1. Operating Systems
Lecture 14

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Agenda for Today
Review of previous lecture
Short-term scheduler
Dispatcher
Reasons for invoking scheduler
Optimization criteria
FCFS, SJF, and SRTF
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Review of Lecture 13
Single and multi-threaded processes
Thread models
User- and kernel-level threads
Pthreads library
Sample multi-threaded code
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Example 1
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
/* Prototype for a function to be passed to our thread */
void* MyThreadFunc(void *arg);
int main() {
pthread_t aThread;
/* Create a thread and have it run the MyThreadFunction */
pthread_create(&aThread, NULL, MyThreadFunc, NULL);
/* Parent waits for the aThread thread to exit */
pthread_join(aThread, NULL);
printf ("Exiting the main function.\n");
return 0; }
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Example 1
void* MyThreadFunc(void* arg) {
printf ("Hello, world! ... The threaded version.\n");
return NULL; }
$ gcc hello.c –o hello –lpthread –D_REENTRANT
$ hello
Hello, world! ... The threaded version.
Exiting the main function. $
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Example 2
#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS 5
void *PrintHello(void *threadid) {
printf("\n%d: Hello World!\n", threadid); pthread_exit(NULL); }
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Example 2
int main (int argc, char *argv[]) {
pthread_t threads[NUM_THREADS];
int rc, t;
for (t=0; t < NUM_THREADS; t++) {
printf("Creating thread %d\n", t);
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
if (rc) {
printf("ERROR; return code is %d\n", rc);
exit(-1); }
pthread_exit(NULL);
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CPU Scheduling
Scheduling processes in the ready queue
Short-term scheduler
Different types of schedulers
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Life of a Process
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10. Histogram of CPU-burst Times
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CPU Scheduler
Short-term scheduler
Selects a process from among the processes in the ready queue
Invokes the dispatcher to have the CPU allocated to the selected process
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Dispatcher
Dispatcher gives control of the CPU to the process selected by the short-term scheduler; this involves:
switching context
switching to user mode
jumping to the proper location in the user program to start (or restart) it
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Dispatcher
Dispatch latency – time it takes for the dispatcher to stop one process and start another running.
Typically, a few microseconds
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CPU Scheduler
CPU scheduling decisions may take place when a process:
Switches from running to waiting state
Switches from running to ready state
Switches from waiting to ready
Terminates
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CPU Scheduler
Scheduling under 1 and 4 is nonpreemptive.
All other scheduling is preemptive.
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Scheduling Criteria
CPU utilization – keep the CPU as busy as possible
Throughput – # of processes that complete their execution per time unit
Turnaround time – amount of time to execute a particular process
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Scheduling Criteria
Waiting time – amount of time a process has been waiting in the ready queue
Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)
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18. Optimization Criteria
Maximize CPU utilization
Maximize throughput
Minimize turnaround time
Minimize waiting time
Minimize response time
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FCFS Scheduling
The process that enters the ready queue first is scheduled first, regardless of the size of its next CPU burst
Example: Process Burst Time P P P
Suppose that processes arrive into the system in the order: P1, P2 , P3
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FCFS Scheduling
Processes are served in the order: P1, P2, P3
The Gantt Chart for the schedule is:
Waiting times P1 = 0; P2 = 24; P3 = 27
Average waiting time: ( )/3 = 17 P1 P2 P3 24 27 30
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FCFS Scheduling
Suppose that processes arrive in the order: P2 , P3 , P1 .
The Gantt chart for the schedule is:
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: ( )/3 = 3
Convoy effect short process behind long process P1 P3 P2 6 3 30
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Recap of Lecture
Short-term scheduler
Examples of Pthreads
Dispatcher
Reasons for invoking scheduler
Optimization criteria
FCFS
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