cs238 cpu scheduling dr. alan r. davis. cpu scheduling the objective of multiprogramming is to have...
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Cs238 CPU Scheduling
Dr. Alan R. Davis
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CPU Scheduling• The objective of multiprogramming is to have some
process running at all times, to maximize CPU utilization.
• A process is executed until it must wait, usually for completion of some I/O request.
• The OS then switches the process with another ready process which can use the CPU.
• Processes alternate between CPU bursts and I/O bursts.
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Alternating Sequence of CPU And I/O Bursts
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Histogram of CPU-burst Times
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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 process:
1.Switches from running to waiting state.
2.Switches from running to ready state.
3.Switches from waiting to ready.
4.Terminates.
• Scheduling under 1 and 4 is nonpreemptive.• All other scheduling is preemptive.
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Dispatcher• Dispatcher module 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 restart
that program
• Dispatch latency – time it takes for the dispatcher to stop one process and start another running.
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Scheduling Algorithms What criteria should be used?
• CPU Utilization: keep the CPU as busy as possible
• Throughput: maximize the number of processes completed per unit of time
• Turnaround time: minimize the time interval between submission and completion of processes
• Waiting time: minimize the time a process remains in the waiting queue
• Response time: minimize the time between submission of a process and the first response
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Optimization Criteria
• Max CPU utilization
• Max throughput
• Min turnaround time
• Min waiting time
• Min response time
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Scheduling Algorithms First-come, First-served (FCFS)
• Works as its name implies.
• Example (Process, Burst Time): (P1, 24), (P2, 3), (P3, 3)
• If they arrive in the order P1, P2, P3, then the average waiting time is (0 + 24 + 27)/3 = 17
• If they arrive in the order P2, P3, P1, then the average waiting time is (6 + 0 + 3)/3 = 3
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First-Come, First-Served (FCFS) Scheduling
• Example: Process Burst Time
P1 24
P2 3
P3 3
• Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
• Waiting time for P1 = 0; P2 = 24; P3 = 27
• Average waiting time: (0 + 24 + 27)/3 = 17
P1 P2 P3
24 27 300
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FCFS Scheduling (Cont.)Suppose that the 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: (6 + 0 + 3)/3 = 3• Much better than previous case.• Convoy effect short process behind long process
P1P3P2
63 300
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Scheduling Algorithms Shortest-Job-First (SJF)
• Choose the job with the next shortest CPU burst
• Example (Process, Burst Time): (P1, 6), (P2, 8), (P3, 7), (P4, 3)
• They are processed in the order P4, P1, P3, P2 then the average waiting time is (3 + 16 + 9 + 0)/4 = 7
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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.
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ProcessArrival TimeBurst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
• SJF (non-preemptive)
• Average waiting time = (0 + 6 + 3 + 7)/4 - 4
Example of Non-Preemptive SJF
P1 P3
P2
73 160
P4
8 12
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Example of Preemptive SJFProcessArrival TimeBurst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
• SJF (preemptive)
• Average waiting time = (9 + 1 + 0 +2)/4 - 3
P1 P3P2
42 110
P4
5 7
P2 P1
16
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Determining Length of Next CPU Burst
• Can only estimate the length.
• Can be done by using the length of previous CPU bursts, using exponential averaging.
:Define 4.
10 , 3.
burst CPU next the for value predicted 2.
burst CPU of lenght actual 1.
1n
thn nt
.t nnn 11
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Examples of Exponential Averaging
=0 n+1 = n
– Recent history does not count.
=1– n+1 = tn
– Only the actual last CPU burst counts.
• If we expand the formula, we get:n+1 = tn+(1 - ) tn -1 + …
+(1 - )j tn -1 + …
+(1 - )n=1 tn 0
• Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor.
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Scheduling Algorithms Priority Scheduling
• Associate a priority with each process, and schedule the process with the highest priority next.
• Equal priority processes can be scheduled FCFS.
• SJF is priority scheduling with shorter jobs having higher priority.
• Example (Process, Burst Time, Priority): (P1, 10, 3), (P2, 1, 1), (P3, 2, 3), (P4, 1, 4), (P5, 5, 2)
• They are processed in the order P2, P5, P1, P3, P4 then the average waiting time is (6 + 0 + 16 + 18 + 1)/5 = 8.2
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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).– Preemptive– nonpreemptive
• 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.
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Scheduling Algorithms 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 (n-1)q time units.
• Performance– q large FIFO
– q small q must be large with respect to context switch, otherwise overhead is too high.
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Example: RR with Time Quantum = 20
Process Burst Time
P1 53
P2 17
P3 68
P4 24
• The Gantt chart is:
• Typically, higher average turnaround than SJF, but better response.P1 P2 P3 P4 P1 P3 P4 P1 P3 P3
0 20
37 57 77 97 117 121 134 154 162
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How a Smaller Time Quantum Increases Context Switches
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Turnaround Time Varies With The Time Quantum
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Multilevel Queue• Ready queue is partitioned into separate queues:
foreground (interactive)background (batch)
• Each queue has its own scheduling algorithm, foreground – RRbackground – 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; i.e.,80% to foreground in RR
– 20% to background in FCFS
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Multilevel Queue Scheduling
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Multilevel Feedback Queue• A process can move between the various queues; aging can
be implemented this way.• Multilevel-feedback-queue scheduler defined by the
following parameters:– number of queues– scheduling algorithms for each queue– method used to determine when to upgrade a process– method used to determine when to demote a process– method used to determine which queue a process will enter when
that process needs service
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Multilevel Feedback Queues
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Example of Multilevel Feedback Queue• Three queues:
– Q0 – time quantum 8 milliseconds
– Q1 – 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. 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. If it still does not complete, it is preempted and moved to queue Q2.
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Multiple-Processor Scheduling• CPU scheduling more complex when multiple CPUs are
available.
• Homogeneous processors within a multiprocessor.
• Load sharing
• Symmetric Multiprocessing (SMP) – each processor makes its own scheduling decisions.
• Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing.
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Multiple-Processor Scheduling• Load sharing schedules processes among several
identical processors.
• Each processor could have its own ready queue, but then some processors must be idle.
• The processors could share a single ready queue.
• One processor could be appointed as the scheduler for all of them (asymmetric multiprocessing).
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Real-Time Scheduling
• Hard real-time systems are required to complete a critical task within a guaranteed amount of time.
• The scheduler must then know that time requirement for each process.
• It then either accepts or rejects the process if it can’t meet the requirement.
• If the system has secondary storage or virtual memory, the scheduler can not know how long the process will take.
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Real-Time Scheduling
• A soft real-time system requires that critical processes receive priority over less important ones.
• The system must have priority scheduling.
• Steps must be taken to prevent starvation of less important processes.
• Priority of real-time processes must not degrade over time.
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Real-Time Scheduling Dispatch Latency
• Dispatch latency must be low.
• System calls must be preemptible.
• Preemption points can be inserted into long duration system calls, so that a ready high priority process can be detected.
• Or we could make the entire kernel preemptible. We would need to make all kernel data structures protected through various synchronization mechanisms.
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Dispatch Latency
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Thread Scheduling
• Local Scheduling – How the threads library decides which thread to put onto an available LWP.
• Global Scheduling – How the kernel decides which kernel thread to run next.
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Solaris 2 Scheduling
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Java Thread Scheduling
• JVM Uses a Preemptive, Priority-Based Scheduling Algorithm.
• FIFO Queue is Used if There Are Multiple Threads With the Same Priority.
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Java Thread Scheduling (cont)JVM Schedules a Thread to Run When:
• The Currently Running Thread Exits the Runnable State.
• A Higher Priority Thread Enters the Runnable State
* Note – the JVM Does Not Specify Whether Threads are Time-Sliced or Not.
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Time-Slicing• Since the JVM Doesn’t Ensure Time-Slicing, the yield()
Method May Be Used:
while (true) {
// perform CPU-intensive task
. . .
Thread.yield();
}
This Yields Control to Another Thread of Equal Priority.
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Thread Priorities• Priority Comment
Thread.MIN_PRIORITY Minimum Thread Priority
Thread.MAX_PRIORITY Maximum Thread Priority
Thread.NORM_PRIORITY Default Thread Priority
Priorities May Be Set Using setPriority() method:
setPriority(Thread.NORM_PRIORITY + 2);
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Algorithm Evaluation• Choose the important criteria, then evaluate the various
algorithms under consideration.• Deterministic modeling: calculate the cost of each algorithm
on a given set of input data• Queuing models: measure the distribution of CPU and I/O
bursts, as well as the arrival times of processes, and use formulas and concepts of queuing-network theory to calculate other quantities
• Simulations: program a model of the computer system, generate test data randomly
• Implementation: code all the algorithms, include them in the OS, and test under real conditions
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Evaluation of CPU Schedulers by Simulation