Transcript Why Computer Architecture is Interesting and Challenging

Why Computer Architecture is
Exciting and Challenging
Daniel Sorin
Department of Electrical & Computer Engineering
Duke University
(C) 2002 Daniel Sorin
Duke Architecture
Computer Architecture 101
• Computer architects design computer systems
– Processors: Intel Pentium 4, IBM PowerPC
– Also: memory systems, interconnections, ???
Pentium
disk
cache
memory
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memory
(DRAM)
ethernet
card
bridge to I/O
CPS 300 – Daniel Sorin
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 (>2 GHz)
– Do more work (execute instructions) at same time
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CPS 300 – Daniel Sorin
Intro to Multiprocessor Architecture
• I design computer systems with multiple processors
Node
Node
Node
Node
Pentium
disk
cache
memory
(DRAM)
ethernet
card
bridge to I/O
Interconnection
Network
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CPS 300 – Daniel Sorin
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:
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Interconnection network design
Protocols for communicating and sharing data
Scalability
Reliability
CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
Technology Trends – Why do I Care?
• We design an architecture for a given technology
• Technology parameters:
parameter
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Number of transistors on a chip
Transistor speed
Amount of memory
Memory speed
Bandwidth between components
Power usage
Applications to be run on system
time
• All of these change dramatically over time!
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CPS 300 – Daniel Sorin
Technology Trends – A Few Examples
• Number of transistors
– Doubles every 18 months (Moore’s 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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CPS 300 – Daniel Sorin
Technology Trends – Good or Bad?
• Pessimist: trends make designs obsolete
– But now I have to re-think everything I’d 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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CPS 300 – Daniel Sorin
What To Do With a Billion Transistors?
A. Make the processors bigger
Proc
chip
B. Make more little processors
Proc Proc Proc
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chip
CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
Prevalence of Computers
• They’re everywhere!
– Desktops, laptops, cars, airplanes, ???
• Optimist/Architect:
– More opportunities to innovate!
• Pessimist: we’re in trouble if:
– They fail
– They use up too much power
Let’s look at these two issues now
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CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
Power Usage
• Used to just be a problem for supercomputers
– E.g., Cray computers came with coolant system
• Now it’s a problem for all computers (tech trend!)
– More computers using more power
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CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
Ways to Evaluate New Architectures
Tradeoff between three desired features
Building
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CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
Modeling
• Mathematically model the system
– Use probabilities and/or queuing models
• Advantages
mem time = hit time + miss rate*penalty
+ 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
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CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
Simulation Challenges
Application
Simulator
Performance results
System
description
Tough problems associated with each arrow!
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CPS 300 – Daniel Sorin
Applications to Simulate
• We care how system does on important applications
• But who defines “important”? (I do!)
• Types of applications
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Scientific (genomics, weather simulation, protein folding)
Commercial (database, web serving, application serving)
Desktop (office productivity software, multimedia)
Portable (voice recognition)
???
CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
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?
– Can’t do it!
– Major problem in architecture research
– JIR = “Journal of Irreproducible Results”
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CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
Why Architects Need Friends
• Architecture is considered both computer engineering
and computer science
• Architects interact with other areas
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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)
CPS 300 – Daniel Sorin
How Architecture Relates to Other Areas
Application Software
Operating Systems, Compilers, Networking Software
Computer Architecture
Circuits, Wires, Network Hardware
• Besides these interactions, also global issues!
– Power, system verification, performance analysis, etc.
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CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
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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CPS 300 – Daniel Sorin
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
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CPS 300 – Daniel Sorin