Virtual Memory:Part 2

Kashyap ShethKishore PuttaBijal ShahKshama Desai

Index

Recap Translation lookaside buffer Segmentation Segmentation with paging Working set model References

Terms & Notions Virtual memory (VM) is

Not a physical device but an abstract concept Comprised of the virtual address spaces (of all

processes) Virtual address space (VAS) (of one

process) Set of visible virtual addresses (Some systems may use a single VAS for all

processes)

Paging

Page: The Virtual Address Space is divided into equal number of units called a Page. Each page is of the same size.

Page frame: The Physical Address Space is divided into equal number of units called Page Frames. Each page frame is of the same size.

Memory Management Unit (MMU): it is used to map the virtual address onto the physical address.

Translation Lookaside Buffer

Each virtual memory reference can cause two physical memory accesses one to fetch the page table one to fetch the data

To overcome this problem a high-speed cache is set up for page table entries called the TLB - Translation Lookaside Buffer

Translation Lookaside Buffer

Contains page table entries that have been most recently used

Functions same way as a memory cache

Translation Lookaside Buffer

Given a virtual address, processor examines the TLB

If page table entry is present (a hit), the frame number is retrieved and the real address is formed

If page table entry is not found in the TLB (a miss), the page number is used to index the process page table

Translation Lookaside Buffer

First checks if page is already in main memory if not in main memory a page fault is issued

The TLB is updated to include the new page entry

Operation of Paging and Translation Lookaside

Buffer(Stallings Fig 8.8)

Use of a Translation Lookaside Buffer(Stallings Fig 8.7)

Segmentation

What is Segmentation? Segmentation: Advantages Segmentation: Disadvantages

Virtual address space

Call stack

Parse tree

Constant table

Source text

Symbol table

Address space allocated to the parse tree

free

Space currently being used by the parse tree

Symbol table has bumped into the source text table

MMU

STBR

STLR

Base

Limit

OtherSegment table

What is Segmentation?

Seg 1(code) Seg 2

(data)Seg 3(stack)

Virtual memory

Segment # Offsetvirtual address

memoryaccess fault

no

Physicalmemory

Seg 2(data)

Seg 1(code)

Seg 3(stack)

0x00

as in paging:valid,

modified,protection,

etc.

offset <limit ?

External fragmentatio

n

Segment Base + Offsetphysical address

yes

Segmentation: Advantages

As opposed to paging: No internal fragmentation (but: external

fragmentation) May save memory if segments are very small and

should not be combined into one page (e.g. for reasons of protection)

Segment tables: only one entry per actual segment as opposed to one per page in VM

Average segment size >> average page size less overhead (smaller tables)

Segmentation: Disadvantages

External fragmentation Costly memory management algorithms

Segmentation: find free memory area big enough (search!)

Paging: keep list of free pages, any page is ok (take first!)

Segments of unequal size not suited as well for swapping

Combined Segmentationand Paging (CoSP)

What is CoSP? CoSP: Advantages CoSP: Disadvantages

CPUs so

Segment limit

Page table base

Segment table( for process)

<

yes

no

+

Logical address

Memory trap

so

p po

Physical memory

Page table (for segment)

ff po

Architecture For Segmentation With Paging

CoSP: Advantages

Reduces memory usage as opposed to pure paging Page table size limited by segment size Segment table has only one entry per actual segment

Simplifies handling protection and sharing of larger modules (define them as segments)

Most advantages of paging still hold Simplifies memory allocation Eliminates external fragmentation Supports swapping, demand paging, prepaging etc.

CoSP: Disadvantages

Internal fragmentation Yet only an average of about ½ page per

contiguous address range

Page 1Page 2

Process requests a 6KB address range(4KB pages)

internal fragmentati

on

Working Sets

• Working set of pages: minimum collection of pages that must be loaded in main memory for a process to operate efficiently without unnecessary page faults.

“Smallest collection of information that must be present in main memory to assure efficient execution of the program.

Process/Working Set: two manifestations of same ongoing computational activity.

Working Set Strategy

W(t,D) = set of pages in memory at time t of that process that have been referenced in the last D virtual time units. Virtual time = time that elapses while process in

execution measured in instruction steps. Working set size: number of pages in W(t,D).

Characteristics of Working Sets

Size: Working set is non decreasing function of window

(D) size. Specifically, W(t, D+1) contains W(t,D).

Prediction: Expect intuitively that immediate past page reference behavior constitutes good prediction of immediate future behavior.

Detecting/Measuring W(t,D)

Hardware mechanism to record if page referenced in last D seconds.

Software: Sample page table entries at intervals of D/K.

Any page that was referenced in these intervals is in working set.

Memory Allocation

A program will not be run unless there is space in memory for its working set.

Using the Working Set Concept

A strategy for resident set size: Monitor the working set of each process Periodically remove from the resident set of a process

those pages that are not in its working set. A process may execute only if its working set is in main

memory (if resident set includes its working set).

Issues With this Strategy

Past does not necessarily predict future. Size and membership of working set change over time.

A true measurement of WS for each process is impractical. Need to time stamp every page reference and keep a time-ordered queue.

Optimal value of D is unknown and would vary.

Alternatively

Look at page fault rate, not exact page references. Page fault rate falls as resident set size

increases. If page fault rate is below some threshold, give a

smaller resident set size If above some threshold, increase resident set

size.

References

Operating Systems,3rd edition-Gary Nutt. Modern Operating systems,2nd edition-

Tanenbaum. World Wide Web.

Operating Systems, William Stallings.

The End