2160 Embedded System Design 11

12 Cooperative Multitasking Improvement to co-routines: hides context switching mechanism; still relies on processes to voluntarily give up CPU. Each process allows a context switch at cswitch() call. Separate scheduler chooses which process runs next. if (x > 2) else sub1(y); sub2(y, 2); cswitch(); proca(a, b, c); Process 1 Student A save_state(current); p = choose_process(); load_and_go(p); Scheduler TA proc_data(r, s, t); cswitch(); If (val1 == 3) abc(val2); rst(val3); Process 2 Student B ECE 1160/2160 Embedded System Design 12

13 Problems with Cooperative Multitasking Programming errors can keep other processes out: process never gives up CPU; process waits too long to switch, missing input. Process2() { x = global1; /* global1 is an input to the process */ while (x < 500) x = aproc(global2); /* subroutine does its work */ cswitch(); } ECE 1160/2160 Embedded System Design 13

14 Preemptive Multitasking No more voluntary release of CPU Operating System (OS) is now in charge Most powerful form of multitasking: OS controls when context switches; OS determines what process runs next. Use periodic timer interrupts to call OS to switch contexts Timer CPU interrupt interrupt interrupt P1 OS P1 OS P2 Flow of control with preemption time ECE 1160/2160 Embedded System Design 14

15 Preemptive Context Switching Timer interrupt gives control to OS, which saves interrupted process s state in an activation record. OS chooses next process to run. OS installs desired activation record as current CPU state. ECE 1160/2160 Embedded System Design 15

16 POSIX IEEE standards designed to provide application portability between Unix variants. IEEE defines a Unix-like OS interface. IEEE defines the shell and utilities IEEE defines real-time extensions. Supported by many operating systems Variants of UNIX: AIX, HP-UX, Solaris, Linux Many commercial RTOS, e.g., VxWorks Windows provides similar services ECE 1160/2160 Embedded System Design 16

17 Processes in POSIX Create a process with fork: parent process keeps executing the old program; child process executes a new program. Process A Process A Process B ECE 1160/2160 Embedded System Design 17

18 Process Management OS keeps track of: process priorities; scheduling state; process control block. Processes may be created: statically before system starts; dynamically during execution. OS controls when context switches and what process runs. ECE 1160/2160 Embedded System Design 18

19 Priority-Driven Scheduling Every process has a priority. CPU goes to the highest-priority ready process Variants Fixed vs. dynamic priority Preemptive vs. non-preemptive ECE 1160/2160 Embedded System Design 19

20 Preemptive Priority Scheduling Most common real-time scheduling approach Real-Time POSIX Real-time priorities in Linux, Solaris, and Windows Most RTOS: VxWorks Not the only possible way Clock-driven scheduling (e.g., round robin) Reservation-based scheduling Proportional share scheduling FIFO scheduling ECE 1160/2160 Embedded System Design 20

21 Example: Space Shuttle Software Error Space Shuttle s first launch was delayed by a software timing error: Primary control system PASS and backup system BFS. BFS failed to synchronize with PASS. A change to one routine added unnoticed delay that threw off start time calculation. 1 in 67 chance of timing problem. ECE 1160/2160 Embedded System Design 21

22 Inter-Process Communication Inter-process communication (IPC) OS provides mechanisms so that processes can pass data. IPC styles Shared memory: processes have some memory in common; must cooperate to avoid destroying/missing messages. Message passing: processes send messages along a communication channel---no common address space. ECE 1160/2160 Embedded System Design 22

23 Shared Memory and Problems Process 1 and 2 take turn to execute on the CPU Problem when two processes try to write the shared memory location: Race condition process 1 reads flag and sees 0. process 2 reads flag and sees 0. process 1 sets flag to one and writes location. process 2 sets flag to one and overwrites the same location. if (flag == 0) /* preempted*/ flag=1; loc=var; /* preempted*/ print(loc); if (flag == 0) var = 5; flag=1; loc=var; memory var = 2; process 1 process 2 if (flag == 0) /* preempted*/ flag=1; loc=var; /* preempted*/ ECE 455/555 Embedded System Design 23

24 Race Conditions Conditions for race conditions to happen Concurrent processes/tasks access shared variables. Preemption/interruption at a wrong time. Atomic section: section of code that cannot be interrupted by another process. Critical section: section of code that must not be concurrently accessed by more than one thread of execution. Mutual exclusion POSIX: preemptive scheduling race among processes. ECE 455/555 Embedded System Design 24

25 Summary OS: manage multiple, concurrent tasks Process Co-routines methodology Multitasking with context switch Processes in TinyOS and POSIX fork(), execv() Process states and management Process scheduling Priority-driven scheduling Space shuttle software error Inter-process communication Shared memory Message passing ECE 1160/2160 Embedded System Design 25

26 Reading Recommended reading: Operating System Concepts, A. Silberschatz, B. Galvin and G. Gagne, 8 th edition, (the famous dinosaur book) ECE 1160/2160 Embedded System Design 26