CGS 3863 Operating Systems Spring 2013

Dan C. MarinescuOffice: HEC 439 BOffice hours: M-Wd 11:30 AM – 1 PM

Last time: Discussion of the paper “Architecture of Complexity” by H. A. Simon Modularity, Abstractions, Layering, Hierarchy

Today: Complexity of Computer Systems Resource Sharing and Complexity Bandwidth and Latency Iteration Names and The Basic Abstractions Memory. Critical Properties of Memory Systems.

Read-Write Coherence Before or After Atomicity.

RAID Next time

Discussion of the paper “Hints for Computer Systems Design” by Butler Lamson.

Lecture 4 – Thursday September 2, 2010

2Lecture 4

Names and fundamental abstractions

The fundamental abstractions1. Storage mem, disk, data struct, File Systems, disk arrays2. Interpreters cpu, programming language e.g. java VM3. Communication wire, Ethernet

rely on names. Naming:

Flat Hierarchical

3Lecture 4

Computers a distinct species of complex systems

The complexity of computer systems not limited by the laws of physics distant bounds on composition Digital systems are noise-free. The hardware is controlled by software

The rate of change unprecedented The cost of digital hardware has dropped in average 30% per

year for the past 35 years

4Lecture 4

Analog, digital, and hybrid systems Analog systems:

the noise from individual components accumulate and the number of components is limited

Digital systems: are noise-free the number of components is not limited regeneration restoration of digital signal levels static discipline the range of the analog values a device

accepts for each input digital value should be wider than the range of analog output values

digital components could fail but big mistakes are easier to detect than small ones!!

Hybrid systems e.g., quantum computers and quantum communication systems

5Lecture 4

Computers are controlled by software

Composition of hardware limited by laws of physics. Composition of software is not physically constrained;

software packages of 107 lines of code exist Abstractions hide the implementation beneath module

interfaces and allow the creation of complex software modification of the modules

Abstractions can be leaky. Example, representation of integers, floating point numbers.

6Lecture 4

Exponential growth of computers Unprecedented:

when a system is ready to be released it may already be obsolete. when one of the parameters of a system changes by a factor of

2 other components must be drastically altered due to the incommensurate scaling.

10 the systems must be redesigned; E.g.; balance CPU, memory, and I/O bandwidth;

does not give pause to developers to learn lessons from existing systems find and correct all errors

negatively affects “human engineering” ability to build reliable and user-friendly systems

the legal and social frameworks are not ready

7Lecture 4

Coping with complexity of computer systems

Modularity, abstraction, layering, and hierarchy are necessary but not sufficient.

An additional technique iteration Iteration

Design increasingly more complex functionality in the system Test the system at each stage of the iteration to convince

yourself that the design is sound Easier to make changes during the design process

8Lecture 4

Iteration – design principles

Make it easy to change the simplest version must accommodate all changes required by

successive versions do not deviate from the original design rationale think carefully about modularity it is very hard to change it.

Take small steps; rebuild the system every day, to discover design flaws and errors. Ask others to test it.

Don’t rush to implementation. Think hard before starting to program.

Use feedback judiciously use alpha and beta versions do not be overconfident from an early success

Study failures understand that complex systems fail for complex reasons.

9Lecture 4

Curbing complexity

In absence of physical laws curb the complexity by good judgment. Easier said than done because: tempted to add new features than in the previous generation competitors have already incorporated the new features the features seem easy to implement the technology has improved human behavior: arrogance, pride, overconfidence…

10Lecture 4

MICROPROCESSORS (1980s)MULTI-CORE MICROPROCESSORS

(2000s)

WORLD WIDE WEB (1990s)

GOOGLE, YouTube (2000s)

FIBER OPTICS (1990s)

WIRELESS (2000s)

SENSORSDIGITAL CAMERAS

(2000s)

COLLECT

PROCESSDISSEMINATE

COMMUNICATE

OPTICAL STORAGEHIGH DENSITY SOLID-STATE

(1990s)SPINTRONICS (2000s)

MILESTONES IN INFORMATION PROCESSING

BOOLEAN ALGEBRA (1854)DIGITAL COMPUTERS (1940s)INFORMATION THEORY (1948)

- Quantum Computing- Quantum Information Theory

STORE

Critical elements of information revolution!

11Lecture 4

Complexity of computing and communication systems

New components

New applications

Interconnectivity + mobility, embedded

devices

Physical constraints

Larger segment of population using

the systems

Optimization of resource

consumption

TimingConstraints

12Lecture 4

The relation between homo sapiens and the computers

The feelings of the homo sapiens: Hate Frustration Lack of understanding

The Operating System A program to “domesticate” the computer. Transforms a “bare machine” into a “user machine” Controls and facilitates access to computing resources;

optimizes the use of resources. The relation went through several stages:

Many-to-one One-to-one Many-to-many Peer-to-peer

13Lecture 4

Resource sharing and complexity

A main function of the OS is resource sharing. Sharing computer resources went through several

stages with different levels of complexity: Many-to-one One-to-one Many-to-many Peer-to-peer

14Lecture 4

Time-Shared Computer System

HSHS

HSHS

HSHS

HS

Switch

HS – Homo Sapiens

HS Personal Computer

A. Many-to-one

B. One-to-one

C. Many-to-many

HS Computer System

Computer System

Computer SystemHS

Internet

CI

HS – Homo SapiensCI- Computing Instrument

CI

15Lecture 4

D. Peer-to-peer

HS

HS

Internet

HS – Homo SapiensPC- Personal Computer I

Host Guest

Host Guest

HSHostGuest

HSHostGuest

16Lecture 4

Lecture 4 1717

Names and the three basic abstractions Memory stores named objects

write(name, value) value READ(name) file system: /dcm/classes/Fall09/Lectures/Lecture5.ppt

Interpreters manipulates named objects machine instructions ADD R1,R2 modules Variables call sort(table)

Communication Links connect named objects HTTP protocol used by the Web and file systems Host: boticelli.cs.ucf.edu put /dcm/classes/Fall09/Lectures/Lecture5.ppt get /dcm/classes/Fall09/Lectures/Lecture5.ppt

Lecture 4 18

Latency and Bandwidth

Important concepts for physical characterization. Applies to all three abstractions. Informal

Bandwidth number of operations per second! Latency to get there

The bandwidth of the CPU, Memory, and I/O sbsystems must be balanced.

18

Lecture 4 1919

Communication latency- time it takes the first bit sent to reach the receiver

Latency

Bandwidth

Bandwidth- number of bits/bytes transmitted per unit of time

t1

t2

t3

t4t5

t6

Time Time

Sender Receiver

Program Storagedevice

t2t3

t1

Operation latency- time it takes the command to read the device

Latency

Bandwidth- number of bits/bytes transmitted per unit of time

Bandwidth

t4

t5

t6

Lecture 4 2020

Memory Hardware memory:

Devices RAM (Random Access Memory) chip Flash memory non-volatile memory that can be erased and

reprogrammed Magnetic tape Magnetic Disk CD and DVD

Systems RAID File systems DBMS (Data Base management Systems)

Lecture 4 2121

Attributes of the storage medium/system

Durability the time it remembers Stability whether or not the data is changed during the

storage Persistence property of data storage system, it keeps

trying to preserve the data

Lecture 4 2222

Critical properties of a storage medium/system Read/Write Coherence the result of a READ of a

memory cell should be the same as the most recent WRITE to that cell.

Before-or-after atomicity the result of every READ or WRITE is as if that READ or WRITE occurred either completely before or completely after any other READ or WRITE

Lecture 4 2323

time

WRITE item A in cell M

AM

READ from cell M

AM

A A

Read/Write Coherence è the result of a READ of a memory cell should be the same as the most recent WRITE to that cell.

Before-or-after atomicity è the result of every READ or WRITE is as if that READ or WRITE occurred either completely before or completely after any other READ or WRITE

Current READ/WRITE

Previous READ/WRITE

Next READ/WRITE

Lecture 4 2424

Why it is hard to guarantee the critical properties? Concurrency multiple threads could READ/WRITE to

the same cell Remote storage The delay to reach the physical

storage may not guarantee FIFO operation Optimizations data may be buffered to increase I/O

efficiency Cell size may be different than data size data may be

written to multiple cells. Replicated storage difficult to maintain consistency.

Lecture 4 2525

Access type; access time

Sequential access Tapes CD/DVD

Random access devices Disk

Seek Search time Read/Write time

RAM

Lecture 4 26

Physical memory organization RAM two dimensional array. To select a flip-flop

provide the x and y coordinates. Tapes blocks of a given length and gaps (special

combination of bits. Disk:

Multiple platters Cylinders correspond to a particular position of the moving arm Track circular pattern of bits on a given platter and cylinder Record multiple records on a track

26

Lecture 4 27

Names and physical addresses Location addressed memory the hardware maps the

physical coordinates to consecutive integers, addresses Associative memory unrestricted mapping; the hardware

does not impose any constraints in mapping the physical coordinates

27

Figure 2.2 from textrbook

Lecture 4 28

RAID – Redundant Array of Inexpensive Disks

The abstraction put to work to increase performance and durability.

Raid 0 allows concurrent reading and writing. Increases performance but does not improve reliability.

Raid 1 increases durability by replication the block of data on multiple disks (mirroring)

Raid 2 Disks are synchronized and striped in very small stripes, often in single bytes/words. Error correction calculated across corresponding bits on disks,

and is stored on multiple parity disks (Hamming codes).

28

Lecture 4 2929

Lecture 4 30

RAID (cont’d)

Raid 3 Striped set with dedicated parity or bit interleaved parity or byte level parity.

Raid 4 improves reliability, it adds error correction

30

Lecture 4 31

RAID (cont’d) Raid 5 striped disks with parity combines three or more

disks to protect data against loss of any one disk. The storage capacity of the array is reduced by one disk

Raid 6 striped disks with dual parity combines four or more disks to protect against loss of any two disks.  Makes larger RAID groups more practical. Large-capacity drives lengthen the time needed to recover from

the failure of a single drive. Single parity RAID levels are vulnerable to data loss until the failed drive is rebuilt: the larger the drive, the longer the rebuild will take. Dual parity gives time to rebuild the array without the data being at risk if a (single) additional drive fails before the rebuild is complete.

31

Lecture 4 3232

Figure 2.3 from textbook