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Thursday, June 15, 2006

Confucius says: He who play in root, eventually kill tree.

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telnet 203.128.0.236

instead of

telnet chand.lums.edu.pk

from outside LUMS

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Another example

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FCFS

Simplest algorithm – easy to implementWhen a running process blocks, it is

placed at the end of queue like a newly arrived process

Non preemptiveDoes not emphasize throughput – long

processes are allowed to monopolize the CPU.

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FCFS

Suffers from convoy effect Penalizes short processes following long ones

Average WT varies if process CPU burst times vary greatly

Not suitable for time sharing systemsTends to favor CPU bound over I/O

bound processes

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Starvation possibleThroughput vs. turnaround time tradeoffIntroduces context switching.Burst sizes known in advance and all

available

SRTN

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Priority SchedulingA priority number (integer) is associated with

each processThe CPU is allocated to the process with the

highest priority (smallest integer highest priority ...may be different on different systems). Preemptive nonpreemptive

SJF is a priority scheduling where priority is the predicted next CPU burst time.

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Example

Processes Burst Time Priority Arrival Time

P1 10 3 0

P2 1 1 1

P3 2 3 2

P4 1 4 3

P5 5 2 4

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Priority SchedulingProblem Starvation – low priority processes

may never execute.Solution Aging – as time progresses increase

the priority of the process.

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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.

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Performance q large FIFO q small q must be large with respect

to context switch, otherwise overhead is too high.

Round Robin (RR)

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Example: RR with Time Quantum = 20

Process Burst Time

P1 53

P2 17

P3 68

P4 24

Typically, higher average turnaround than SJF, but better response.

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P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

The Gantt chart is:

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How a Smaller Time Quantum Increases Context Switches

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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 – RR background – FCFS

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Multilevel Queue

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

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Multilevel Feedback QueueMultilevel-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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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. 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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Multilevel Feedback Queues

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Processes Arrival time Burst time

P1 0 17

P2 12 25

P3 28 8

P4 36 32

P5 46 18

Multilevel feed back queue example

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Multilevel feed back queue exampleMultilevel feedback queue scheduling with three queues

Q1, Q2, Q3. The scheduler first executes processes in Q1, which is

given a time quantum of 8ms. If a process does not finish within this time, it is moved to tail of Q2.

The scheduler executes processes in Q2 only if Q1 is empty. The process at the head of Q2 is given a quantum of 16ms. If it does not complete, it is preempted and put in Q3.

Processes in Q3 are run in FCFS basis, only when Q1 and Q2 are empty.

A process in Q1 will preempt a process in Q2, a process that arrives in Q2 will preempt a process in Q3.

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User level thread with 50msec process quantum and threads that run 5msec per CPU burst

THREAD SCHEDULING

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User level thread with 50msec process quantum and threads that run 5msec per CPU burst

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Kernel level thread with 50msec process quantum and threads that run 5msec per CPU burst

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Kernel level thread with 50msec process quantum and threads that run 5msec per CPU burst

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Threads

Goal for threads: Allow each to use blocking calls but prevent a blocked thread from affecting other threads.

Threads in user space: Conflict with this goal.

One compelling reason for threads in user space: Work with existing operating systems

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Threads

System calls can be made non-blocking select system call

• checking code: jacket / wrapper Changes to system call library Inelegant solution Conflict with our goal Changing semantics of calls means changing

existing user programs

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We want:

Combine the advantage of user threads with those of kernel threads.

We want good performance and flexibility but without having to make special non-blocking system calls or checking for conditions.

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Scheduler Activations

Many to many models: User threads multiplexed onto kernel threads.

Main idea:

Avoid unnecessary transitions between user and kernel space

• If a thread a waiting locally for another one, then no need to involve the kernel

• Some number of virtual processors assigned to each process by the kernel (LWP: data structure between user and kernel threads)

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Scheduler Activations

• Some number of virtual processors assigned to each process by the kernel (LWP: data structure between user and kernel threads)

• LWPs can be requested or released by each process

• User process can schedule user threads onto available virtual processors.

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Scheduler Activations

When a kernel sees that a thread has blocked it informs the process run-time system of this occurrence by starting it at a well known address (Upcall)

Now the process can reschedule its threads.

When the data for blocked thread becomes available kernel makes another upcall

The process will decide whether to run the previously blocked thread or put it in ready queue.

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Scheduler Activations

CPU-bound: maybe one LWP

I/O bound: multiple LWPs

• One LWP for each concurrent blocking system call

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Thread Scheduling

• Many to many model: Thread library schedules user-level threads on available LWPs (PCS)

• Decision among threads of same process

• Kernel decides which kernel thread to schedule onto a CPU (SCS)

• One to one model systems use only SCS

• Windows, Linux, Solaris 9

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Scheduling in Unix - other versions also possible

Designed to provide good response to interactive processes

Uses multiple queues

Each queue is associated with a range of non-overlapping priority values

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Scheduling in Unix - other versions also possible

Processes executing in user mode have positive values

Processes executing in kernel mode (doing system calls) have negative values

Negative values have higher priority and large positive values have lowest

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Scheduling in Unix

Only processes that are in memory and ready to run are located on queues

Scheduler searches the queues starting at highest priority

first process is chosen on that queue and started. It runs for one time quantum (say 100ms) or until it blocks.

If the process uses up its quantum it is blocked Processes within same priority range share

CPU in RR