operating systems lecture 38 frame allocation read ch. 10.5 - 10.6

Silberschatz, Galvin and Gagne 2002Modified for CSCI 399, Royden, 2005

7.1Operating System Concepts

Operating Systems

Lecture 38Frame Allocation

Read Ch. 10.5 - 10.6



Page Replacement Algorithms

Page replacement algorithms select the page to be replaced.

Want lowest page-fault rate. Evaluate algorithm by running it on a particular string of

memory references (reference string) and computing the number of page faults on that string.

In all our examples, the reference string is

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5. Last time:

FIFO algorithm Optimal algorithm



Least Recently Used (LRU) Algorithm

Replace the page that has not been used for the longest period of time.

Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

Considered a good replacement algorithm. Question: How do we implement it?



LRU Counter implementation

Add a logical clock or counter that is incremented with each memory reference.

When a reference is made to a page, the clock register is copied into the time-of-use field in the page table entry.

Replace the page with the smallest time value. This requires a search of the page table to find the page

with the lowest clock value. It also requires an extra write to memory (of the clock

value) for each memory access.



LRU Algorithm, Stack Implementation

Keep a stack of page numbers. If a page is referenced, move it to the top of the stack. The LRU page ends up on the bottom of the stack. No search is required to find the LRU page. Because of the need to remove a page from the middle of

the stack, implement the stack with a doubly linked list. Example: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5



Use Of A Stack to Record The Most Recent Page References



Allocation of Frames

Each process has a maximum number of frames allocated--we cannot allocate more frames than there are in memory.

Each process needs minimum number of pages. As the number of frames per process decreases, the fault rate

increases. When a page fault occurs before an instruction is complete, we must

restart the instruction. There must be enough frames to hold all the pages that a single

instruction can reference. Example: IBM 370 – 6 pages to handle Storage to Storage MOVE

instruction (SS MOVE): instruction is 6 bytes, might span 2 pages. 2 pages to handle from. 2 pages to handle to.

Two major allocation schemes. fixed allocation priority allocation



Fixed Allocation

Equal allocation – e.g., if 100 frames and 5 processes, give each 20 pages.

Proportional allocation – Allocate according to the size of process.

mSs

pa

m

sS

ps

iii

i

ii

×==

=∑=

=

for allocation

frames of number total

process of size



Priority Allocation

Use a proportional allocation scheme using priorities rather than size.

If process Pi generates a page fault, either select for replacement one of its frames. OR select for replacement a frame from a process with

lower priority number.



Global vs. Local Allocation

Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another.

Local replacement – each process selects from only its own set of allocated frames.

In global replacement a process cannot control its own page fault rate. It's turnaround time may vary depending on the behavior of other processes.

With local replacement, a process may be hindered because it does not have access to other, less used pages in memory.

Global replacement generally has greater system throughput, so it is more commonly used.



Thrashing

If a process does not have “enough” pages, the page-fault rate is very high.

Thrashing a process is busy swapping pages in and out. Cause of thrashing:

If using a global replacement scheme, if one process starts needing more frames it will take them from other processes.

These other processes may start faulting more. There could end up being many processes in queue waiting

for pager to swap in needed pages. This leads to low CPU utilization. operating system thinks that it needs to increase the degree

of multiprogramming to increase CPU utilization. another process added to the system, making the problem

worse.



Thrashing

Why does paging work?Locality model Process migrates from one locality to another. Localities may overlap.

Why does thrashing occur? size of locality > total memory size



Locality In A Memory-Reference Pattern



Working-set model

The working-set model is a way of estimating the size of the current locality for a process.

working-set window a fixed number of page references The working set is the set of pages in the most recent page

references.



Working-Set Model

WSSi (working set of Process Pi) =total number of pages referenced in the most recent (varies in time) if too small will not encompass entire locality. if too large will encompass several localities. if = will encompass entire program.

D = WSSi total demand frames

if D > m Thrashing Policy if D > m, then suspend one of the processes.



Page-Fault Frequency Scheme

The Page-Fault Frequency scheme is an alternative to the working-set model.

Establish “acceptable” page-fault rate. If actual rate too low, process loses frame. If actual rate too high, process gains frame.

operating systems lecture 38 frame allocation read ch. 10.5 - 10.6

Documents