Module 6: CPU Scheduling 10/19/03
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Transcript of Module 6: CPU Scheduling 10/19/03
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.1
Module 6: CPU Scheduling10/19/03
• Basic Concepts
• Scheduling Criteria
• Scheduling Algorithms
• Multiple-Processor Scheduling
• Real-Time Scheduling
• Algorithm Evaluation
NOTE: Instructor annotations in BLUE
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.2
Basic Concepts
• Objective: Maximum CPU utilization obtained with multiprogramming
• A process is executed util it must wait - typically for completion if I/O request, or a time quanta expires.
• CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait.
• CPU burst distribution
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.3
Alternating Sequence of CPU And I/O Bursts
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.4
Histogram of CPU-burst Times
“Tune” the scheduler to these statistics
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.5
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. Runs until it Switches from running to waiting state … stop
executing only when a needed resource or service is currently unavailable.
2. Switches from running to ready state in the middle of a burst – can stop execution at any time
3. Switches from waiting to ready. … ?4. Runs until it Terminates.
• Scheduling under 1 and 4 is nonpreemptive.
• All other scheduling is preemptive.
• Under nonpremptive scheduling, once a CPU is assigned to a process, the process keeps the CPU until it releases the CPU either by terminating or switching to wait state - “naturally” stop execution.
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.6
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.
• Both scheduler and dispatcher are performance “bottlenecks” in the OS, and must be made as fast and efficient as possible.
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.7
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 processOr: the time from time of submission to time of completion – includes waits in queues in addition to execute time.
• Waiting time – amount of time a process has been waiting in the ready queue – sum of the times in ready queue - this is from the point of view scheduler - scheduler does not look at CPU time or I/O wait time (only ready queue time) if it minimizes “waiting time”. .. Get a process through the ready queue as soon as possible.
• 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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.8
Optimization Criteria
• Max CPU utilization
• Max throughput
• Min turnaround time
• Min waiting time
• Min response time
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.9
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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.10
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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.11
Shortest-Job-First (SJF) 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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.12
Process Arrival Time Burst Time
P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
• SJF (non-preemptive)
• FCFS is “tie breaker” if burst times the same.
• 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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.13
Example of Preemptive SJF(Also called Shortest-Remaining-Time-First (SRTF) )
Process Arrival Time Burst 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
In order for a newarrival to preempt,its burst must be strictly less than current remainingtime
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.14
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
n+1 = tn + (1- )n
n stores past history tn is “recent history is a weighting factor… recursive in n
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.15
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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.16
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 - a “reward” for waiting in line a long time - let the old geezers move ahead!
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.17
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
• RR is a preemptive algorithm.
• 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. ==> “processor sharing” - user thinks it has its own processor running at 1/n speed (n processors)
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.18
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
FCFS is tie breaker Assume all arrive at 0 timein the order given.
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.19
How a Smaller Time Quantum Increases Context Switches
Context switch overhead very critical for 3rd case - since overhead is independent of quanta time
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.20
Turnaround Time Varies With The Time Quantum
TAT can be improved if most processes finish each burst in one quantaEX: if 3 processes each have burst of 10, then for q = 1, avg_TAT = 29, but for q = burst = 10, avg_TAT = 20.==> design tip: tune quanta to average burst.
No strict correlation of TATand time quanta size -except for below
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.21
Multilevel Queue (no Feedback - see later)
• Ready queue is partitioned into separate queues:foreground queue (interactive)background queue (batch)
• Each queue has its own scheduling algorithm, foreground queue – RRbackground queue – FCFS
• Scheduling must be done between the queues.– Fixed priority scheduling; i.e., serve all from foreground then
from background. All higher priority queues must be empty before given queue is processed. Possibility of starvation. Assigned queue is for life of the process.
– Time slice between queues – 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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.22
Multilevel Queue Scheduling
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.23
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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.24
Multilevel Feedback Queues
Fig 6.7
Higher priority queuespre-empt lower priority queues on new arrivals
Queue 0 - High priority
Queue 1
Queue 2Low priority
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.25
Example of Multilevel Feedback Queue
• Three queues: – Q0 – time quantum 8 milliseconds– Q1 – time quantum 16 milliseconds– Q2 – FCFS– Q2 longest jobs, with lowest priority, Q1 shortest jobs with
highest priority.
• 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(demoted).
– 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.
– Again, Qn is not served until Qn-1 empty
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.26
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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.27
Real-Time Scheduling
• Hard real-time systems – required to complete a critical task within a guaranteed amount of time.
• Soft real-time computing – requires that critical processes receive priority over less fortunate ones.
• Deadline scheduling used.
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.28
Dispatch Latency
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.29
Solaris 2 Thread Scheduling
• 3 scheduling priority classes– Timesharing/interactive – lowest – for users
Within this class: Multilevel feedback queues longer time slices in lower priority queues
– System – for kernel processes– Real time – Highest
• Local Scheduling – How the threads library decides which thread to put onto an available LWP. Remember threads are time multiplexed on LWP’s
• Global Scheduling – How the kernel decides which kernel thread to run next.
– The local schedules for each class are “globalized” from the scheduler's point of view – all classes included.
– LPW’s scheduled by kernel
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.30
Solaris 2 Scheduling
Fig. 6.10 Reserved for kernel use.Ex: Scheduler &paging daemon
Only a few in this classReal time
User processes Go here
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.31
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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.32
Java Thread Scheduling (cont)-omit
JVM Schedules a Thread to Run When:
1. The Currently Running Thread Exits the Runnable State.
2. 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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.33
Time-Slicing (Java) - omit
• 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.
• Cooperative multi-tasking possible using yield
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.34
Java Thread Priorities - omit
• 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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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.35
Algorithm Evaluation
• Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload - what we’ve been doing in the examples - optimize various criteria.
– Easy, but success depends on accuracy of input
• Queuing models - statistical - need field measurements of statistics in various compouting environments
• Implementation - Costly - OK is a lot of pre-implementation done first.
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Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 6.36
Evaluation of CPU Schedulers by Simulation
-need good models- drive it with field data and/orstatistical data- could be slow.