Real Time System

1. Real Time System Multitasking

2. Layers of abstraction for concurrency in programs

3. E.g. Furnace Control • Three task • Furnace Heater Control • Set Threshold Temperature • Refresh Display with current temperature • Single execution thread • Multiple threads of execution

4. Multitasking • fundamental mechanism for an application to control and react to multiple, discrete real-world events • Improve responsiveness by avoiding situations where long-running task can block a task that responds to external stimuli • Improved responsiveness reduces latency • improve performance by allowing a program to run simultaneously on multiple processors or cores • MultiCore processors

5. Multitasking • directly control the timing of external interactions • E.g display refresh • Task states

6. Multithreading • In broad sense, threads exist in the form of interrupts on almost all microprocessors • Most operating systems provide a higher-level mechanism than interrupts to realize imperative programs that share memory • Typically they conform to standardized API • which makes it possible to write programs that are portable • Eg. Pthreads or POSIX threads

7. Implementing threads- scheduling • cooperative multitasking • does not interrupt a thread unless the thread itself calls a certain procedure or make a system call • Disadvantage – a task may execute without making any OS system call • Other thread will be starved • Most OS have a ISR routing triggered by a TIMER after every time interval called JIFFY which schedule different threads.

8. Simple Multithreaded C Program • #include <pthread.h> • #include <stdio.h> • void* printN(void* arg) { • for (int i = 0; i < 10; i++) { • printf("My ID: %d\n", *(int*)arg); } • return NULL; • } • int main(void) { • pthread_t threadID1, threadID2; • void* exitStatus; • int x1 = 1, x2 = 2, x3 = 3; • pthread_mutex_init( mymutex,0); • pthread_create(&threadID1, NULL, printN, &x1); • pthread_create(&threadID2, NULL, printN, &x2); • printf("Started threads.\n"); • pthread_join(threadID1, &exitStatus); • pthread_join(threadID2, &exitStatus); • return 0; • }

9. Output of the simple program SStSatrtatraredttthredthredeadste. hrMyads.ea ds. MIyD :ID: 1 1 MMyyID: 2I MyD: ID: 2 M y2 M Iy ID:D: 1 My 2 I MDy: I D1: M2 y My ID: I1D: My I2 MDy I:D: 1 M1y My IDID: 2 : M1 y MIyD: 2 I DM:y 1 MID: y2 MID: y ID1: M 2y ID: 2

10. Mutual Exclusion • Race condition • Two concurrent pieces of code race to access the same resource, and the exact order in which their accesses occurs affects the results of the program • Critical Section • To prevent MUTEX(Mutual Exclusion Locks) are used

11. Simple Multithreaded C Program • pthread_mutex_t *mymutex= (pthread_mutex_t *)PTHREAD_MUTEX_DEFAULT; • void* printN(void* arg) { • pthread_mutex_lock(mymutex); • for (int i = 0; i < 10; i++) {printf("My ID: %d\n", *(int*)arg);} • pthread_mutex_unlock(mymutex); • return NULL; • } • int main(void) { • pthread_t threadID1, threadID2; • void* exitStatus; • int x1 = 1, x2 = 2, x3 = 3; • pthread_mutex_init( mymutex,0); • pthread_create(&threadID1, NULL, printN, &x1); • pthread_create(&threadID2, NULL, printN, &x2); • printf("Started threads.\n"); • pthread_join(threadID1, &exitStatus); • pthread_join(threadID2, &exitStatus); • return 0; • }

12. SSttarartteedtdhrtehreads.a Myds I. D: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2

13. Simple Multithreaded C Program • void* printN(void* arg) { • inti; • for (i = 0; i < 10; i++) { • pthread_mutex_lock(mymutex); • printf("My ID: %d\n", *(int*)arg); • pthread_mutex_unlock(mymutex); • } • return NULL; • } • int main(void) { • pthread_t threadID1, threadID2; • void* exitStatus; • int x1 = 1, x2 = 2,x3=3; • pthread_mutex_init( mymutex,0); • pthread_create(&threadID1, NULL, printN, &x1); • pthread_create(&threadID2, NULL, printN, &x2); • pthread_mutex_lock(mymutex); • printf("Started threads.\n"); • pthread_mutex_unlock(mymutex); • pthread_join(threadID1, &exitStatus); • pthread_join(threadID2, &exitStatus); • return 0; • }

14. Started threads. My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 1 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2 My ID: 2

15. Memory model of C

16. Mutual Exclusion • As mutex locks proliferate in programs, the risk of deadlock increases

17. Deadlocks • A deadlock occurs when some threads become permanently blocked trying to acquire locks • There are a set of necessary conditions and if any one of them is avoided deadlock can be avoided. • Mutual exclusion condition: a resource that cannot be used by more than one process at a time • Hold and wait condition: processes already holding resources may request new resources held by other processes • No preemption condition: No resource can be forcibly removed from a process holding it, resources can be released only by the explicit action of the process. • Circular wait condition: two or more processes form a circular chain where each process waits for a resource that the next process in the chain holds.

18. Deadlock • Prevention • By avoiding any of the necessary condition • Deadlock avoidance • Bankers algorithm • most available techniques either impose severe constraints on the programmer or require considerable sophistication to apply

19. Memory consistency models • Sequential consistency Thread A x = 1; w = y; Thread B: y = 1; z = x;

20. Disadvantage of Multithreading • Difficulty of writing code • Difficulty of managing concurrency • Difficulty of testing and Debugging • Difficult to identify and verify root causes when errors occur • Defects are generally time related • Difficult to reproduce

21. Multiprocessing • Processes are imperative programs with their own memory spaces • Cannot refer to each other variable • consequently they do not exhibit the same difficulties as threads • Require support from OS - MMU • Mechanisms for communication provided by the operating system • Simple- File system • Message Passing

22. Message passing • a computation comprises one or more processes that communicate by calling library routines to send and receive messages • Communication is cooperative • data is sent by calling a routine, and the data is not received until the destination process calls a routine to receive the data • Programs written are highly portable • Provide programmer with explicit control over the location of memory in parallel programs • Library have routines for point to point and collective operations.

23. Example

24. Semaphore • Counting Semaphore • Semaphore are like mutex which count instead of lock • Is a variable whose value is a non-negative integer • Two basic operation • inc/acquire • Dec/release • Only the thread that locks the mutex can unlock it whereas for a semaphore is acquired by one task and released by another task

25. Semaphore Conditional variables Conditional variaables generalize the idea of semaphore They support arbitrary condition than zero or non-zero as gating criteria for blocking Message passing pitfalls Message queue size Program may create message passing program that deadlock

27. Multithreading Vs. Multiprocessing • Approaches to design and develop parallel prog. • Distinction is based on how they • share data and • Implicit in multithreading using shared memory • Message queues are used in multiprocessing • synchronize shared data access • Synch. Using mutexes • Synch implicit due to different time of sending and receiving data

28. Model-Driven Development • modeling software help us to overcome complexity • primary artifacts of development are models rather than programs • programs are still the ultimate product, but they are derived automatically from the models • Model Driven Architecture • UML

29. Advantage of Model Driven Develop. • Abstraction • helps us understand a complex system or situation by removing or hiding from view irrelevant detail, allowing us to better focus on essentials. • Comprehensibility • Are expressed in a form (e.g., a notation) that directly appeals to human intuition, providing a shortcut for understanding complex phenomena. • Accuracy • provides a true–to-life representation of those aspects of the modeled system that are of concern. • Predictive-ness • Accurate models can be used to correctly predict interesting characteristics of the modeled system before the system is constructed • Low cost • models based development take less effort