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Fundamentals of Computer Design

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Outline

• Performance Evolution• The Task of a Computer Designer• Technology and Computer Usage Trends• Cost and Trends in Cost• Measuring and Reporting Performance• Quantitative Principles of Computer Design

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Computer Architecture Is …

• The attributes of a [computing] system as seen by the programmer, i.e., the conceptual structure and functional behavior, as distinct from the organization of the data flows and controls, the logic design, and the physical implementation. (Amdahl, Blaaw, and Brooks, 1964)

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Computer Architecture’s Changing Definition

• 1950s to 1960s: Computer Architecture Course– Computer Arithmetic

• 1970s to mid 1980s: Computer Architecture Course – Instruction Set Design, especially ISA appropriate for compilers.

[ISA includes Instruction set, word size, memory address, processor registers, address and data formats]

• 1990s to 2000s: Computer Architecture Course– Design of CPU, memory system, I/O system, Multiprocessors

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Performance Evolution

• 1970s– performance improved 25—30%/yr– Mostly due to improved architecture + some technology aids

• 1980s– VLSI + microprocessor became the foundation– Technology improves at 35%/yr– Mostly with UNIX and C in mid-80’s

• Even most system programmers gave up assembly language • With this came the need for efficient compilers

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Performance Evolution (Cont.)

• 1980s (Cont.)– Compiler focus brought on the great CISC vs. RISC debate

• With the exception of Intel – RISC won the argument• RISC performance improved by 50%/year initially• Of course RISC is not as simple anymore and the compiler is a

key part of the game– Does not matter how fast your computer is, if the compiler

wastes most of it due to the inability to generate efficient code

– With the exploitation of instruction-level parallelism (pipeline + super-scalar) and the use of caches, performance is further enhanced

CISC: Complex Instruction Set ComputingRISC: Relegate Important Stuff to the Compiler (Reduced Instruction Set Computing)

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Growth in Performance (Figure 1.1)

Mainly due to advanced architecture ideas

Technology driven

MIPS: Microprocessor without Interlocked Pipeline

IBM: International Business Machines

HP: Hewlett Packard

DEC: Digital Equipment Corporation

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The changing Face of Computing and the Task of the Computer Designer

• Changes in computer use have led to three different computing markets

• Each characterized by different applications, requirements, and computing technologies:– Desktop Computing– Servers– Embedded Computers

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Desktop Computing• The first, and still the largest market in dollar terms, is

desktop computing.

• Desktop top systems often are where the newest, highest-performance ,microprocessors appear.

• Recently cost-reduced microprocessors and systems appear.

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Servers• For servers different characteristics are important:

– Availability• Which means that the system can reliably and effectively provide

service.– Scalability

• Means server systems often grow over their lifetime in response to a growing demand for the services they support (or) an increase in functional requirements.

• The ability to scale up the computing capacity, the memory, the storage and the I/O bandwidth of server.

– Efficient Throughput• That is, the overall performance of the server, in terms of

transactions per minute or web page served per second.

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Embedded Computers

• Embedded computers have the widest range of processing power and cost.

• Performance requirement in an embedded application is a real –time requirement.– A real-time performance requirement is one where a segment of the

application has an absolute maximum execution time that is allowed.

• Two other key characteristics exist in many embedded applications:– The need to minimize memory– The need to minimize power.

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The Task of A Computer Designer

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• The task of computer designer faces is a complex one:– Determine what attributes are important for a new machine.– Then design a machine to maximize performance.

• This aspects, including » Instruction set design» Functional Organization» Logic design and implementation

The Task of A Computer Designer

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Condt…

• Instruction set architecture refers to the actual programmer-visible instruction set.– The instruction set architecture servers as the boundary between the

software and hardware.

• The term Organization includes the high-level aspects of a computer design, such as – The Memory system, – The bus structure and – Design of the internal CPU

(CPU-where arithmetic, logic, branching, and data transfer are implemented)

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• Hardware is used to refer to the specifics of a machine, including:– The detailed logic design– The packaging technology of the machine.

• Computer architects must design a computer to meet functional requirements as well as price, power, and performance goal.

• In addition to performance and cost, performance must be aware of important trends in both the implementation technology and the use of computers.

Condt…..

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Technology Trends

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Technology Trends

• To plan for the evolution of a machine, the designer must be especially aware of rapidly occurring changes in implementation technology.

• 4 implementation technologies, which change at a dramatic pace, are critical to modern implementations:– Integrated Circuit Logic Technology– Semiconductor DRAM– Magnetic disk Technology– Network Technology

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Technology Trends (Cont..)

• Integrated Circuits– Density increases at 35%/yr.– Die size increases 10%-20%/yr– Combination is a chip complexity growth rate of 55%/yr– Transistor speed increase is similar but signal propagation does not

track this curve - so clock rates don’t go up as fast

• Semiconductor DRAM– Density quadruples every 3 years (approx. 60%/yr) [4x steps]– Cycle time decreases slowly - 33% in 10 years– Interface changes have improved bandwidth

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Technology Trends (Cont.)

• Magnetic Disk– Currently density improves at 100%/yr– Access time has improved by 33% in 10 years

• Network Technology– Depends both on the performance of switches and transmission

system– 1GB Ethernet becomes available about 5 years after 100MB– Doubling in bandwidth every year

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Cost, Price, and Their Trends

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Cost

• Clearly a market place issue -- profit as a function of volume• Price is what you sell a finished good for, and Cost is the

amount spent to produce it.• Let’s focus on hardware costs• Factors impacting cost

– Learning curve – manufacturing costs decrease over time– Yield – the percentage of manufactured devices that survives the

testing procedure– Volume is also a key factor in determine cost– Commodities are products that are sold by multiple vendors in large

volumes and are essentially identical.

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Integrated Circuits Costs

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Remember This Comic…?

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Cost of an Integrated Circuit

• The cost of a packaged integrated circuit is

Cost of die+Cost of testing die+Cost of packaging and final test

Cost of IC=-----------------------------------------------------------------------------

Final test yield

Cost of die=(Cost of wafer) / (Dies per wafer Die yield)

(Wafer diameter/2)2 Wafer diameter

Dies per wafer=------------------------------ - -------------------------

Die area (2 Die area) 0.5

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Cost of an Integrated Circuit

• The fraction or percentage of good dies on a wafer number (die yield):

Defects per unit area Die area -

Die yield=Wafer yield { 1 +------------------------------------------}

Where is a parameter that corresponds roughly to the number of masking level, a measure on manufacturing complexity, critical to die yield ( = 4.0 is a good estimate).

Die Cost goes roughly with die area5

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Cost versus Price

• The relationship between price and volume can increase the impact of changes in cost, especially at the low end of the market.

• Factors determine price, and to typical ranges for these factors:– Direct Costs (add 10% to 30%): costs directly related to making a

project• Labor, purchasing, scrap, warranty

– Gross Margin (add 10% to 45%): the company’s overhead that cannot be billed directly to one project

• R&D, marketing, sales, equipment maintenance, rental, financing cost, pretax profits, taxes

– Average Discount to get List Price (add 33% to 66%)• Volume discounts and/or retailer markup

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Cost/Price Illustration

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Cost/Price for Different Kinds of Systems

0%

20%

40%

60%

80%

100%

Mini W/S PC

Average Discount

Gross Margin

Direct Costs

Component Costs

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Measuring and Reporting Performance

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Performance

• When we can say that one computer is faster than another?– The user of a desktop machine may say a computer is faster when a

program runs in less time.– The computer center manger running a large server system may say

a computer is faster when it completes more jobs in an hour.

• The computer user is interested in reducing response time[ The time between the start and the completion of an event also

referred to as execution time]

• The manager of large data processing center may be interested in increasing throughput.

[The total amount of work done in given time].

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Performance (Cont.)

• If we want to relate the performance of 2 different machines X and Y.

• In particular “X is n times faster than Y” which means

• Execution time is the reciprocal of performance.

i. e performance = 1/execution time• Improved performance decreasing execution time

X

Y

imeExecutionT

imeExecutionTN

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Measuring Performance

• Several kinds of time– Wall-clock time: response time, or elapsed time

[which is the latency to complete a task, including• Disk access, memory access, input/output activities, operating

systems overhead.– CPU time

• CPU time can be further divided into the – User CPU time: CPU time spent in the program– System CPU time: CPU time spent in the operating system

performing tasks requested by the program.

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Choosing Programs to Evaluate Performance

• Real applications – clearly the right choice – Porting and eliminating system-dependent activities– User burden -- to know which of your programs you really care about

• Modified (or scripted) applications– Enhance portability or focus on particular aspects of system

performance

• Kernels – small, key pieces of real programs– Best used to isolate performance of individual features to explain the

reasons from differences in performance of real programs– Not real programs however -- no user really uses them

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Choosing Programs to Evaluate Performance (Cont.)

• Toy benchmarks – quicksort, puzzle– Beginning programming assignment

• Synthetic benchmarks– Try to match the average frequency of operations and operands of a

large set of programs– No user really runs them -- not even pieces of real programs– They typically reside in cache & don’t test memory performance– At the very least you must understand what the benchmark code is

in order to understand what it might be measuring– Companies thrive or bust on benchmark performance

• Hence they optimize for the benchmark

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Benchmark Suites

• One of the most successful attempts to create standardized benchmarks application suits has been the SPEC.

• SPEC (Standard Performance Evaluation Corporation)– http://www.spec.org

• Desktop benchmarks• Server benchmarks• Embedded benchmarks

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Reporting Performance Results

• The guiding principle of reporting performance measurements should be reproducibility

– List everything another experimenter would need to duplicate the results.

– A SPEC benchmark report requires a fairly complete descriptions of the machine and the compiler flags, as well as the publication of the both the baseline and optimized results.

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Quantitative Principles of Computer Design

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Make the Common Case Fast

• Most pervasive principle of design is to make the common case fast.• In making a design trade-off, favor the frequent case over the infrequent

case.• Improving the frequent event, rather than the rare event, will obviously

help performance • The frequent case is simpler and can be done faster than the infrequent

case.Ex:

– adding two numbers in the CPU, we can expect the overflow in the rare cases.

– Therefore improve the performance by optimizing the more common case of no overflow.

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Amdahl’s Law

• To calculate the performance that can be obtained by improving some portion of computer.

• It states that the performance improvement to be gained from using some faster mode of execution is limited by the fraction of the time the faster mode can be used.

• It also defines the speed up that can be gained by using a particular feature,

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Amdahl’s Law (Condt…)

• Defines speedup gained from a particular feature

• Depends on 2 factors– Fraction of original computation time that can take advantage of the

enhancement – Level of improvement gained by the feature

• Amdahl’s law

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Simple Example

• Important Application:– FPSQRT 20%– FP instructions account for 50%– Other 30%

• Designers say same cost to speedup:– FPSQRT by 40x – FP by 2x – Other by 8x

• Which one should you invest?• Straightforward plug in the numbers & compare BUT what’s

your guess??

Amdahl’s Law says nothing about cost

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And the Winner Is…?

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Calculating CPU Performance

• Essentially all computers are constructed using a clock running at a constant rate.– These discrete time events are called tricks, clock tricks, clock

periods, clocks, cycles, or clock cycles.

• CPU time for a program can be expressed 2 ways:

rateClock

programaforcyclesclockCPUTimeCPU

OR

timecycleClockprogramaforcyclesclockCPUTimeCPU

_

______

__*______

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Calculating CPU Performance (Cont.)

• In addition to the number of clock cycles needed to execute a program, we can also count the number of instructions executed.– The instruction path length or instruction count (IC).

• Now we can calculate the average number of cycles per instruction (CPI).

rateClock

CPIICtimecycleClockCPIICTimeCPU

IC

programaforcyclesclockCPUCPI

_

*__**_

_____

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Calculating CPU Performance (Cont.)

• As the formula demonstrates, CPU performance Focus on 3 Factors -- Cycle Time, CPI, IC– Sadly - they are interdependent and making one better often makes

another worse (but small or predictable impacts)• Cycle time depends on Hardware technology and organization• CPI depends on organization (pipeline, caching...) and ISA• IC depends on ISA and compiler technology

• Often CPI’s are easier to deal with on a per instruction basis

n

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countnInstructio

ICCPI

countnInstructio

ICCPI

CPIOverall

ICCPIcyclesclockCPU

1

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Calculating CPU Performance (Cont.)

• Some times it is useful in designing the CPU to calculate the number of total CPU cycles as

n

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ii

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iii

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countnInstructio

ICCPI

countnInstructio

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