Teaching Operating Systems With Programming and Freeware Lecture 2: Concurrency Issues, processor scheduling (Lab2 and 3) A workshop by Dr. Junaid Ahmed Zubairi Visiting Associate Professor CIT, Agriculture University, Rawalpindi

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Workshop Overview Operating Systems Course Outline Topics Suited for Programming Assignments Process Model and IPC (Lab1) Concurrency Issues (Lab2) Processor Scheduling (Lab3) Disk Scheduling and RAID Programming Project

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Concurrency Concurrent processing involves a set of two or more processes that run at the same time (apparently or physically) Multiple applications run concurrently through Multiprogramming whereas a structured application may be a set of concurrent processes

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Difficulties with Concurrency Sharing global resources with read/write permissions Management of allocation of resources to avoid starvation and deadlocks Programming errors become difficult to locate (and fix)

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 A Simple Example void echo() { chin = getchar(); chout = chin; putchar(chout); }

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 A Simple Example Process P1Process P2. chin = getchar();.. chin = getchar(); chout = chin; putchar(chout);.. putchar(chout);.

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Operating System Concerns Keep track of active processes Allocate and deallocate resources Processor time Memory Files I/O devices Protect data and resources Result of process must be independent of the speed of execution of other concurrent processes

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Process Interaction Processes unaware of each other Processes indirectly aware of each other Process directly aware of each other

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Competition Among Processes for Resources Mutual Exclusion Critical sections Only one program at a time is allowed in its critical section Example only one process at a time is allowed to send command to the printer Deadlock Starvation

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Cooperation Among Processes by Sharing Writing must be mutually exclusive Critical sections are used to provide data integrity

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Cooperation Among Processes by Communication Messages are passes Mutual exclusion is not a control requirement Possible to have deadlock Each process waiting for a message from the other process Possible to have starvation Two processes sending message to each other while another process waits for a message

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Requirements for Mutual Exclusion Only one process at a time is allowed in the critical section for a resource A process that halts in its non-critical section must do so without interfering with other processes No deadlock or starvation

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Requirements for Mutual Exclusion A process must not be delayed access to a critical section when there is no other process using it No assumptions are made about relative process speeds or number of processes A process remains inside its critical section for a finite time only

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 First Attempt Busy Waiting Process is always checking to see if it can enter the critical section Process can do nothing productive until it gets permission to enter its critical section

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Second Attempt Each process can examine the other’s status but cannot alter it When a process wants to enter the critical section it checks the other processes first If no other process is in the critical section, it sets its status for the critical section This method does not guarantee mutual exclusion Each process can check the flags and then proceed to enter the critical section at the same time

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Third Attempt Set flag to enter critical section before checking other processes If another process is in the critical section when the flag is set, the process is blocked until the other process releases the critical section Deadlock is possible when two process set their flags to enter the critical section. Now each process must wait for the other process to release the critical section

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Fourth Attempt A process sets its flag to indicate its desire to enter its critical section but is prepared to reset the flag Other processes are checked. If they are in the critical region, the flag is reset and later set to indicate desire to enter the critical region. This is repeated until the process can enter the critical region.

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Fourth Attempt It is possible for each process to set their flag, check other processes, and reset their flags. This scenario will not last very long so it is not deadlock. It is undesirable

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Correct Solution Each process gets a turn at the critical section If a process wants the critical section, it sets its flag and may have to wait for its turn

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Mutual Exclusion: Hardware Support Special Machine Instructions Performed in a single instruction cycle Not subject to interference from other instructions Reading and writing Reading and testing

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Mutual Exclusion: Hardware Support Test and Set Instruction boolean testset (int i) { if (i == 0) { i = 1; return true; } else { return false; } }

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Mutual Exclusion: Hardware Support Exchange Instruction void exchange(int register, int memory) { int temp; temp = memory; memory = register; register = temp; }

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Mutual Exclusion Machine Instructions Advantages Applicable to any number of processes on either a single processor or multiple processors sharing main memory It is simple and therefore easy to verify It can be used to support multiple critical sections

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Mutual Exclusion Machine Instructions Disadvantages Busy-waiting consumes processor time Starvation is possible when a process leaves a critical section and more than one process is waiting. Deadlock If a low priority process has the critical region and a higher priority process needs it, the higher priority process will obtain the processor to wait for the critical region

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Semaphores Special variable called a semaphore is used for signaling If a process is waiting for a signal, it is suspended until that signal is sent Wait and signal operations cannot be interrupted Queue is used to hold processes waiting on the semaphore

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Semaphores Semaphore is a variable that has an integer value May be initialized to a nonnegative number Wait operation decrements the semaphore value Signal operation increments semaphore value

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Wait Pseudocode struct semaphore { int count; queueType queue; } void wait(semaphore s) { s.count--; if (s.count < 0) { place this process in s.queue; block this process } }

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Signal Pseudocode void signal(semaphore s) { s.count++; if (s.count <= 0) { remove a process P from s.queue; place process P on ready list; } }

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Semaphore Example Semaphore door=1; Process p1 executes wait (door), the final value of door=0 and p1 enters the critical section Process p2 now executes wait (door), the value of door = -1 and p2 is blocked outside the critical section

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Semaphore Example Process p1 completes its critical section and executes signal (door), thus incrementing door to 0. When a signal operation results in a value less than the initial value (1), it means some processes are sleeping on this semaphore. Thus the system will wake up the process at the head of the queue (i.e. p2)

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Using BACI BACI (Ben-Ari Concurrent Interpreter) is developed to help in teaching concurrency Obtain and install BACI from BACI supports C, Pascal and C++ It is originally based on Pascal BACI uses some special concurrent constructs

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Using BACI Multiple blocks can be run concurrently by using cobegin {…} construct cobegin construct appears in the main function and all processes listed within cobegin{…} start running at the same time BACI includes semaphores, binary semaphores and monitors

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Using BACI initialsem(sem,int) initializes the semaphore sem to the value int p(sem) and wait(sem) calls decrement sem by one if sem is greater than zero and block the caller if sem is already zero v(sem) or signal(sem) calls increment sem by 1 or wake up a process if sem is found to be zero empty(sem) returns true if there are no processes waiting on sem and false otherwise

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Using BACI atomic keyword: if a function is defined as atomic, then the function is non-preemptible. void suspend ( void ): puts the calling thread to sleep. void revive ( int process_id ): revives the process with the given id. int which_proc( void ): returns the process number of the current thread. int random (int range): returns a "randomly chosen" integer between 0 and range -1, inclusive

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Example Source Code semaphore door; void coordinate() { initialsem(door,1); } void myturn() { int ct; wait(door); cout<<"MANGO: My turn is now\n"; signal(door); } void yourturn() { wait(door); cout<<"APPLE: No it is my turn\n"; signal(door); }

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Example Source Code void theirturn() { wait(door); cout<<"ORANGE: No it is now my game\n"; signal(door); } void main() { coordinate(); cobegin { myturn(); yourturn(); theirturn(); }

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 How to Run the Program Enter and save the sample source code in a file whose name ends in.cm Use “bacc filename” to compile and obtain a pcode file whose name ends in.pco Run the interpreter by using “bainterp filename”

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Programming Assignment 1 Lab 2 Modify the example program by defining a global variable int count. The critical section in each function will modify the count based on user input. Show, with displayed messages, how one process acquires the critical section and the other process blocks for the critical section

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Barbershop Problem

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Barbershop Problem The number of barber chairs is 3 so the semaphore chair will be initialized to 3 Only four customers can sit on the sofa so the semaphore sofa will be initialized to 4 Complete the barbershop program (limit chairs to 1, sofa to 3, customers to 5 and barbers to 1) Use messages to see what is going on and don’t panic on barber’s deadlock condition as far as it does not stop the customers from getting haircut

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Readers/Writers Problem Any number of readers may simultaneously read the file Only one writer at a time may write to the file If a writer is writing to the file, no reader may read it It is left as an exercise problem for the participants

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Processor Scheduling

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002

Short-Term Scheduling Known as the dispatcher Executes most frequently Invoked when an event occurs Clock interrupts I/O interrupts Operating system calls Signals

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002

Decision Mode Nonpreemptive Once a process is in the running state, it will continue until it terminates or blocks itself for I/O Preemptive Currently running process may be interrupted and moved to the Ready state by the operating system Allows for better service since any one process cannot monopolize the processor for very long

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Process Scheduling Example

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 First-Come-First-Served (FCFS) Each process joins the Ready queue When the current process ceases to execute, the oldest process in the Ready queue is selected

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 First-Come-First-Served (FCFS) A short process may have to wait a very long time before it can execute Favors CPU-bound processes I/O processes have to wait until CPU- bound process completes

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Round-Robin Uses preemption based on a clock An amount of time is determined that allows each process to use the processor for that length of time

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Round-Robin Clock interrupt is generated at periodic intervals When an interrupt occurs, the currently running process is placed in the read queue Next ready job is selected Known as time slicing

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Shortest Process Next Nonpreemptive policy Process with shortest expected processing time is selected next Short process jumps ahead of longer processes

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Shortest Process Next Predictability of longer processes is reduced If estimated time for process not correct, the operating system may abort it Possibility of starvation for longer processes

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Shortest Remaining Time Preemptive version of shortest process next policy Must estimate processing time

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Highest Response Ratio Next (HRRN) Choose next process with the lowest ratio time spent waiting + expected service time expected service time

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Feedback Penalize jobs that have been running longer Don’t know remaining time process needs to execute

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002

Traditional UNIX Scheduling Multilevel feedback using round robin within each of the priority queues Priorities are recomputed once per second Base priority divides all processes into fixed bands of priority levels Adjustment factor used to keep process in its assigned band

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Programming Assignment Lab3 Use the sample code provided to generate random numbers and store them in an array. These numbers are assumed to indicate the length of a process Use FCFS and SPN to display which process will be scheduled next

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002 Sample Code #include void main(void) { const int MAX_LIMIT=55; int key; srand( (unsigned)time( NULL ) ); key = rand()%MAX_LIMIT; printf("%d\n",key); }