CPU SCHEDULING SONALI CHAUHAN SYBSc IT 2008-09 UDIT
BASIC CONCEPTS Main objective of multiprogramming is to keep on running processes all the time for maximum CPU utilization. In uniprocess system only one process run at a time, others process have to wait for cpu time.
Scheduling is fundamental function of OS.
CPU-I/O BURST CYCLE CPU–I/O Burst Cycle – Process execution
consists of a cycle of CPU execution and I/O
CPU SCHEDULER Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them CPU scheduling decisions may take place when a 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready Scheduling under 1 and 4 is nonpreemptive
All other scheduling is preemptive
Selects one process from ready queue to run on CPU
Once a process is allocated the CPU, it does not leave unless: it has to wait, e.g., for I/O request or for a child to terminate OS can force (preempt) a process from CPU at anytime CONT…
Say, to allocate CPU to another higher-priority process
DISPATCHER Scheduler : selects one process to run on CPU Dispatcher : allocates CPU to the selected process, which involves: jumping to the proper location (in the selected process) and restarting it Dispatcher should be fast enough.
Dispatch latency – time it takes for the dispatcher to stop one process and start another running.
How does scheduler select a process to run?
SCHEDULING CRITERIA CPU utilization – keep the CPU as busy as possible Throughput – No of processes that complete their execution per time unit
Turnaround time – amount of time to execute a particular process (time from submission to termination) i.e (waiting in memory + waiting time in ready queue + executing time in CPU + doing IO)
Waiting time – amount of time a process has been waiting in the ready queue (sum of time waiting in ready queue) CONT…
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)
FIRST COME FIRST SERVED SCHEDULING
Suppose that the processes arrive in the order: The Gantt Chart for the schedule is: Waiting time for P1 = 0; P2 = 24; P3 = 27 CONT… First come, First served… P 1 P 2 P 3 24 27 30 0
Average waiting time: (0 + 24 + 27)/3 = 17
Suppose that the processes arrive in the order : P 2 , P 3 , P 1 ( P1: 24, P2: 3, P3: 3) The Gantt chart for the schedule is: Waiting time for P 1 = 6 ; P 2 = 0 ; P 3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case CONT… First come, First served… P 1 P 3 P 2 6 3 30 0
Convoy effect short process behind long process
CONT… SHORTEST-JOB-FIRST SCHEDULING
Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF) CONT… SHORTEST-JOB-FIRST
SJF is optimal – gives minimum average waiting time for a given set of processes
CONT… SHORTEST-JOB-FIRST… Process Arrival Time Burst Time Example of Non-Preemptive SJF P 1 P 3 P 2 7 3 16 0 P 4 8 12
Average waiting time = (0 + 6 + 3 + 7)/4 - 4
Process Arrival Time Burst Time CONT… SHORTEST-JOB-FIRST… Example of Preemptive SJF P 1 P 3 P 2 4 2 11 0 P 4 5 7 P 2 P 1 16
Average waiting time = (9 + 1 + 0 +2)/4 - 3
A priority number (integer) is associated with each process Lager the CPU burst lower the priority The CPU is allocated to the process with the highest priority (smallest integer highest priority) Problem Starvation (Infinity blocking) – low priority processes may never execute CONT… PRIORITY…
Solution Aging – as time progresses increase the priority of the process
Each process gets a small unit of CPU time ( time quantum ), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. If there are n processes in the ready queue and the time quantum is q , then each process gets 1/ n of the CPU time in chunks of at most q time units at once. No process waits more than ( n -1) q time units. CONT… ROUND-ROBIN…
q small q must be large with respect to context switch, otherwise overhead is too high
CONT… ROUND-ROBIN… P 1 P 2 P 3 P 1 P 1 P 1 P 1 P 1 0 4 7 10 14 18 22 26 30
Typically, higher average turnaround than SJF, but better response