Transcript Operating_system_5

CPU Scheduling
Chapter 5
CPU Scheduling
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Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Multiple-Processor Scheduling
Thread Scheduling
UNIX example
Basic Concepts
• Maximum CPU utilization obtained
with multiprogramming
• CPU–I/O Burst Cycle – Process
execution consists of a cycle of CPU
execution and I/O wait
• CPU times are generally much
shorter than I/O times.
CPU-I/O Burst Cycle
Process B
Process A
Histogram of CPU-burst Times
Schedulers
• Process migrates among several queues
– Device queue, job queue, ready queue
• Scheduler selects a process to run from these queues
• Long-term scheduler:
– load a job in memory
– Runs infrequently
• Short-term scheduler:
– Select ready process to run on CPU
– Should be fast
• Middle-term scheduler
– Reduce multiprogramming or memory consumption
CPU Scheduler
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CPU scheduling decisions may take place when a
process:
1. Switches from running to waiting state (by sleep).
2. Switches from running to ready state (by yield).
3. Switches from waiting to ready (by an interrupt).
4. Terminates (by exit).
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Scheduling under 1 and 4 is nonpreemptive.
All other scheduling is preemptive.
Dispatcher
• Dispatcher module gives control of the
CPU to the process selected by the shortterm scheduler; this involves:
– switching context
– switching to user mode
– jumping to the proper location in the user
program to restart that program
• Dispatch latency – time it takes for the
dispatcher to stop one process and start
another running
Scheduling Criteria
• CPU utilization – keep the CPU as busy as possible
• Throughput – # of processes that complete their execution
per time unit
• Turnaround time (TAT) – amount of time to execute a
particular process
• 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)
“The perfect CPU scheduler”
• Minimize latency: response or job completion time
• Maximize throughput: Maximize jobs / time.
• Maximize utilization: keep I/O devices busy.
– Recurring theme with OS scheduling
• Fairness: everyone makes progress, no one starves
Scheduling Algorithms FCFS
• First-come First-served (FCFS) (FIFO)
– Jobs are scheduled in order of arrival
– Non-preemptive
• Problem:
– Average waiting time depends on arrival order
– Troublesome for time-sharing systems
• Convoy effect short process behind long process
• Advantage: really simple!
First Come First Served
Scheduling
• Example: Process
Burst Time
P1
24
P2
3
P3
3
• Suppose that the processes arrive in the order: P1 ,
P2 , P3
P1
0
24
Waiting time for P1 = 0; P2 = 24; P3 = 27
P3
P2
27
30
Average waiting time: (0 + 24 + 27)/3 = 17
• Suppose that the processes arrive in the order: P2 ,
P3 , P1 .
Waiting time for P1 = 6; P2 = 0; P3 = 3
P2
0
P3
3
P1
6
Average waiting time: (6 + 0 + 3)/3 = 3
30
Shortest-Job-First (SJR)
Scheduling
• Associate with each process the length of its
next CPU burst. Use these lengths to schedule
the process with the shortest time
• Two schemes:
– 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)
• SJF is optimal – gives minimum average waiting
time for a given set of processes
Shortest Job First Scheduling
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Example:
Process
P1
P2
P3
P4
Non preemptive SJF
P1
0
Average waiting time = (0 + 6 + 3 + 7)/4 = 4
P3
4 5
2
Arrival Time Burst Time
0
7
2
4
4
1
5
4
P1(7)
7
P2
8
P4
12
16
P1‘s wating time = 0
P2‘s wating time = 6
P2(4)
P3(1)
P4(4)
P3‘s wating time = 3
P4‘s wating time = 7
Shortest Job First Scheduling
Cont’d
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Example:
Process Arrival Time Burst Time
P1
0
7
P2
2
4
P3
4
1
P4
5
4
Average waiting time = (9 + 1 + 0 +2)/4 = 3
Preemptive SJF
P1
P2 P3 P2
P4
P1
P1(7)
0
2
P1(5)
4
P2(4) P2(2)
P3(1)
P4(4)
5
7
11
16
P1‘s wating time = 9
P2‘s wating time = 1
P3‘s wating time = 0
P4‘s wating time = 2
Shortest Job First Scheduling
Cont’d
• Optimal scheduling
• However, there are no accurate
estimations to know the length of the next
CPU burst
Shortest Job First Scheduling
Cont’d
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Optimal for minimizing queueing time, but impossible to
implement. Tries to predict the process to schedule based on
previous history.
Predicting the time the process will use on its next schedule:
t( n+1 ) =
Here:
w * t( n )
+ ( 1 - w ) * T( n )
t(n+1) is time of next burst.
t(n)
is time of current burst.
T(n)
is average of all previous bursts .
W
is a weighting factor emphasizing current or previous
bursts.
Priority Scheduling
• A priority number (integer) is associated with each process
• The CPU is allocated to the process with the highest priority
(smallest integer  highest priority in Unix but lowest in Java).
– Preemptive
– Non-preemptive
• SJF is a priority scheduling where priority is the predicted next CPU
burst time.
• Problem  Starvation – low priority processes may never execute.
• Solution  Aging – as time progresses increase the priority of the
process.
Round Robin (RR)
• 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 (n1)q time units.
Round Robin Scheduling
• time quantum = 20
Process
P1
P2
P3
P4
Burst Time
53
17
68
24
Wait Time
57 +24 = 81
20
37 + 40 + 17= 94
57 + 40 = 97
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20 37 57 77 97 117 121 134 154 162
P1(33)
24P1(13)
P1(53)
57
P2(17) 20
P3(68)
P4(24)
37
57
P3(48)
40
40
17
P3(28) P3(8)
P4(4)
Average wait time = (81+20+94+97)/4 = 73
Round Robin Scheduling
• Typically, higher average turnaround than SJF, but better
response.
• Performance
– q large  FCFS
– q small  q must be large with respect to context switch,
otherwise overhead is too high.
Turnaround Time Varies With
The Time Quantum
TAT can be improved if most process
finish their next CPU burst in a single
time quantum.
Multilevel Queue
• Ready queue is partitioned into separate queues:
EX:
• foreground (interactive)
background (batch)
• Each queue has its own scheduling algorithm
EX
– foreground – RR
– background – FCFS
• Scheduling must be done between the queues
– Fixed priority scheduling; (i.e., serve all from foreground then from
background). Possibility of starvation.
– Time slice – each queue gets a certain amount of CPU time which it can
schedule amongst its processes;
EX
80% to foreground in RR
20% to background in FCFS
Multilevel Queue Scheduling
Multi-level Feedback Queues
Implement multiple ready queues
– Different queues may be scheduled using different algorithms
– Just like multilevel queue scheduling, but assignments are not static
• Jobs move from queue to queue based on feedback
– Feedback = The behavior of the job,
• EX does it require the full quantum for computation, or
• does it perform frequent I/O ?
• Need to select parameters for:
– Number of queues
– Scheduling algorithm within each queue
– When to upgrade and downgrade a job
Example of Multilevel Feedback
Queue
• Three queues:
– Q0 – RR with time quantum 8 milliseconds
– Q1 – RR time quantum 16 milliseconds
– Q2 – FCFS
• Scheduling
– A new job enters queue Q0 which is served FCFS.
When it gains CPU, job receives 8 milliseconds (RR).
If it does not finish in 8 milliseconds, job is moved to
queue Q1.
– At Q1 job is again served FCFS and receives 16
additional milliseconds (RR). If it still does not
complete, it is preempted and moved to queue Q2.
– AT Q2 job is served FCFS
Multilevel Feedback Queues
Multiple-Processor Scheduling
• CPU scheduling more complex when multiple
CPUs are available
 Different rules for homogeneous or heterogeneous
processors.
• Load sharing in the distribution of work, such that
all processors have an equal amount to do.
• Asymmetric multiprocessing – only one processor
accesses the system data structures, alleviating
the need for data sharing
• Symmetric multiprocessing (SMP) – each
processor is self-scheduling
 Each processor can schedule from a common ready
queue OR each one can use a separate ready queue.
Thread Scheduling
• On operating system that support threads the
kernel-threads (not processes) that are being
scheduled by the operating system.
• Local Scheduling (process-contention-scope
PCS )– How the threads library decides which
thread to put onto an available LWP
• PTHREAD_SCOPE_PROCESS
• Global Scheduling (system-contention-scope
SCS )– How the kernel decides which kernel
thread to run next
• PTHREAD_SCOPE_PROCESS
Linux Scheduling
• Two algorithms: time-sharing and real-time
• Time-sharing
– Prioritized credit-based – process with most credits is
scheduled next
– Credit subtracted when timer interrupt occurs
– When credit = 0, another process chosen
– When all processes have credit = 0, recrediting occurs
• Based on factors including priority and history
• Real-time
– Defined by Posix.1b
– Real time Tasks assigned static priorities. All other tasks
have dynamic (changeable) priorities.
The Relationship Between Priorities and
Time-slice length
List of Tasks Indexed According to
Prorities
Conclusion
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We’ve looked at a number of different
scheduling algorithms.
• Which one works the best is application
dependent.
– General purpose OS will use priority based,
round robin, preemptive
– Real Time OS will use priority, no
preemption.
References
• Some slides from
– Text book slides
– Kelvin Sung - University of Washington,
Bothell
– Jerry Breecher - WPI
– Einar Vollset - Cornell University