1 BİL 542 Parallel Computing

2 Parallel Programming Chapter 1

3 Why Use Parallel Computing? Main Reasons: Save time and/or money: In theory, throwing more resources at a task will shorten its time to completion, with potential cost savings. Parallel clusters can be built from cheap, commodity components. Solve larger problems: Many problems are so large and/or complex that it is impractical or impossible to solve them on a single computer, especially given limited computer memory. For example:  Web search engines/databases processing millions of transactions per second

4 Demand for Computational Speed Continual demand for greater computational speed from a computer system than is currently possible Areas requiring great computational speed include numerical modeling and simulation of scientific and engineering problems. Computations must be completed within a “reasonable” time period.

5 Grand Challenge Problems One that cannot be solved in a reasonable amount of time with today’s computers. Obviously, an execution time of 10 years is always unreasonable. Examples  Databases, data mining  Oil exploration  Web search engines, web based business services  Medical imaging and diagnosis  Pharmaceutical design  Management of national and multi-national corporations  Financial and economic modeling  Advanced graphics and virtual reality, particularly in the entertainment industry  Networked video and multi-media technologies  Collaborative work environments  Modeling large DNA structures  Global weather forecasting  Modeling motion of astronomical bodies.

6 Weather Forecasting Atmosphere modeled by dividing it into 3- dimensional cells. Calculations of each cell repeated many times to model passage of time.

7 Global Weather Forecasting Example Suppose whole global atmosphere divided into cells of size 1 mile  1 mile  1 mile to a height of 10 miles (10 cells high) - about 5  10 8 cells. Suppose each calculation requires 200 floating point operations. In one time step, floating point operations necessary. To forecast the weather over 7 days using 1-minute intervals, a computer operating at 1Gflops (10 9 floating point operations/s) takes 10 6 seconds or over 10 days. To perform calculation in 5 minutes requires computer operating at 3.4 Tflops (3.4  floating point operations/sec).

8 Modeling Motion of Astronomical Bodies Each body attracted to each other body by gravitational forces. Movement of each body predicted by calculating total force on each body. With N bodies, N - 1 forces to calculate for each body, or approx. N 2 calculations. (N log 2 N for an efficient approx. algorithm.) After determining new positions of bodies, calculations repeated.

9 A galaxy might have, say, stars. Even if each calculation done in 1 ms (extremely optimistic figure), it takes 10 9 years for one iteration using N 2 algorithm and almost a year for one iteration using an efficient N log 2 N approximate algorithm. Modeling Motion of Astronomical Bodies

10 Astrophysical N-body simulation by Scott Linssen (undergraduate UNC-Charlotte student).

11 Parallel Computing Using more than one computer, or a computer with more than one processor, to solve a problem. Motives Usually faster computation - very simple idea - that n computers operating simultaneously can achieve the result n times faster - it will not be n times faster for various reasons. Other motives include: fault tolerance, larger amount of memory available,...

12 Parallel Computing vs Traditional Computing

13 Background Parallel computers - computers with more than one processor - and their programming - parallel programming - has been around for more than 40 years.

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15 Speedup Factor where t s is execution time on a single processor and t p is execution time on a multiprocessor. S(p) gives increase in speed by using multiprocessor. Use best sequential algorithm with single processor system. Underlying algorithm for parallel implementation might be (and is usually) different. S(p) = Execution time using one processor (best sequential algorithm) Execution time using a multiprocessor with p processors tsts tptp =

16 Speedup factor can also be cast in terms of computational steps: Can also extend time complexity to parallel computations. S(p) = Number of computational steps using one processor Number of parallel computational steps with p processors

17 Maximum Speedup Maximum speedup is usually p with p processors (linear speedup). Possible to get superlinear speedup (greater than p) but usually a specific reason such as: Extra memory in multiprocessor system

COMPE472 Parallel Computing 18 Speedup against number of processors f = 20% f = 10% f = 5% f = 0% Number of processors,p

COMPE472 Parallel Computing 19 Maximum Speedup Factors limiting speedup  Communication time  Extra computations in the parallel algorithm (reevaluation of constants locally)  Idle time of some processors

20 Maximum Speedup Amdahl’s law Serial section Parallelizable sections (a) One processor (b) Multiple processors ft s (1- f)t s t s - f)t s /p t p p processors

COMPE472 Parallel Computing 21 Speedup factor is given by: This equation is known as Amdahl’s law Amdahl’s Law

22 Amdahl’s law Even with infinite number of processors, maximum speedup is limited : Example With only 5% of computation being serial, maximum speedup is 20, irrespective of number of processors.

23 Superlinear Speedup example - Searching (a) Searching each sub-space sequentially t s t s /p StartTime  t Solution found xt s /p Sub-space search x indeterminate

24 (b) Searching each sub-space in parallel Solution found  t

COMPE472 Parallel Computing 25 Speed-up then given by S(p)S(p) x t s p t  + t  =

26 Worst case for sequential search when solution found in last sub-space search. Then parallel version offers greatest benefit.

27 Least advantage for parallel version when solution found in first sub-space search of the sequential search, i.e. Actual speed-up depends upon which subspace holds solution but could be extremely large. S(p) = t  t  = 1

28 Scalability Architecturally scalable system  Increase in number of processors leading to increase in speedup Architectural/Algorithmic scalability  Increase in data size can be accomodated by the increase in number of processors

29 Message-Passing Computations In a message passing environment, computation time consists of two parts: The ratio below can be used as a metric:

30 Types of Parallel Computers Two principal types: Shared memory multiprocessor Distributed memory multicomputer

Type of parallel systems 31 Shared-memory Distributed-memory

32 Shared Memory Multiprocessor

33 Conventional Computer Consists of a processor executing a program stored in a (main) memory: Each main memory location located by its address. Addresses start at 0 and extend to 2 b - 1 when there are b bits (binary digits) in address. Main memory Processor Instructions (to processor) Data (to or from processor)

34 Shared Memory Multiprocessor System Natural way to extend single processor model - have multiple processors connected to multiple memory modules, such that each processor can access any memory module : Processors Interconnection network Memory module One address space

35 Simplistic view of a small shared memory multiprocessor Examples: Dual Pentiums Quad Pentiums ProcessorsShared memory Bus

36 Quad Pentium Shared Memory Multiprocessor Processor L2 Cache Bus interface L1 cache Processor L2 Cache Bus interface L1 cache Processor L2 Cache Bus interface L1 cache Processor L2 Cache Bus interface L1 cache Memory controller Memory I/O interface I/O bus Processor/ memory bus Shared memory

37 Programming Shared Memory Multiprocessors Threads - programmer decomposes program into individual parallel sequences, (threads), each being able to access variables declared outside threads. Example: Pthreads (unix) Sequential programming language with preprocessor compiler directives to declare shared variables and specify parallelism. Example: OpenMP (needs OpenMP compiler)

38 Sequential programming language with added syntax to declare shared variables and specify parallelism. Example UPC (Unified Parallel C) - needs a UPC compiler. Parallel programming language with syntax to express parallelism - compiler creates executable code for each processor (not now common) Sequential programming language and ask parallelizing compiler to convert it into parallel executable code. (not now common)

39 Message-Passing Multicomputer Complete computers connected through an interconnection network: Processor Interconnection network Local Computers Messages memory

40 Interconnection Networks Limited and exhaustive interconnections 2- and 3-dimensional meshes Hypercube (not now common) Using Switches:  Crossbar  Trees  Multistage interconnection networks Peer-to-peer

41 Two-dimensional array (mesh) Links Computer/ processor

42 Three-dimensional hypercube In a d-dim hypercube, each node connects to one node in each dimension. Above a 3-d hypercube is shown. Each node is assigned a 3 bit address. Address difference between nodes is only 1 bit.

43 Crossbar switch Provides exhaustive connections using one switch for each connection. Used in shared memory systems.

44 Tree Switch element Root Links Processors

45 Multistage Interconnection Network Example: Omega network

46 Communication Methods Circuit switching  Establish the path  Maintain/Reserve links for message passing  Simple telephone system is an example  Used in early multicomputers (INTEL IPSC-2) Packet switching  Divide message into “packets”  Packet = Source/Dest addresses + Data  Packet max size is known  Mail system is an example

47 Flynn’s Classifications Flynn (1966) created a classification for computers based upon instruction streams and data streams:  Single instruction -single data (SISD) computer Single processor computer - single stream of instructions generated from program. Instructions operate upon a single stream of data items.

48 Single Instruction, Single Data (SISD): A serial (non-parallel) computer Single instruction: only one instruction stream is being acted on by the CPU during any one clock cycle Single data: only one data stream is being used as input during any one clock cycle Deterministic execution This is the oldest and until recently, the most prevalent form of computer Examples: most PCs, single CPU workstations and mainframes

49 SISD

50 Multiple Instruction Stream-Multiple Data Stream (MIMD) Computer General-purpose multiprocessor system - each processor has a separate program and one instruction stream is generated from each program for each processor. Each instruction operates upon different data. Both the shared memory and the message- passing multiprocessors so far described are in the MIMD classification.

51 MIMD Multiple Instruction, Multiple Data (MIMD): Currently, the most common type of parallel computer. Most modern computers fall into this category. Multiple Instruction: every processor may be executing a different instruction stream Multiple Data: every processor may be working with a different data stream Examples: most current supercomputers, networked parallel computer "grids" and multi-processor SMP computers - including some types of PCs.

52 MIMD

53 Single Instruction Stream-Multiple Data Stream (SIMD) Computer A specially designed computer - a single instruction stream from a single program, but multiple data streams exist. Instructions from program broadcast to more than one processor. Each processor executes same instruction in synchronism, but using different data. Developed because a number of important applications that mostly operate upon arrays of data.

COMPE472 Parallel Computing 54 SIMD Single Instruction, Multiple Data (SIMD): A type of parallel computer Single instruction: All processing units execute the same instruction at any given clock cycle Multiple data: Each processing unit can operate on a different data element This type of machine typically has an instruction dispatcher, a very high- bandwidth internal network, and a very large array of very small-capacity instruction units. Best suited for specialized problems characterized by a high degree of regularity,such as image processing.

COMPE472 Parallel Computing 55 SIMD

56 Networked Computers as a Computing Platform A network of computers became a very attractive alternative to expensive supercomputers and parallel computer systems for high-performance computing in early 1990’s. Several early projects. Notable:  Berkeley NOW (network of workstations) project.  NASA Beowulf project.

COMPE472 Parallel Computing 57 Key advantages: Very high performance workstations and PCs readily available at low cost. The latest processors can easily be incorporated into the system as they become available. Existing software can be used or modified.

58 Software Tools for Clusters Based upon Message Passing Parallel Programming: Parallel Virtual Machine (PVM) - developed in late 1980’s. Became very popular. Message-Passing Interface (MPI) - standard defined in 1990s. Both provide a set of user-level libraries for message passing. Use with regular programming languages (C, C++,...).

59 Beowulf Clusters* A group of interconnected “commodity” computers achieving high performance with low cost. Typically using commodity interconnects - high speed Ethernet, and Linux OS. * Beowulf comes from name given by NASA Goddard Space Flight Center cluster project.

60 Cluster Interconnects Originally fast Ethernet on low cost clusters Gigabit Ethernet - easy upgrade path More Specialized/Higher Performance Myrinet Gbits/sec - disadvantage: single vendor cLan SCI (Scalable Coherent Interface) QNet Infiniband - may be important as infininband interfaces may be integrated on next generation PCs