COT 4600 Operating Systems Spring 2011

COT 4600 Operating Systems Spring 2011

Dan C. MarinescuOffice: HEC 304 Office hours: Tu-Th 5:00-6:00 PM

Lecture 21

Lecture 21 - Thursday April 7, 2011 Last time:

Scheduling Today:

The scheduler Multi-level memories

Next Time: Memory characterization Multilevel memories management using virtual memory Adding multi-level memory management to virtual memory Page replacement algorithms

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Lecture 20

The scheduler

The system component which manages the allocation of the processor/core.

It runs inside the processor thread and implements the scheduling policies.

Other functions Determines the burst Manages multiple queues of threads

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Lecture 20

CPU burst

CPU burst the time required by the thread/process to execute

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Lecture 20

Estimating the length of next CPU burst

Done using the length of previous CPU bursts, using exponential averaging

10 , 3.burst CPUnext for the valuepredicted 2.

burst CPU oflength actual 1.

1

n

thn nt

.1 1 nnn t

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Lecture 20

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 -j + … +(1 - )n +1 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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Lecture 20

Predicting the length of the next CPU burst

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Lecture 20

Multilevel queue

Ready queue is partitioned into separate queues each with its own scheduling algorithm : foreground (interactive) RR background (batch) FCFS

Scheduling 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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Lecture 20

Multilevel queue scheduling

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Lecture 20

Multilevel feedback queue

A process can move between the various queues; aging can be implemented this way

Multilevel-feedback-queue scheduler characterized by: number of queues scheduling algorithms for each queue strategy when to upgrade/demote a process strategy to decide the queue a process will enter when it needs service

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Lecture 20

Example of a multilevel feedback queue exam

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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Lecture 20

Multilevel feedback queues

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Lecture 20

Unix scheduler

The higher the number quantifying the priority the lower the actual process priority.

Priority = (recent CPU usage)/2 + base Recent CPU usage how often the process has used the CPU since the

last time priorities were calculated. Does this strategy raises or lowers the priority of a CPU-bound

processes? Example:

base = 60 Recent CPU usage: P1 =40, P2 =18, P3 = 10

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Lecture 20

Comparison of scheduling algorithmsRound Robin FIFO MFQ

Multi-LevelFeedback Queue

SFJShortest Job First

SRJNShortest Remaining Job Next

Throughput

Responsetime

May be low is quantum is too small

Shortest average responsetime if quantum chosen correctly

Not emphasized

May be poor

May be low is quantum is too small

Good for I/O bound but poor for CPU-bound processes

High

Good for short processesBut maybe poor for longer processes

High

Good for short processesBut maybe poor for longer processes

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Lecture 20

Round Robin

FIFO MFQMulti-LevelFeedback Queue



IO-bound

Infinite postponement

No distinctionbetweenCPU-bound andIO-bound

Does not occur


Does not occur

Gets a high priority if CPU-bound processes are present

May occur for CPU bound processes


May occur for processes with long estimated running times


May occur for processes with long estimated running times

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Lecture 20

Round Robin

FIFO MFQMulti-LevelFeedback Queue



Overhead

CPU-bound

Low


The lowest


Can be high Complex data structures and processing routines

Gets a low priority if IO-bound processes are present

Can be high Routine to find to find the shortest job for each reschedule


Can be high Routine to find to find the minimum remaining time for each reschedule


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Lecture 21

Memory virtualization

A process runs in its own address space; multiple threads may share an address space.

Process/tread management and memory management are important functions of an operating system

Virtual memory Allows programs to run on systems with different sizes of real memory It may lead to performance penalty. A virtual address space has a fixed size determined by the number of bits

in an address., e.g., if n=32 then size = 232 ~ 4 GB. Swap area: image on a disk of the virtual memory of a process. Page: a group of consecutive virtual addresses e.g., of size 4 K is brought

from the swap area to the main memory at once. Not all pages of a process are in the real memory at the same time

Caching Blocks of virtual memory addresses are brought into faster memory. Leads to performance improvement.

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Lecture 21

Locality of reference

Locality of reference: when a memory location is referenced then the next references are likely to be in close proximity. The programs include sequential code The data structures include often data that are processed together; e.g.,

an array. Spatial and temporal locality of reference. Thus it makes sense to group a set of consecutive addresses into

units and transfer such units at once between a slow but larger storage space to a faster but smaller storage. The sixe of the units is different Virtual memory Page size: 2 - 4 KB Cache 16 – 256 words.

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Lecture 21

Virtual memory Several strategies

Paging Segmentation Paging+ segmentation

At the time a process/thread is created the system creates a page table for it; an entry in the page table contains The location in the swap area of the page The address in main memory where the page resides if the page has

been brought in from the disk Other information e.g. dirty bit.

Page fault a process/thread references an address in a page which is not in main memory

On demand paging a page is brought in the main memory from the swap area on the disk when the process/thread references an address in that page.

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Lecture 21 20

Lecture 21 21

Lecture 20

Dynamic address translation

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Documents

COT 4600 Operating Systems Spring 2011