CS 258 Parallel Computer Architecture Lecture 1 Introduction to Parallel Architecture

1. CS 258 Parallel Computer ArchitectureLecture 1Introduction to Parallel Architecture January 23, 2002 Prof John D. Kubiatowicz

2. 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 SOFTWARE

3. Instruction Set Architecture (ISA) • Great picture for uniprocessors…. • Rapidly crumbling, however! • Can this be true for multiprocessors??? • Much harder to say. software instruction set hardware

4. What is Parallel Architecture? • A parallel computer is a collection of processing elements that cooperate to solve large problems fast • Some broad issues: • Models of computation: PRAM? BSP? Sequential Consistency? • Resource Allocation: • how large a collection? • how powerful are the elements? • how much memory? • Data access, Communication and Synchronization • how do the elements cooperate and communicate? • how are data transmitted between processors? • what are the abstractions and primitives for cooperation? • Performance and Scalability • how does it all translate into performance? • how does it scale?

5. Computer Architecture Topics (252+) Input/Output and Storage Disks, WORM, Tape RAID Emerging Technologies Interleaving Bus protocols DRAM Coherence, Bandwidth, Latency Memory Hierarchy L2 Cache Network Communication Other Processors L1 Cache Addressing, Protection, Exception Handling VLSI Instruction Set Architecture Pipelining and Instruction Level Parallelism Pipelining, Hazard Resolution, Superscalar, Reordering, Prediction, Speculation, Vector, Dynamic Compilation

6. Computer Architecture Topics (258) Shared Memory, Message Passing, Data Parallelism P M P M P M P M ° ° ° Network Interfaces S Interconnection Network Processor-Memory-Switch Topologies, Routing, Bandwidth, Latency, Reliability Multiprocessors Networks and Interconnections Everything in previous slide but more so!

7. What will you get out of CS258? • In-depth understanding of the design and engineering of modern parallel computers • technology forces • Programming models • fundamental architectural issues • naming, replication, communication, synchronization • basic design techniques • cache coherence, protocols, networks, pipelining, … • methods of evaluation • from moderate to very large scale • across the hardware/software boundary • Study of REAL parallel processors • Research papers, white papers • Natural consequences?? • Massive Parallelism  Reconfigurable computing? • Message Passing Machines  NOW  Peer-to-peer systems?

8. SuperServers Departmenatal Servers Workstations Workstations Personal Computers Will it be worthwhile? • Absolutely! • even through few of you will become PP designers • The fundamental issues and solutions translate across a wide spectrum of systems. • Crisp solutions in the context of parallel machines. • Pioneered at the thin-end of the platform pyramid on the most-demanding applications • migrate downward with time • Understand implications for software • Network attachedstorage, MEMs, etc?

9. Why Study Parallel Architecture? • Role of a computer architect: • To design and engineer the various levels of a computer system to maximize performance and programmability within limits of technology and cost. • Parallelism: • Provides alternative to faster clock for performance • Applies at all levels of system design • Is a fascinating perspective from which to view architecture • Is increasingly central in information processing • How is instruction-level parallelism related to course-grained parallelism??

10. Is Parallel Computing Inevitable? This was certainly not clear just a few years ago Today, however: • Application demands: Our insatiable need for computing cycles • Technology Trends: Easier to build • Architecture Trends: Better abstractions • Economics: Cost of pushing uniprocessor • Current trends: • Today’s microprocessors have multiprocessor support • Servers and workstations becoming MP: Sun, SGI, DEC, COMPAQ!... • Tomorrow’s microprocessors are multiprocessors

11. Can programmers handle parallelism? • Humans not as good at parallel programming as they would like to think! • Need good model to think of machine • Architects pushed on instruction-level parallelism really hard, because it is “transparent” • Can compiler extract parallelism? • Sometimes • How do programmers manage parallelism?? • Language to express parallelism? • How to schedule varying number of processors? • Is communication Explicit (message-passing) or Implicit (shared memory)? • Are there any ordering constraints on communication?

12. Granularity: • Is communication fine or coarse grained? • Small messages vs big messages • Is parallelism fine or coarse grained • Small tasks (frequent synchronization) vs big tasks • If hardware handles fine-grained parallelism, then easier to get incremental scalability • Fine-grained communication and parallelism harder than coarse-grained: • Harder to build with low overhead • Custom communication architectures often needed • Ultimate course grained communication: • GIMPS (Great Internet Mercenne Prime Search) • Communication once a month

13. CS258: Staff Instructor:Prof John D. Kubiatowicz Office: 673 Soda Hall, 643-6817 kubitron@cs Office Hours: Thursday 1:30 - 3:00 or by appt. Class: Wed, Fri, 1:00 - 2:30pm 310 Soda Hall Administrative: Veronique Richard, Office: 676 Soda Hall, 642-4334 nicou@cs Web page: http://www.cs/~kubitron/courses/cs258-S02/ Lectures available online <11:30AM day of lecture Email: cs258@kubi.cs.berkeley.edu Clip signup link on web page (as soon as it is up)

14. Why Me? The Alewife Multiprocessor • Cache-coherence Shared Memory • Partially in Software! • User-level Message-Passing • Rapid Context-Switching • Asynchronous network • One node/board

15. TextBook: Two leaders in field Text: Parallel Computer Architecture: A Hardware/Software Approach, By: David Culler & Jaswinder Singh Covers a range of topics We will not necessarily cover them in order.

16. Lecture style • 1-Minute Review • 20-Minute Lecture/Discussion • 5- Minute Administrative Matters • 25-Minute Lecture/Discussion • 5-Minute Break (water, stretch) • 25-Minute Lecture/Discussion • Instructor will come to class early & stay after to answer questions Attention 20 min. Break “In Conclusion, ...” Time

17. Research Paper Reading • As graduate students, you are now researchers. • Most information of importance to you will be in research papers. • Ability to rapidly scan and understand research papers is key to your success. • So: you will read lots of papers in this course! • Quick 1 paragraph summaries will be due in class • Students will take turns discussing papers • Papers will be scanned and on web page.

18. How will grading work? • No TA This term! • Tentative breakdown: • 20% homeworks / paper presentations • 30% exam • 40% project (teams of 2) • 10% participation

19. New Applications More Performance Application Trends • Application demand for performance fuels advances in hardware, which enables new appl’ns, which... • Cycle drives exponential increase in microprocessor performance • Drives parallel architecture harder • most demanding applications • Programmers willing to work really hard to improve high-end applications • Need incremental scalability: • Need range of system performance with progressively increasing cost

20. Time (1 processor) Time (p processors) Speedup • Speedup (p processors) = • Common mistake: • Compare parallel program on 1 processor to parallel program on p processors • Wrong!: • Should compare uniprocessor program on 1 processor to parallel program on p processors • Why? Keeps you honest • It is easy to parallelize overhead.

21. Amdahl's Law Speedup due to enhancement E: ExTime w/o E Performance w/ E Speedup(E) = ------------- = ------------------- ExTime w/ E Performance w/o E Suppose that enhancement E accelerates a fraction F of the task by a factor S, and the remainder of the task is unaffected

22. Amdahl’s Law for parallel programs? Best you could ever hope to do: Worse: Overhead may kill your performance!

23. Metrics of Performance Application Answers per month Operations per second Programming Language Compiler (millions) of Instructions per second: MIPS (millions) of (FP) operations per second: MFLOP/s ISA Datapath Megabytes per second Control Function Units Cycles per second (clock rate) Transistors Wires Pins

24. Commercial Computing • Relies on parallelism for high end • Computational power determines scale of business that can be handled • Databases, online-transaction processing, decision support, data mining, data warehousing ... • TPC benchmarks (TPC-C order entry, TPC-D decision support) • Explicit scaling criteria provided • Size of enterprise scales with size of system • Problem size not fixed as p increases. • Throughput is performance measure (transactions per minute or tpm)

25. TPC-C Results for March 1996 • Parallelism is pervasive • Small to moderate scale parallelism very important • Difficult to obtain snapshot to compare across vendor platforms

26. Scientific Computing Demand

27. Engineering Computing Demand • Large parallel machines a mainstay in many industries • Petroleum (reservoir analysis) • Automotive (crash simulation, drag analysis, combustion efficiency), • Aeronautics (airflow analysis, engine efficiency, structural mechanics, electromagnetism), • Computer-aided design • Pharmaceuticals (molecular modeling) • Visualization • in all of the above • entertainment (films like Toy Story) • architecture (walk-throughs and rendering) • Financial modeling (yield and derivative analysis) • etc.

28. Applications: Speech and Image Processing • Also CAD, Databases, . . . • 100 processors gets you 10 years, 1000 gets you 20 !

29. Is better parallel arch enough? • AMBER molecular dynamics simulation program • Starting point was vector code for Cray-1 • 145 MFLOP on Cray90, 406 for final version on 128-processor Paragon, 891 on 128-processor Cray T3D

30. Summary of Application Trends • Transition to parallel computing has occurred for scientific and engineering computing • In rapid progress in commercial computing • Database and transactions as well as financial • Usually smaller-scale, but large-scale systems also used • Desktop also uses multithreaded programs, which are a lot like parallel programs • Demand for improving throughput on sequential workloads • Greatest use of small-scale multiprocessors • Solid application demand exists and will increase

31. Technology Trends • Today the natural building-block is also fastest!

32. Can’t we just wait for it to get faster? • Microprocessor performance increases 50% - 100% per year • Transistor count doubles every 3 years • DRAM size quadruples every 3 years • Huge investment per generation is carried by huge commodity market 180 160 140 DEC 120 alpha Integer FP 100 IBM HP 9000 80 RS6000 750 60 540 MIPS MIPS 40 M2000 Sun 4 M/120 20 260 0 1987 1988 1989 1990 1991 1992

33. Proc $ Interconnect Technology: A Closer Look • Basic advance is decreasing feature size ( ) • Circuits become either faster or lower in power • Die size is growing too • Clock rate improves roughly proportional to improvement in  • Number of transistors improves like (or faster) • Performance > 100x per decade • clock rate < 10x, rest is transistor count • How to use more transistors? • Parallelism in processing • multiple operations per cycle reduces CPI • Locality in data access • avoids latency and reduces CPI • also improves processor utilization • Both need resources, so tradeoff • Fundamental issue is resource distribution, as in uniprocessors

34. Growth Rates • 30% per year 40% per year

35. Architectural Trends • Architecture translates technology’s gifts into performance and capability • Resolves the tradeoff between parallelism and locality • Current microprocessor: 1/3 compute, 1/3 cache, 1/3 off-chip connect • Tradeoffs may change with scale and technology advances • Understanding microprocessor architectural trends => Helps build intuition about design issues or parallel machines => Shows fundamental role of parallelism even in “sequential” computers

36. Phases in “VLSI” Generation

37. Architectural Trends • Greatest trend in VLSI generation is increase in parallelism • Up to 1985: bit level parallelism: 4-bit -> 8 bit -> 16-bit • slows after 32 bit • adoption of 64-bit now under way, 128-bit far (not performance issue) • great inflection point when 32-bit micro and cache fit on a chip • Mid 80s to mid 90s: instruction level parallelism • pipelining and simple instruction sets, + compiler advances (RISC) • on-chip caches and functional units => superscalar execution • greater sophistication: out of order execution, speculation, prediction • to deal with control transfer and latency problems • Next step: thread level parallelism? Bit-level parallelism?

38. How far will ILP go? • Infinite resources and fetch bandwidth, perfect branch prediction and renaming • real caches and non-zero miss latencies

39. Proc Proc Proc Proc Threads Level Parallelism “on board” • Micro on a chip makes it natural to connect many to shared memory • dominates server and enterprise market, moving down to desktop • Alternative: many PCs sharing one complicated pipe • Faster processors began to saturate bus, then bus technology advanced • today, range of sizes for bus-based systems, desktop to large servers MEM No. of processors in fully configured commercial shared-memory systems

40. What about Multiprocessor Trends?

41. Bus Bandwidth

42. What about Storage Trends? • Divergence between memory capacity and speed even more pronounced • Capacity increased by 1000x from 1980-95, speed only 2x • Gigabit DRAM by c. 2000, but gap with processor speed much greater • Larger memories are slower, while processors get faster • Need to transfer more data in parallel • Need deeper cache hierarchies • How to organize caches? • Parallelism increases effective size of each level of hierarchy, without increasing access time • Parallelism and locality within memory systems too • New designs fetch many bits within memory chip; follow with fast pipelined transfer across narrower interface • Buffer caches most recently accessed data • Processor in memory? • Disks too: Parallel disks plus caching

43. Economics • Commodity microprocessors not only fast but CHEAP • Development costs tens of millions of dollars • BUT, many more are sold compared to supercomputers • Crucial to take advantage of the investment, and use the commodity building block • Multiprocessors being pushed by software vendors (e.g. database) as well as hardware vendors • Standardization makes small, bus-based SMPs commodity • Desktop: few smaller processors versus one larger one? • Multiprocessor on a chip?

44. Can anyone afford high-end MPPs??? • ASCI (Accellerated Strategic Computing Initiative)ASCI White: Built by IBM • 12.3 TeraOps, 8192 processors (RS/6000) • 6TB of RAM, 160TB Disk • 2 basketball courts in size • Program it??? Message passing

45. Consider Scientific Supercomputing • Proving ground and driver for innovative architecture and techniques • Market smaller relative to commercial as MPs become mainstream • Dominated by vector machines starting in 70s • Microprocessors have made huge gains in floating-point performance • high clock rates • pipelined floating point units (e.g., multiply-add every cycle) • instruction-level parallelism • effective use of caches (e.g., automatic blocking) • Plus economics • Large-scale multiprocessors replace vector supercomputers

46. Raw Uniprocessor Performance: LINPACK

47. Raw Parallel Performance: LINPACK • Even vector Crays became parallel • X-MP (2-4) Y-MP (8), C-90 (16), T94 (32) • Since 1993, Cray produces MPPs too (T3D, T3E)

48. 350 319 313 u n 284 300 u 239 250 MPP u u PVP n 200 198 n SMP u s 187 Number of systems 150 110 106 s n n 100 106 s s 73 50 63 0 s 11/93 11/94 11/95 11/96 500 Fastest Computers

49. Summary: Why Parallel Architecture? • Increasingly attractive • Economics, technology, architecture, application demand • Increasingly central and mainstream • Parallelism exploited at many levels • Instruction-level parallelism • Multiprocessor servers • Large-scale multiprocessors (“MPPs”) • Focus of this class: multiprocessor level of parallelism • Same story from memory system perspective • Increase bandwidth, reduce average latency with many local memories • Spectrum of parallel architectures make sense • Different cost, performance and scalability

50. Where is Parallel Arch Going? Old view: Divergent architectures, no predictable pattern of growth. Application Software System Software Systolic Arrays SIMD Architecture Message Passing Dataflow Shared Memory • Uncertainty of direction paralyzed parallel software development!