Memory Management

Malik Rizwan Yasin – Ph.d Researcher in Advance Network System

Memory Management

Malik Rizwan Yasin -00923009289949 1

4.1 Basic memory management4.2 Swapping4.3 Virtual memory4.4 Page replacement algorithms4.5 Modeling page replacement algorithms4.6 Design issues for paging systems4.7 Implementation issues4.8 Segmentation

Memory Management


• Ideally programmers want memory that is– large– fast– non volatile

• Memory hierarchy– small amount of fast, expensive memory – cache – some medium-speed, medium price main memory– gigabytes of slow, cheap disk storage

• Memory manager handles the memory hierarchy

Requirements of MM

• Relocation: cannot be sure where program will be loaded in memory

• Protection: avoiding unwanted interference by other processes

• Efficient use of CPU and main memory• Sharing: data shared by cooperating processes


CPU Utilization

Degree of multiprogramming


Multiprogramming with Fixed Partitions


• Fixed memory partitionsa) separate input queues for each partitionb) single input queue

Multiprogramming with Fixed Partitions• Memory is allocated according to some

algorithm, e.g. using best fit• Strength: easy implementation• Weakness: inefficient use of memory because

of internal fragmentation (partitions may not be full); limited number of active processes


Swapping or Dynamic Partitioning


Memory allocation changes by swapping processes in and out

Shaded regions are unused memory - external fragmentation

Problem with growing segments


• Allocating space for growing data segment• Allocating space for growing stack & data segment

Virtual Memory


• Problem: some programs are too big for main memory; large programs in memory limit the degree of multiprogramming

• Solution: keep only those parts of the programs in main memory that are currently in use

• Basic idea: a map between program-generated addresses (virtual address space) and main memory

• Main techniques: paging and segmentation

Paging (1)

The position and function of the MMUMalik Rizwan Yasin -

00923009289949 10

Paging (2)


The relation betweenvirtual addressesand physical memory addres-ses given bypage table

Page Tables (1)

Malik Rizwan Yasin -00923009289949 12Internal operation of MMU with 16 4 KB pages

Page Tables (2)Second-level page tables

Top-level page table


• 32 bit address with 2 page table fields• Two-level page tables

Page Tables (3)

Typical page table entry


TLBs – Translation Lookaside Buffers

A TLB to speed up pagingMalik Rizwan Yasin -

00923009289949 15

Inverted Page Tables

Comparison of a traditional page table with an inverted page tableMalik Rizwan Yasin -

00923009289949 16

Page Replacement

• Page fault: referencing a page that is not in main memory

• Page fault forces choice– which page must be removed to make room for

incoming page• Modified page must first be saved

– unmodified just overwritten• Better not to choose an often used page

– will probably need to be brought back in soon


Page Fault Handling (1)

Hardware traps to kernelGeneral registers savedOS chooses page frame to freeIf selected frame is dirty, writes it to disk


Page Fault Handling (2)

OS brings scheduled new page in from diskPage tables updatedFaulting instruction backed up to when it began Faulting process scheduledRegisters restoredProgram continues


Optimal Page Replacement Algorithm

• Replace page needed at the farthest point in future– Optimal but unrealizable

• Estimate by …– logging page use on previous runs of process– although this is impractical


Not Recently Used Page Replacement Algorithm• Each page has Reference bit, Modified bit

– bits are set when page is referenced, modified• Pages are classified

1. not referenced, not modified2. not referenced, modified3. referenced, not modified4. referenced, modified

• NRU removes page at random– from lowest numbered non empty class

• Macintosh v.m. uses a variant of NRUMalik Rizwan Yasin -

00923009289949 21

FIFO Page Replacement Algorithm

• Maintain a linked list of all pages– in order they came into memory

• Page at beginning of list (the oldest page) is replaced

• Disadvantage– page in memory the longest may be often used


Second Chance Page Replacement Algorithm


• Operation of a second chance– pages sorted in FIFO order– Page list if fault occurs at time 20, A has R bit set

(numbers above pages are loading times)

The Clock Page Replacement Algorithm


Least Recently Used (LRU)

• Assume pages used recently will used again soon– throw out page that has been unused for longest time

• Must keep a linked list of pages– most recently used at front, least at rear– update this list every memory reference !!

• Alternatively, keep counter in each page table entry indicating the time of last reference– choose page with lowest value counter


Implementation of LRU


LRU using a matrix – pages referenced in order0,1,2,3,2,1,0,3,2,3

Simulating LRU in Software

• The aging algorithm simulates LRU in software


The Working Set

• Working set: the set of pages currently used by the process – Changes over time.

• Locality of reference: during any phase of execution, the process references only a relatively small fraction of its pages.

• Thrashing: a program causing page faults at every few instructions.


The Working Set Page Replacement Algorithm

The working set algorithmMalik Rizwan Yasin -

00923009289949 29

The WSClock Page Replacement Algorithm

Malik Rizwan Yasin -00923009289949 30Operation of the WSClock algorithm

Review of Page Replacement Algorithms


Modeling Page Replacement AlgorithmsBelady's Anomaly


a) FIFO with 3 page framesb) FIFO with 4 page frames• P's show which page references show page faults

Design Issues for Paging SystemsLocal versus Global Allocation Policies (1)


a) Original configurationb) Local page replacementc) Global page replacement

Local versus Global Allocation Policies (2)


Page fault rate as a function of the number of page frames assigned

Load Control

• Despite good designs, system may still thrash

• When PFF algorithm indicates – some processes need more memory – but no processes need less

• Solution :Reduce number of processes competing for memory– swap one or more to disk, divide up pages they held– reconsider degree of multiprogramming


Page Size

Small page size• Advantages

– less internal fragmentation – better fit for various data structures, code sections– less unused program in memory

• Disadvantages– programs need many pages, larger page tables


Cleaning Policy

• Need for a background process, paging daemon– periodically inspects state of memory

• When too few frames are free– selects pages to evict using a replacement algorithm

• It can use same circular list (clock) as regular page replacement algorithm


Segmentation (1)


• One-dimensional address space with growing tables• One table may bump into another

Segmentation (2)


Allows each table to grow or shrink, independently

Segmentation (3)


Comparison of paging and segmentation

Implementation of Pure Segmentation


(a)-(d) Development of external fragmentation(e) Compaction

Segmentation with Paging: MULTICS (1)


• Descriptor segment points to page tables• Segment descriptor – numbers are field lengths


A 34-bit MULTICS virtual address




Conversion of a 2-part MULTICS address into a main memory address



Simplified version of the MULTICS TLB

Date post:	12-Nov-2014
Category:	Documents
Upload:	malik-m-rizwan-yasin
View:	796 times
Download:	2 times

Memory Management

Documents