Why Computer Architecture is Exciting and Challenging

1
Why Computer Architecture is Exciting and
Challenging

• Daniel Sorin
• Department of Electrical Computer Engineering
• Duke University

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Computer Architecture 101

• Computer architects design computer systems
• Processors Intel Pentium 4, IBM PowerPC
• Also memory systems, interconnections, ???

Pentium
disk
ethernet card
cache memory
memory (DRAM)
bridge to I/O
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Intro to Microarchitecture (CPS 220 in 1 slide)

• Microarchitects design processors
• Goals for processors
• Faster!!!!
• Higher bandwidth communication with memory system
• Backward-compatible with previous models
• How do we make processors faster?
• Faster clocks (gt2 GHz)
• Do more work (execute instructions) at same time

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Intro to Multiprocessor Architecture

• I design computer systems with multiple
  processors

Node
Node
Node
Node
Interconnection Network
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What Multiprocessor Architects Do All Day

• How do we make processors work together?
• Exploit parallelism in applications
• Example web server
• Each processor handles different requests
• Processors communicate occasionally to synch up
• Some challenges
• Interconnection network design
• Protocols for communicating and sharing data
• Scalability
• Reliability

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Outline

• What is computer architecture?
• Why is architecture exciting?
• Fast-paced Technology trends
• Important Prevalence of computers
• Why is architecture challenging?

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Technology Trends Why do I Care?

• We design an architecture for a given technology
• Technology parameters
• Number of transistors on a chip
• Transistor speed
• Amount of memory
• Memory speed
• Bandwidth between components
• Power usage
• Applications to be run on system
• All of these change dramatically over time!

parameter
time
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Technology Trends A Few Examples

• Number of transistors
• Doubles every 18 months (Moores Law)
• Memory size
• 1992 I bought an extra 512Kbytes for my desktop
• 2002 My desktop came with 512 Mbytes
• Power usage
• Pentium 4 can draw 50 amps of current and burn 50
  W
• Important applications
• Word processing, spreadsheets ? multimedia, web
  surfing

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Technology Trends Good or Bad?

• Pessimist trends make designs obsolete
• But now I have to re-think everything Id already
  solved!
• Optimist/Architect trends offer opportunities
• What can do we do with a billion transistors?
• Good design ideas ?? Bad design ideas
• E.g., was good idea to scale up processor sizes
• But, it now uses too much power and is too complex

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What To Do With a Billion Transistors?

1. Make the processors bigger

Proc
chip

1. Make more little processors

Proc
Proc
Proc
chip
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Outline

• What is computer architecture?
• Why is architecture exciting?
• Fast-paced Technology trends
• Important Prevalence of computers
• Why is architecture challenging?

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Prevalence of Computers

• Theyre everywhere!
• Desktops, laptops, cars, airplanes, ???
• Optimist/Architect
• More opportunities to innovate!
• Pessimist were in trouble if
• They fail
• They use up too much power
• Lets look at these two issues now

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Computer Availability

• Availability probability that computer works
  correctly
• More parts ? more parts that can fail
• How do we protect computer from failing?
• Redundancy (e.g., double bagging your groceries)
• Recovery mechanisms (e.g., Undo button in
  PowerPoint)
• My research explores how to detect errors and
  recover from them

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Power Usage

• Used to just be a problem for supercomputers
• E.g., Cray computers came with coolant system
• Now its a problem for all computers (tech
  trend!)
• More computers using more power

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Outline

• What is computer architecture?
• Why is architecture exciting?
• Why is architecture challenging?
• Difficult to evaluate solutions
• Inter-related with other fields

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Ways to Evaluate New Architectures
Tradeoff between three desired features
Ease of development
Flexibility
Modeling
Simulating
Runtime speed
Precision
Building
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Building

• Construct a hardware prototype
• Advantages
• Way cool to show off hardware to friends
• Runs quickly
• Disadvantages
• Takes long time (grad student time!) to build
• Expensive
• Not flexible
• ? Generally too labor intensive for research
  studies

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Modeling

• Mathematically model the system
• Use probabilities and/or queuing models
• Advantages
• Very flexible
• Very quick to develop
• Runs quickly
• Disadvantages
• Cannot capture effects of system details
• Architects are skeptical of models
• ? Generally OK for back of the envelope estimates

mem time hit time miss ratepenalty
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Simulating

• Write a program that mimics system behavior
• Advantages
• Very flexible
• Relatively quick to develop
• Disadvantages
• Runs slowly (e.g., 30,000 times slower than
  hardware)
• ? Method of choice for most architectural
  research

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Simulation Challenges
Application
Simulator
Performance results
System description

• Tough problems associated with each arrow!

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Applications to Simulate

• We care how system does on important applications
• But who defines important? (I do!)
• Types of applications
• Scientific (genomics, weather simulation, protein
  folding)
• Commercial (database, web serving, application
  serving)
• Desktop (office productivity software,
  multimedia)
• Portable (voice recognition)
• ???

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Describing Simulated System

• How detailed must our simulator be?
• Model every transistor in the processor?
• Would take too long
• Abstract away details of processor organization?
• Could miss important effects of processor
  features
• Could achieve wrong conclusion
• Need balance
• Model in detail only where necessary
• E.g., I model memory system in detail, but
  abstract disks

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Performance Results

• How do tell if our design is good?
• Need metrics for evaluation
• Performance metrics
• Clock speed (gigahertz)?
• Instructions per cycle?
• Database transactions per second
• Compare results to other published research?
• Cant do it!
• Major problem in architecture research
• JIR Journal of Irreproducible Results

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Outline

• What is computer architecture?
• Why is architecture exciting?
• Why is architecture challenging?
• Difficult to evaluate solutions
• Inter-related with other fields

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Why Architects Need Friends

• Architecture is considered both computer
  engineering and computer science
• Architects interact with other areas
• Circuit design (Electrical Engineering)
• Transmission lines (EE)
• Power (EE, Mechanical Engineering)
• Compilers (Comp Sci)
• Operating systems (CS)
• Networking (EE, CS)
• Databases (CS)
• Queuing theory (CS, EE, Industrial Engineering)

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How Architecture Relates to Other Areas

• Besides these interactions, also global issues!
• Power, system verification, performance analysis,
  etc.

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How Architecture Relates to Hardware (EE)
Application Software
Operating Systems, Compilers, Networking Software
Computer Architecture
Circuits, Wires, Network Hardware

• Architecture should enable efficient hardware
  design
• Avoid huge hardware structures
• Avoid cross-chip wires

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How Architecture Relates to System Software
Application Software
Operating Systems, Compilers, Networking Software
Computer Architecture
Circuits, Wires, Network Hardware

• Architecture should support system software
• Provide good target for compiler (unlike IA-64?)
• Support important OS features (such as
  synchronization)

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How Architecture Relates to User Software
Application Software
Operating Systems, Compilers, Networking Software
Computer Architecture
Circuits, Wires, Network Hardware

• Architecture should efficiently run important
  apps
• Intel added MMX hardware to support media apps
• Sun IBM design multiprocessors for commercial
  apps

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Conclusions

• Computer architecture is way cool, but not easy
• If it was easy, everyone would do it. - my Mom
• Students interested in architecture should
• Take ECE 259 / CPS 221 in the spring
• Attend architecture/systems seminars in CS
  department
• Take courses in related fields