Computer Science 213 © 2006 Donald Acton 139 The “Plan” Build a model of –A computer system –How the CPU works –Memory –What the software sees Explore

1Computer Science 213© 2006 Donald Acton

The “Plan”The “Plan”

• Build a model of– A computer system– How the CPU works– Memory– What the software sees

• Explore our model to see how to go from hardware to the view application software has


How is software organized to tie the hardware together?

How is software organized to tie the hardware together?

MainmemoryBus interface

ALU

Register file

CPU

Disk controller

Graphicsadapter

USBcontroller

Mouse Keyboard Display

Disk

I/O bus

PC

From Computer Systems: A Programmer’s Perspective

I/O bridge

System bus Memory bus

Expansion slots forother devices suchas network adapters


Types of SoftwareTypes of Software

There are 2 broad classes of software:– Operating, or Systems software– Application Software

The operating system code is The operating system code is often referred to as the often referred to as the kernelkernel


LayeringLayering

Application programs

Processor Main memory I/O devices

Operating system

Software

Hardware



Role of the OSRole of the OS

• Provide services commonly used by applications

• Arbitrate requests for resources• Resources are anything managed by

the OS:– Memory– Devices– CPU cycles


Role of the OS cont’d…Role of the OS cont’d…

• Protect applications from each other• Create a virtual machine to program

to (i.e. hide the hardware details as much as possible from the application programmer.)


Virtual MachineVirtual Machine

• An idealized version of what a computer should look like to an application

• The OS realizes the virtual machine• System calls are the access points to the

virtual machine• A collection of system calls for a particular

class of services (e.g. file system manipulation) form an Application Programming Interface (API)


Typical Virtual Machine Services

Typical Virtual Machine Services

• Processes Processor– System calls to create manage and

manipulate processes

• Virtual Memory Physical memory– System calls to allocate and free

memory

• Files I/O devices– System calls to create, open, close, read

and write devices


Layering - AgainLayering - Again



Operating system

Software

Hardware



Virtual Machine AdvantagesVirtual Machine Advantages

• Applications don’t have to know how the hardware really works – interacting with hardware is UGLY

• If new hardware is added only the virtual machine software needs to be updated


DisadvantagesDisadvantages

• The extra software makes manipulating the hardware slower

• It may not be possible to access all the features of the hardware


System CallsSystem Calls

• They are like subroutine/function calls from the application to the OS

• Collections of system calls form an API

• Example APIs– win32 API (Microsoft Windows)– Unix sockets (networking)– POSIX (a collection of “UNIX” APIs)


POSIXPOSIX

• Portable Operating System Interface• Basically yet another attempt (mid

80s) to unify the collection of UNIX OSes

• POSIX sub standards:– POSIX.1 – system interfaces and headers– POSIX.1b – realtime extensions– POSIX.1c – threads– POSIX.2 – shell and utilities


System CallsSystem Calls

Application program


Operating System


Another System Call ViewAnother System Call View

Application Operating System

Application code

Time

OS code

Interacts with hardware

OS code

Application code

Adapted from: Computer Systems: A Programmer’s Perspective


Typical System CallsTypical System Calls

• System calls are the only way to access shared resources or request a service

• Example Unix system calls:– fork() – create a new process that is an

exact copy of the current process– execve() – replace the current running

process image with a new one– exit() – causes the process to terminate

after cleaning up


Example System Calls cont’d

Example System Calls cont’d

– creat() – create a new file– getrlimit() – determine system resource

consumption– mkdir() – make a directory– open() – open a file– stime() – set system time and date– wait() for a child process to exit


Waiting for HardwareWaiting for Hardware

• What should the OS do when it interacts with hardware and there is a delay?

• Hardware is fast, right, so shouldn’t the CPU just wait?


Consider a Disk AccessConsider a Disk Access

• Typical read of a disk takes 10ms• How many instructions could a

3.2GHz P4 execute in that time?


How to Avoid Wasting Cycles

How to Avoid Wasting Cycles

• Run another process• This requires the OS to remember:

– All the processes and what they are doing so a new one can be selected

– What system call (i.e. process) is associated with a hardware request

– What hardware requests are outstanding so the requests can be checked


How Is this Done?How Is this Done?

• Process – the OS abstraction of a running program (application)

• Context – all the state information needed to run a process

• Context Switch – stopping or suspending the currently running process for another


Content of a ContextContent of a Context

• PC – indicates where the process is executing

• Registers – capture the current state of the CPU

• Memory – defines the code and data the process is working with

• System Resources – assigned to process by OS


ResourcesResources

• Open files• Devices• Buffers to hold I/O data• Held locks/semaphores


Performing Context SwitchPerforming Context Switch

Process A Process B

User code

Kernel code

User code

Kernel code

User code

TimeSystem call

System Call/InterruptReturn

from system call



Process SchedulingProcess Scheduling

• Scheduling is the act of “saving” the context of the running process and selecting a new process to run

• Can only happen when kernel is executing

• Kernel will execute when:1. Application makes a system call2. An interrupt occurs


SchedulingScheduling

System perspective

User

Kernel

Application A’s perspective

Active

Inactive

B BAA A



Scheduling PoliciesScheduling Policies

• The rules the kernel uses to select a process to run

• Most common is Round Robin– Process is running– Clock interrupt occurs – Kernel stops current process and places

it on end of queue of ready processes– Selects next process in queue to start


Process StatesProcess States

• Using the CPU• Waiting for the CPU• Waiting for the kernel to finish

providing a service• The process could be finished• The process could be newly created


Process State DiagramProcess State Diagram

• Consider a snapshot of system at a given time

• States are all the legal states any process could be in

• Transitions are the arcs describing all the legal states a process could immediately transition to


Process State DiagramProcess State Diagram

New

Ready

Blocked

DeadRunning


Role of main()Role of main()

• main() is just a well known function• When linking a program think of the

linker as having an existing program that always needs to call main()

• You supply the main()• The code before main() is setup code• The code executed after main()

returns performs cleanup


Calling main()Calling main()

main()

Process startupcode

User suppliedmain() code

Process cleanup code


Process TerminationProcess Termination

• A process ends if it – Returns from main()– Calls exit() or _exit() explicitly

• Returning from main() results in exit() being called


Just what does the OS do?Just what does the OS do?

• We have:– A layering model from the hardware to the

application– Basic model of CPU and a computer system

• We know about applications• We know how hardware signals the CPU• Remaining issue:

– How does the OS move information from the hardware to the application and back?


From Disks to ApplicationsFrom Disks to Applications

• Disk hardware model• OS view of a disk

– File system organization on disk

• Application view of a file– OS supplied disk services

• Sharing files• Transactions


Layering Yet Again!Layering Yet Again!


Operating system

Hardware

General Layering

Structure

Application

Unix I/O

File System

Disk Drive

File System

Layering


Disk ConstructionDisk Construction

• Platters – the actual recording surface, there are two recording surfaces per platter

• Surface is divided into tracks• Each track is divided into sectors• A sector is the smallest amount of

data that can be written/read to/from a disk


Surface LayoutSurface Layout

Spindle

SurfaceTracks

Track k

Sectors

Gaps



Platter ViewPlatter View

Surface 0

Surface 1

Surface 2

Surface 3

Surface 4

Surface 5

Cylinder k

Spindle

Platter 0

Platter 1

Platter 2



Disk DriveDisk Drive


Head CrashHead Crash

http://www.bolhuijo.com/gallery/diskdrive/aaa


Disk in ActionDisk in Action

Spindle


Performance MeasurementPerformance Measurement

• Seek time – the time to position the head to the appropriate track

• Rotational Latency -the length of time it takes for the spot on the disk to move under the head

• Transfer time – the amount of time to read the data from the sector once the reading has begun


Calculating Average Access Time

Calculating Average Access Time

7200 RPM, Tavg seek 10ms, 600 sectors/track

Tavg rotation

Tavg transfer

Taccess


Scheduling Disk TransfersScheduling Disk Transfers

• FCFS – first come first served

• SCAN

• SSTF – shortest seek time first

Fairest

Highest Throughput


Data Transfer: (Non DMA)Data Transfer: (Non DMA)

Mainmemory

ALU

Register file

CPU chip

Disk controller

Graphicsadapter

USBcontroller

Mouse Keyboard Monitor

Disk

I/O bus

Bus interface


Write (out)Read (in)


Evolution to DMAEvolution to DMA

• Why should the CPU be involved in touching all the data?– Compared to CPU memory access speeds, going

to the device memory is very time consuming

• Direct Memory Access (DMA) allows the device to access:– Main memory– Another device on the bus

• DMA can cause bus contention (more to come)


Controlling the Disk (DMA)Controlling the Disk (DMA)

Mainmemory

ALU

Register file

CPU chip

Disk controller

Graphicsadapter

USBcontroller


Disk

I/O bus

Bus interface


Interrupt


CPU – Disk InteractionCPU – Disk Interaction

• CPU writes commands to instruct disk controller which sector to read or write

• Disk controller positions head and initiates transfer, typically using DMA

• DMA – Direct Memory Access allows disk controller to transfer data directly to or from main memory

• Controller signals CPU via interrupt (through PIC) that operation is complete

• If read, CPU can get result directly from main memory


Bus ContentionBus Contention

• While OS is waiting for one device to finish it could instruct additional devices to perform operations

• Could result in 2 or more devices attempting to access bus at the same time


ContentionContention

Mainmemory

ALU

Register file

CPU chip

Disk controller

Graphicsadapter

USBcontroller


Disk

I/O bus

Bus interface



Are All Sectors the Same?Are All Sectors the Same?

Spindle


Things to ConsiderThings to Consider

• The circumference of the outer track is greater than the inner track therefore:– The disk is moving faster under the head– There is more surface area to a track

• Zoned bit encodings permit more sectors per track on outer tracks than inner tracks

• Some sectors are bad • Modern disk drives abstract the

track/head/sector information out and present an image of logical sectors


Disk Sectors/TrackDisk Sectors/Track

From: http://www.storagereview.com/

Fixed sectors per trackZoned-bit encoding

Variable Sectors per track

5 tracks 16 sectors/track



3 tracks 11 sectors/track3 tracks 9 sectors/track


Cheap vs Expensive DisksCheap vs Expensive Disks

How do cheap and expensive disks differ?

• People pay for performance • Features that improve a disk’s

performance or reliability cost more to incorporate into a disk


Disk failure Disk failure

Affected by– Duty cycle (how much time it is involved

in data transfer)– Temperature– Number of platters– Number of hours drive is on– Physical environment

• Mean time between failures • Typical service life 3 – 5 years


Disk LifeDisk Life

• High initial failure rates• Stable failure rate during the drives

service life (3 – 5 years)• Gradual increase in failures as

components wear out• Separate failure rates for power-on events• Mean time between failure – average

aggregate number of power on hours of a collection of disks before observing a failure


More Disks More HeadachesMore Disks More Headaches

• Network Appliance sells a compartment with 336 drives in it

• If MTBF of 100,000 hours how many drives would be expected to be replaced in a year?


Onto the File SystemOnto the File System

• Works directly with the view of the disk provided by the controller

• Provides a device independent view of a disk

• Further levels of abstraction on file system are what the application sees

Application

Unix I/O

File System

Disk Drive

File System

Layering


Purpose of a file systemPurpose of a file system

• Provides an abstraction so that applications don’t access disk directly

• Hides complexity of different drive types

• Manages the device• Provides structure for the

organization of the data• Protection


File System PropertiesFile System Properties

• Persistent – survive reboots• Correct – reflect the operations

performed• Robust – must be able to be

recovered in crash• Efficient – must make good use of

disk space and must be fast


Unix File System SemanticsUnix File System Semantics

• A file is simply a sequence of m bytes– B0, B1, .... , Bk , .... , Bm-1

• Everything is a file– All I/O devices are represented as files

• /dev/ad0s4e (/var disk partition)• /dev/ttyd0 (terminal)• /dev/rmt0 (tape drive)

– Even the running kernel is represented as a file:• /dev/kmem


Unix File TypesUnix File Types

• Regular files:– Binary/text files they are all the same

• Directory file – It’s still a file but it contains names and

locations of other files

• Special files– Block (disks) character (terminal)

• Interprocess communication– Named pipes– Sockets


Blocks – the basic unitBlocks – the basic unit

• A file is a sequence of bytes stored in a set of fixed sized blocks

• Block sizes are multiples of the sector size (e.g. 512, 1024, 4096, 8192)

• The sectors comprising a particular block are contiguous

• All blocks in a particular file system are the same size

• With respect to a file, the smallest amount of data the file system transfers to or from a disk is a block

• Blocks are the virtualization of a disk’s sectors


Needed File System Components and Services

Needed File System Components and Services

• Must be able to:– Keep track of which blocks are free– Determine which blocks belong to a

particular file– Maintain administrative data like file

permissions, creation and modified times– Find the list of free blocks, root directory,

and some “other stuff” when the system is started


The Primitive PiecesThe Primitive Pieces

• Super block – helps locate everything• Inode – maintains the file’s

administrative data, tracks the file’s data blocks

• Data blocks – multipurpose blocks used for – File’s data blocks– Indirect blocks


File System Data StructuresFile System Data Structures

• Inode list – list of free inodes (inodes are just numbers)

• Inode map – given an inode can locate the disk block the inode is in

• Free block list


Single Indirect Block

Unix InodeUnix InodeType/mode Link count

File Size

UID GID

Various modification

access and creation

times

Direct Data Block 0

Direct Data Block 1

Direct Blocks 2 - 9

Single Indirect Block

Double Indirect Block

Additional bookkeeping

information

Data Block

Data Block

0

……127

Data Block

Data Block

0

……127

0

……127

Data Block

Double Indirect Block

0

……127

0

……127

0

……127

0

……127

0

……127

Data Block

Direct Data Block 1

Direct Data Block 0Data Block

0

……127

Triple Indirect BlockTriple Indirect Block

The indirect block is exactly

the size of a block

Data Block


(Some) Inode Parts(Some) Inode Parts

• Type/Mode– Type (regular file, directory, device, etc)– Permissions

• Group, user, other - 3 bits each (e.g. rwxr-x—x, rw-r-----)

• Link count– How many directory entries this file has

• UID – User ID number – identifies owner of file

• GID– Group ID number – identifies group of file


How big is the largest file that could be created in the original Unix

filesystem?

How big is the largest file that could be created in the original Unix

filesystem?Compute the number

of blocks in the fileBlocks Blocks

Direct Blocks 10 10

Single Indirect 128 128

Double Indirect 1282

Triple Indirect 1283

Total


Block Address RangesBlock Address Ranges

• Direct– 0 to 5,119 (10 x 512 – 1)

• Single indirect– 5120 to 70,655 (5,120 + 128 x 512 – 1)

• Double indirect– 70,656 to 8,459,263

(70,656 + 1282 x 512 - 1)

• Triple indirect– 8,459,264 to 1,082,201,087

(8,459,264 + 1283 x 512 – 1)


Locate byte 1,000,000,033Locate byte 1,000,000,033

• Convert to blocks– 1,000,000,033 / 512 = 1,953,125– Remainder of 33 is offset into the block– Based on result determine if direct

access, single, double, or triple indirect– This case is triple indirect – Determine which of the 1283 blocks it is

• 1,953,125 – (10 + 128 + 1282) = 1,936,603


1 D 8 C D B

0001 1101 1000 1100 1101 1011

Continuing the huntContinuing the hunt

• 1,936,603 0x1D8CDB• Observe that 128 is 27 so if we look at

the above in binary and then group 7 bits at a time we can quickly get the indexes to the needed indirect blocks


struct ufs1_dinode {

u_int16_t di_mode; /* 0: IFMT, permissions; see below. */

int16_t di_nlink; /* 2: File link count. */

union {

u_int16_t oldids[2]; /* 4: Ffs: old user and group ids. */

} di_u;

u_int64_t di_size; /* 8: File byte count. */

int32_t di_atime; /* 16: Last access time. */

int32_t di_atimensec; /* 20: Last access time. */

int32_t di_mtime; /* 24: Last modified time. */

int32_t di_mtimensec; /* 28: Last modified time. */

int32_t di_ctime; /* 32: Last inode change time. */

int32_t di_ctimensec; /* 36: Last inode change time. */

ufs1_daddr_t di_db[NDADDR]; /* 40: Direct disk blocks. */

ufs1_daddr_t di_ib[NIADDR]; /* 88: Indirect disk blocks. */

u_int32_t di_flags; /* 100: Status flags (chflags). */

int32_t di_blocks; /* 104: Blocks actually held. */

int32_t di_gen; /* 108: Generation number. */

u_int32_t di_uid; /* 112: File owner. */

u_int32_t di_gid; /* 116: File group. */

int32_t di_spare[2]; /* 120: Reserved; currently unused */

};

FreeBSD 6.0 dinode.hFreeBSD 6.0 dinode.h


SuperblockSuperblock

• Located in first available sector after boot sector

• Maintains global file system information such as:– Location of inode map and free inode list– Location of the inode that corresponds to the

root directory– Location of list of free blocks– Block size being used– Sector size (anything except 512 is unusual)– Dirty flag

• Loss of the super block is a catastrophe


From the FactoryFrom the Factory

• As shipped the disk does not contain a file system (old disk drives didn’t even contain tracks and sectors)

• When disk is installed must:– Lay down tracks and sectors– Determine if all the sectors are good– Put down a superblock – Create an inode map and free inode list– Build the list of free blocks


FormattingFormatting

• Referred to as low level formatting in the Microsoft/PC world

• Laid down sectors and tracks (controller was external to drive)

• Wandering tracks• Mapped out bad sectors• Modern drives are shipped formatted

and deal with the bad sectors for us


Bad SectorsBad Sectors

• Over time sectors may become unreliable• Sectors may be unreliable initially due to

manufacturing flaws• Modern drives maintain tables mapping

bad sectors to good sectors• On older systems done manually or by

running a special program (bad144) • Example console error message:

NOTICE: sdsk: Unrecoverable error reading SCSI disk 2 dev 1/64 (ha=0 id=1 lun=0) block=219102 Medium error: Unrecovered read error


DisklabelDisklabel

• Defines Unix partitions on a disk • A partition is almost like a logical

disk within a (potentially logical) disk • Partitions break the “disk” into

logical functional regions• Example:

– Swap space– /var– /usr

size offset fstype

a: 2097152 0 4.2BSD

b: 5120000 2097152 swap

c: 61432560 0 unused # "raw"

d: 2560000 7217152 4.2BSD

e: 8388608 9777152 4.2BSD

f: 43266800 18165760 4.2BSD


newfsnewfs

• High level formatting to Microsoft/PC crowd

• Within Unix partition disk creates:– Superblock and duplicates– Inode list and map– Free block list

• Example newfs output /dev/ad0s2e: 4096.0MB (8388608 sectors) block size 16384,

fragment size 2048 using 23 cylinder groups of 183.77MB, 11761 blks,

23552 inodes.

super-block backups at: 160, 376512, … 8279904


MetadataMetadata

• Data about data• File metadata:

– Basically information in inode– Describes owner of data, file size, etc

• File system metadata:– Superblock information– Block size, inode map, block lists, etc


Directory FileDirectory File

• A directory is just a file that contains information to locate other files and directories

typedef struct dirent32 {

ino32_t d_ino; /* "inode number" of entry */

off32_t d_off; /* offset of disk directory

entry */

uint16_t d_reclen; /* length of this record */

char d_name[1];/* name of file */

} dirent32_t;

From: FreeBSD 5.2


Superblock + Inodes + data blocks and directories

Superblock + Inodes + data blocks and directories

Inode Map

Free Blocks

Inode of ‘/’

SuperblockInode Inode Inode Inode

Inode Map

Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Inode

Data

Block

vmunix

Inode

Data

Block

Data

Block

Indirect

Data

Block

Data

BlockData

BlockData

Block

Data

Block


How it might look on diskHow it might look on disk

Spindle


Creating a fileCreating a file

• When a file is created it is empty– Locate a free inode – Fill in inode fields– Remove inode from free list

• Locate directory to add file to – Add a new directory entry which

includes• Filename• Inode of file


Creating a FileCreating a File

Free Inodes

Inode Map

Free Blocks

…

Superblock

Inode Inode Inode Inode

Inode Map

Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Some Directory

•

••

FileA

FileC

NewFile

Second Directory

•

••

Link Count = 1Link Count = 2

NewFile2

Data

Block


Steps in Writing a File BlockSteps in Writing a File Block

• If data goes into an existing block– Make changes to in memory copy, write

changed block

• Otherwise– Locate a free block, write data– Add free block to inode or indirect block– May have to get a block to use as

indirect block first


Writing a File BlockWriting a File Block

Free Inodes

Inode Map

Free Blocks

…

Superblock


Inode Map

Data

Block Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Some Directory

•

••

FileA

FileC

NewFile

Indirect

Block


Steps in Deleting a FileSteps in Deleting a File

• Directory entry is removed and directory updated on disk

• Link count on file being removed decremented

• If link count 0– Return disk blocks to free list– Return inode to free inode list


Deleting a FileDeleting a File

Free Inodes

Inode Map

Free Blocks

…

Superblock


Inode Map

Data

Block Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Some Directory

•

••

FileA

FileC

NewFile

Indirect

Block

Link Count = 1Link Count = 0


Order of Writing Meta DataOrder of Writing Meta Data

• If data not written in proper order file system could be inconsistent if system crashes

• Examples:– Block claimed by two inodes

• (block added to inode before old inode updated)

– Inode referenced from two directories when not a link

• Inode freed and then immediately reused but directory of initial reference not update


Meta Data Update Requirements

Meta Data Update Requirements

• Indirect blocks and inodes written synchronously upon deallocation

• Directory updates consisting of:– Deleting a file– Adding a file– Changing (renaming) a file are all done

synchronously• Causes significant file system

slowdown


Ordering Operations (BAD)Ordering Operations (BAD)

Free Inodes

Inode Map

Free Blocks

…

Superblock


Inode Map

Data

Block Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Some Directory

•

••

FileA

FileC

NewFile

Indirect

Block


Ordering Operations (Good)Ordering Operations (Good)

Free Inodes

Inode Map

Free Blocks

…

Superblock


Inode Map

Data

Block Data

Block

Data

Block

Data

Block

Data

Block

Data

Block

Some Directory

•

••

FileA

FileC

NewFile

Indirect

Block


Checking a File SystemChecking a File System

• Need to determine– Which inodes are in use– Which blocks are in use– Which inodes and blocks are

unaccounted for so that they can be recovered for use


Files - SummaryFiles - Summary

• The inode does not contain the file’s name• A file can be associated with several

different names• When the inode’s reference count goes to

0 the resources used by the file can be reclaimed

• All the interesting information about a file (e.g. owner, permissions etc.) is contained in the inode


Actions to Read File DataActions to Read File Data

• To open a file and read data the system must:– Read the inode map– Read the inode for the file– Read 0 – 3 indirect blocks– Read the data block

• With this approach 3 – 6 reads are needed


Actions to Write File DataActions to Write File Data

• To open a file and write data the system must:– Read the inode map– Read the inode for the file– Read 0 – 3 indirect blocks

• Allocate and insert 0 – 3 indirect blocks

• Read and or allocate the data block, change it, write it back


The Case for CachesThe Case for Caches

• Assume IDE disk with 16KB blocks– Transfer time for 16KB ≈16x10-5ms– Average access time 10ms (really about

12.5ms)

• Time to read 16KB of data– 10ms * 3(6) = 30(60)ms

• Effective transfer rate– 16KB/30(60)ms = 533 KB/s (267KB/s)

• Manufacturer claimed transfer rate 150MB/s (SATA)


Speeding things upSpeeding things up

• Obvious problem is all the disk accesses

• Idea– Save (i.e. cache) copies of important

data to avoid going to disk– Could cache:

• Inode map• Inodes• Even individual disk blocks (indirect blocks

would be an especially good candidate)


Where are caches used?Where are caches used?


Registers

On-chip L1cache (SRAM)

Main memory(DRAM)

Local secondary storage(local disks)

Larger, slower,

and cheaper (per byte)storagedevices

Remote secondary storage(distributed file systems, Web servers)

Local disks hold files retrieved from disks on remote network servers.

Main memory holds disk blocks retrieved from local disks.

Off-chip L2cache (SRAM)

L1 cache holds cache lines retrieved from the L2 cache.

CPU registers hold words retrieved from cache memory.

L2 cache holds cache lines retrieved from memory.

L0:

L1:

L2:

L3:

L4:

L5:

Smaller,faster,and

costlier(per byte)storage devices


The Cost of CachingThe Cost of Caching

• It requires storage, that storage could be used for something else

• If changes are made to the cached copy then the copy and entity being cached are inconsistent

• Keeping things consistent can be a complex task especially if there are multiple cached copies


Buffering vs CachingBuffering vs Caching

• Buffering is used to deal with speed mismatches between a data producer and data consumer. The buffer contains the data.

• Caching is keeping a copy of data to speed up access to that data

• Some systems merge aspects of these two (e.g. a buffer of a changed inode can also be the inode’s cached copy)

Documents

Computer Science 213 © 2006 Donald Acton 139 The “Plan” Build a model of –A computer system –How the CPU works –Memory –What the software sees Explore