Thrashing & Working Set

8/7/2019 Thrashing & Working Set

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Operating SystemsProf. Navneet Goyal

Department of Computer Science & Information Systems

BITS, Pilani


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Topics for Today Thrashing

Working Set


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Principle of Locality

Program and data references within aprocess tend to cluster

Only a few pieces of a process will beneeded over a short period of time

Possible to make intelligent guesses aboutwhich pieces will be needed in the future

This suggests that virtual memory maywork efficiently


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Locality In A Memory-Reference Pattern


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Thrashing If a process does not have enough

pages, the page-fault rate is very high.This leads to:

low CPU utilization operating system thinks that it needs to

increase the degree of multiprogramming

another process added to the system

Thrashing | a process is busy swappingpages in and out


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Thrashing (Cont.)


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Demand Paging and Thrashing

Why does demand paging work?Locality model

Process migrates from onelocality to another

Localities may overlap

Why does thrashing occur?7 size of locality > total memorysize


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Working-Set Model ( |working-set window| a fixed number of

page referencesExample: 10,000 instruction

WSSi (working set size ofProcess Pi) =

total number of pages referenced in the mostrecent ( (varies in time)

if( too small will not encompass entirelocality

if( too large will encompass several localities

if( = g will encompass entire program

D = 7 WSSi | total demand frames

ifD > m Thrashing

Policy ifD > m, then suspend one of the

processes


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Keeping

Track of t

heW

orking Set Approximate with interval timer + a reference

bit

Example: ( = 10,000

Timer interrupts after every 5000 time units Keep in memory 2 bits for each page

Whenever a timer interrupts copy and sets thevalues of all reference bits to 0

If one of the bits in memory = 1 page inworking set

Why is this not completely accurate?

Improvement = 10 bits and interrupt every 1000time units


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Page Size

Smaller page size, less amount of internalfragmentation

Smaller page size, more pages required

per process More pages per process means larger

page tables Larger page tables means large portion of

page tables in virtual memory Secondary memory is designed to

efficiently transfer large blocks of data soa large page size is better


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Page Size

Small page size, large number of pageswill be found in main memory

As time goes on during execution, thepages in memory will all contain portionsof the process near recent references.Page faults low.

Increased page size causes pages tocontain locations further from any recentreference. Page faults rise.


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Frame Allocation: Issues Smaller no. of frames to a process more

processes can reside in MM increases probability

that OS will find at least one ready process

Despite Principle of Locality, Page faults rate will behigh

Beyond a certain size, additional allocation will have

no noticeable effect on page fault rate because of

Principle of Locality


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Frame Allocation Schemes

Fixed Allocation

Equal Allocation

Proportional Allocation Priority Allocation


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Fixed AllocationEqual allocation e.g., if 100 frames and 5 processes,

give each 20 pages.

Proportional allocation Allocate according to the size

of process.

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Priority Allocation

Use a proportional allocation scheme

using priorities rather than size.

If process Pigenerates a page fault,

select for replacement one of its frames.

select for replacement a frame from a process

with lower priority number.


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Global vs. Local Allocation

Global replacement processselects a replacement frame fromthe set of all frames; one processcan take a frame from another.

Local replacement each processselects from only its own set of

allocated frames.

Thrashing & Working Set

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