1. (C) 2002 Daniel SorinDuke Architecture Why Computer Architecture is Exciting and Challenging Daniel Sorin Department of Electrical & Computer Engineering Duke University

2. CPS 300 – Daniel Sorin slide 2 Computer Architecture 101 Computer architects design computer systems –Processors: Intel Pentium 4, IBM PowerPC –Also: memory systems, interconnections, ??? Pentium cache memory (DRAM) bridge to I/O disk ethernet card

3. CPS 300 – Daniel Sorin slide 3 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

4. CPS 300 – Daniel Sorin slide 4 Intro to Multiprocessor Architecture I design computer systems with multiple processors Node Interconnection Network Interconnection Network Pentium cache memory (DRAM) bridge to I/O disk ethernet card Node

5. CPS 300 – Daniel Sorin slide 5 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

6. CPS 300 – Daniel Sorin slide 6 Outline What is computer architecture? Why is architecture exciting? –Fast-paced: Technology trends –Important: Prevalence of computers Why is architecture challenging?

7. CPS 300 – Daniel Sorin slide 7 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! time parameter

8. CPS 300 – Daniel Sorin slide 8 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

9. CPS 300 – Daniel Sorin slide 9 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

10. CPS 300 – Daniel Sorin slide 10 What To Do With a Billion Transistors? A.Make the processors bigger Proc B.Make more little processors Proc chip Proc

11. CPS 300 – Daniel Sorin slide 11 Outline What is computer architecture? Why is architecture exciting? –Fast-paced: Technology trends –Important: Prevalence of computers Why is architecture challenging?

12. CPS 300 – Daniel Sorin slide 12 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

13. CPS 300 – Daniel Sorin slide 13 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

14. CPS 300 – Daniel Sorin slide 14 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

15. CPS 300 – Daniel Sorin slide 15 Outline What is computer architecture? Why is architecture exciting? Why is architecture challenging? –Difficult to evaluate solutions –Inter-related with other fields

16. CPS 300 – Daniel Sorin slide 16 Ways to Evaluate New Architectures Simulating Modeling Building Tradeoff between three desired features

17. CPS 300 – Daniel Sorin slide 17 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

18. CPS 300 – Daniel Sorin slide 18 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 rate*penalty

19. CPS 300 – Daniel Sorin slide 19 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

20. CPS 300 – Daniel Sorin slide 20 Simulation Challenges Simulator Application System description Performance results Tough problems associated with each arrow!

21. CPS 300 – Daniel Sorin slide 21 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) –???

22. CPS 300 – Daniel Sorin slide 22 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

23. CPS 300 – Daniel Sorin slide 23 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”

24. CPS 300 – Daniel Sorin slide 24 Outline What is computer architecture? Why is architecture exciting? Why is architecture challenging? –Difficult to evaluate solutions –Inter-related with other fields

25. CPS 300 – Daniel Sorin slide 25 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)

26. CPS 300 – Daniel Sorin slide 26 How Architecture Relates to Other Areas Computer Architecture Operating Systems, Compilers, Networking Software Circuits, Wires, Network Hardware Application Software Besides these interactions, also global issues! –Power, system verification, performance analysis, etc.

27. CPS 300 – Daniel Sorin slide 27 How Architecture Relates to Hardware (EE) Computer Architecture Operating Systems, Compilers, Networking Software Circuits, Wires, Network Hardware Application Software Architecture should enable efficient hardware design –Avoid huge hardware structures –Avoid cross-chip wires

28. CPS 300 – Daniel Sorin slide 28 How Architecture Relates to System Software Computer Architecture Operating Systems, Compilers, Networking Software Circuits, Wires, Network Hardware Application Software Architecture should support system software –Provide good target for compiler (unlike IA-64?) –Support important OS features (such as synchronization)

29. CPS 300 – Daniel Sorin slide 29 How Architecture Relates to User Software Computer Architecture Operating Systems, Compilers, Networking Software Circuits, Wires, Network Hardware Application Software Architecture should efficiently run important apps Intel added MMX hardware to support media apps Sun & IBM design multiprocessors for commercial apps

30. CPS 300 – Daniel Sorin slide 30 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