Chapter 9: Virtual Memory Joe McCarthy CSS 430: Operating Systems - Virtual Memory1

Chapter 9:Virtual Memory

Joe McCarthy

CSS 430: Operating Systems - Virtual Memory 1

Chapter 9: Virtual Memory• Background• Demand Paging• Copy-on-Write• Page Replacement• Allocation of Frames • Thrashing• Memory-Mapped Files• Allocating Kernel Memory• Other Considerations• Operating-System Examples

Material derived, in part, fromOperating Systems Concepts with Java, 8th Ed.

2CSS 430: Operating Systems - Virtual Memory

Virtual Memory

Background

• Virtual memory – separation of logical memory from physical memory– Only part of the program needs to be in memory

for execution– Logical address space vs. physical address space?

Background

for execution– Logical address space >> physical address space– Degree of multiprogramming?

Background

for execution– Logical address space >> physical address space– Higher degree of multiprogramming

Swapping vs. Paging

Swapping Paging

Resident & Non-resident Pages

Resident: A, C, F

Non-resident: B, D, E, G, H

Demand Paging• Bring a page into memory only when it is needed

– Alternative: anticipatory or pre-paging• When page is needed but not [yet] in memory

– page fault– not-in-memory bring into memory– Swapper that deals with pages is a pager

• Advantages of demand vs. anticipatory paging?

Demand Paging• Bring a page into memory only when it is needed

– Alternative: anticipatory or pre-paging• When page is needed but not [yet] in memory

– page fault– not-in-memory bring into memory– Swapper that deals with pages is a pager

• Advantages of demand vs. anticipatory paging:– Less I/O needed– Less memory needed – Faster response– More users / programs

Handling a Page Fault

Page fault:Reference tonon-resident page

[Page table hasentries for all pages,but all pages maynot be resident]

1. Reference to non-resident page2. Trap to OS; look at PCB:

If invalid reference abortOtherwise, bring page into memory

3. Find empty frame4. Swap page into frame5. Update tables (page table, PCB)

Set valid-invalid bit = v6. Restart the instruction that caused the page fault

Copy on Write

• Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory

• Shared page is copied only if either process modifies it• COW allows more efficient process creation as only

modified pages are copied

What if there is no free frame?

3. Find empty frame

4. Swap page into frame

5. Update tables (page table, PCB)Set valid-invalid bit = v

6. Restart the instruction that caused the page fault

What if there is no free frame?

3. Find empty frame(or select victim frame)

4. Swap page into frame(possibly swap victim out)

5. Update tables (page table, PCB)Set valid-invalid bit = v

6. Restart the instruction that caused the page fault

Effective Access Time

• Page Fault Rate 0 p 1.0

• Effective Access TimeEAT =

Effective Access Time

• Page Fault Rate 0 p 1.0

• Effective Access TimeEAT = (1 – p) x memory access time+ p (page fault overhead + swap page out + swap page in + restart overhead)

Effective Access Time Example• Memory access time = 200 nanoseconds• Average page-fault service time = 8 milliseconds

• EAT = (1 – p) * 200 ns + p (8 ms) = (1 – p) * 200 ns + p * 8,000,000 ns

= 200 + p * 7,999,800 ns• If 1 access out of 1,000 (0.1%, p = 0.001)

causes a page fault, then EAT = 8.2 microseconds This is a slowdown by a factor of 40 (!)

Page Fault rates & Effective Access Times

Page Replacement• Prevent over-allocation of memory by modifying

page-fault service routine to include page replacement

• Use a modify (“dirty”) bit to reduce overhead of page transfers – only modified pages are written to disk

• Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory

Need For Page Replacement

Page Replacement

Page Replacement Algorithms

• Goal: low page-fault rate

• Evaluation?

Page Replacement Algorithms

• Goal: low page-fault rate

• Evaluation: simulate algorithm using a particular string of memory references (reference string) and computing the number of page faults on that string

• Example reference string:

7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1

FIFO Page Replacement

Replace frame which has been in memory for longest time

FIFO Page Replacement

Replace frame which has been in memory for longest time

First-In-First-Out (FIFO) Algorithm• Reference string:

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5• 3 frames

• 4 frames

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5• 3 frames

• 4 frames

9 page faults

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5• 3 frames

• 4 frames

9 page faults

5 10 page faults

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5• 3 frames

• 4 frames

• Belady’s Anomaly: more frames more page faults

9 page faults

5 10 page faults

Optimal Page Replacement

Replace frame which will not be used for longest time

Replace frame which will not be used for longest timeIs this knowable?

Is this useful?

LRU Page Replacement

Replace frame which has not been used for longest time

LRU Page Replacement

Replace frame which has not been used for longest time

LRU Implementations

• Counter implementation– Every page table entry has a counter– When page is referenced, counter = current time– When a page needs to be replaced, look at the

counters to determine which is oldest

• Advantages / disadvantages?

LRU Implementations

• Stack implementation– Maintain a stack of page numbers– When page is referenced, move it to the top– When page needs to be replaced, select bottom– Stack: doubly linked list

• Advantages / disadvantages?

LRU Approximations• Reference bit

– Associated with each page table entry– When page is referenced, set ref bit = 1– Replace a page with ref bit = 0 (if it exists)

• pages with ref bit = 0 less recently used than pages with ref bit = 1

• Second chance algorithm– aka “Clock” replacement– For each page,

• If ref bit = 0, replace this page, stop

• Else set ref bit = 0 (for next round), continue

Second View of Second Chance

Operating Systems Internals & Design Principles, 7th Edition

by William Stallings

Find a frame for page 727

Second View of Second Chance

Find a frame for page 727

Counting Algorithms

• Keep a counter of the number of references that have been made to each page

Counting Algorithms

• Keep a counter of the number of references that have been made to each page

• Least Frequently Used (LFU) Algorithm: – Replace page with smallest count

• Most Frequently Used (MFU) Algorithm: – Replace page with largest count– Idea: the pages with the smallest counts were

probably just brought in and have yet to be used

Allocation of Frames

• Each process needs a minimum number of pages– Example: IBM 370

• May require 6 pages to handle a single SS MOVE instruction• Instruction is 6 bytes, might span 2 pages• from block might span 2 pages• to block might span 2 pages

• Two major allocation schemes– fixed allocation– priority allocation

Fixed Allocation

Fixed Allocation• Equal allocation: all processes get the same

number of frames– E.g., if there are 100 frames and 5 processes,

give each process 20 frames.• Proportional allocation: differentially allocate

according to the size of process (in pages)

for allocation

frames of number total

process of size

5964137127

56413710

Fixed Allocation

for allocation

frames of number total

process of size

5964137127

56413710

• Equal allocation: all processes get the same number of frames– E.g., if there are 100 frames and 5 processes,

give each process 20 frames.• Proportional allocation: differentially allocate

according to the size of process (in pages)

Priority Allocation

• Proportional allocation scheme using priorities rather than size

• If process Pi generates a page fault,– select for replacement one of its frames– 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– Priority allocation must use global

replacement

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

Sufficient Allocation• If a process is not allocated enough pages,

the page-fault rate is very high. This leads to:– ? CPU utilization

the page-fault rate is very high. This leads to:– low CPU utilization

the page-fault rate is very high. This leads to:– low CPU utilization– OS decides it needs to increase the degree of

multiprogramming

the page-fault rate is very high. This leads to:– low CPU utilization– OS decides it needs to increase the degree of

multiprogramming– another process added to the system

Thrashing• If a process does not have “enough” pages, the

page-fault rate is very high. This leads to:– low CPU utilization– OS decides it needs to increase the degree of

multiprogramming– another process added to the system

• Thrashing more time spent paging than executing user processes

Memory Reference Patterns

Demand Paging & Thrashing

• Why does demand paging work?– Locality model

• Memory references tend to cluster together• Process migrates from one locality to another• Localities may overlap

• Why does thrashing occur?– size of localities > total memory size allocated

Working-Set Model• working-set window sequence of page references (e.g., 10,000)• 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 ( suspend a process)

Working-Set Model• working-set window sequence of page references (e.g., 10,000)• 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 ( suspend a process)

Page-Fault Frequency Scheme

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

End of Chapter 9

Keeping Track of the Working 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 & set the values of

all reference bits to 0– If one of the bits in memory = 1 page in working set

• Why is this not completely accurate?• Improvement = 10 bits and interrupt every 1000

time units

Working Sets and Page Fault Rates

Memory-Mapped Files• Memory-mapped file I/O allows file I/O to be treated

as routine memory access by mapping a disk block to a page in memory.

• A file is initially read using demand paging. A page-sized portion of the file is read from the file system into a physical page. Subsequent reads/writes to/from the file are treated as ordinary memory accesses.

• Simplifies file access by treating file I/O through memory rather than read() write() system calls.

• Also allows several processes to map the same file allowing the pages in memory to be shared.

Memory Mapped Files

Memory-Mapped Shared Memoryin Windows

Memory-Mapped Files in Java

Allocating Kernel Memory

• Treated differently from user memory

• Often allocated from a free-memory pool– Kernel requests memory for various structures,

many of which have fixed sizes– Some kernel memory needs to be contiguous– All kernel memory accesses needs to be fast

Buddy System• Allocates memory from

fixed-size segment consisting of physically-contiguous pages

• Power-of-2 allocator– Satisfies requests in units sized as

power of 2 (rounded up)– When smaller allocation needed

than is available, split a chunk into two buddies of next-lower power of 2

• Continue as needed• Coalescing:

– Combine adjacent chunks when freed

Buddy System

Tree Representation

Slab Allocation

Slab Allocator• Alternate strategy• Slab is one or more physically contiguous pages• Cache consists of one or more slabs• Single cache for each unique kernel data structure

– Each cache filled with objects – instantiations of the data structure• When cache created, filled with objects marked as free• When structures stored, objects marked as used• If slab is full of used objects, next object allocated from empty slab

– If no empty slabs, new slab allocated• Benefits?

Slab Allocator• Alternate strategy• Slab is one or more physically contiguous pages• Cache consists of one or more slabs• Single cache for each unique kernel data structure

– Each cache filled with objects – instantiations of the data structure• When cache created, filled with objects marked as free• When structures stored, objects marked as used• If slab is full of used objects, next object allocated from empty slab

– If no empty slabs, new slab allocated• Benefits include no fragmentation, fast memory request

satisfaction

Other Issues

• Pre-paging:– [when] is pre-paging worthwhile?

• Page size– What factors influence page size?– What factors are influenced by page size?

Other Issues -- Prepaging• Prepaging

– To reduce the large number of page faults that occurs at process startup

– Prepage all or some of the pages a process will need, before they are referenced

– But if prepaged pages are unused, I/O and memory was wasted

– Assume s pages are prepaged and α of the pages is used

• Is cost of s * α save pages faults > or < than the cost of prepaging s * (1- α) unnecessary pages?

• α near zero prepaging loses

Other Issues – Page Size

• Page size selection must take into consideration:– fragmentation– table size – I/O overhead– locality

Other Issues – TLB Reach • TLB Reach - The amount of memory accessible from the TLB• TLB Reach = (TLB Size) X (Page Size)• Ideally, the working set of each process is stored in the TLB

– Otherwise there is a high degree of page faults

• Increase the Page Size– This may lead to an increase in fragmentation as not all

applications require a large page size

• Provide Multiple Page Sizes– This allows applications that require larger page sizes the

opportunity to use them without an increase in fragmentation

Other Issues – Program Structure• Program structure

– Int[128,128] data;– Each row is stored in one page – Program 1

for (j = 0; j < 128; j++) for (i = 0; i < 128; i++) data[i,j] = 0;

Page faults:

– Program 2 for (i = 0; i < 128; i++) for (j = 0; j < 128; j++) data[i,j] = 0;

Page faults:

Other Issues – Program Structure• Program structure

– Int[128,128] data;– Each row is stored in one page – Program 1

for (j = 0; j < 128; j++) for (i = 0; i < 128; i++) data[i,j] = 0;

Page faults: 128 x 128 = 16,384

– Program 2 for (i = 0; i < 128; i++) for (j = 0; j < 128; j++) data[i,j] = 0;

Page faults: 1 x 128 = 128

Operating System Examples

• Windows XP

• Solaris

Windows XP• Uses demand paging with clustering. Clustering brings in pages

surrounding the faulting page.• Processes are assigned working set minimum and working set

maximum (typically 50 and 345 pages)• Working set minimum is the minimum number of pages the process

is guaranteed to have in memory.• A process may be assigned as many pages up to its working set

maximum.• When the amount of free memory in the system falls below a

threshold, automatic working set trimming is performed to restore the amount of free memory (clock or FIFO algorithm)

• Working set trimming removes pages from processes that have pages in excess of their working set minimum.

Solaris • Maintains free list with

thresholds– Lotsfree: start paging

(1/64 size of phys. memory)– Desfree: increase paging– Minfree: start swapping

• Pageout process• 2-handed clock algorithm

– Handspread– Scanrate: slowscan

fastscan

Chapter 9: Virtual Memory Joe McCarthy CSS 430: Operating Systems - Virtual Memory1

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