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Parallel Computing

Parallel Computing

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Parallel Computing

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  1. Parallel Computing Parallel PVM (Parallel Virtual Machine) MPI (Message Passing Interface) CUDA (Compute Unified Device Architecture) Parallel C# - Lightweight Parallelism

  2. There are several parallel programming models in common use: Shared Memory Threads Message Passing Data Parallel Hybrid

  3. Parallel Programming Models are Abstract Although it might not seem apparent, these models are NOT specific to a particular type of machine or memory architecture. In fact, any of these models can (theoretically) be implemented on any underlying hardware. Two examples: 1. Shared memory model on a distributed memory machine: Kendall Square Research (KSR) ALLCACHE approach. 2. Message passing model on a shared memory machine: MPI on SGI Origin. Machine memory was physically distributed, but appeared to the user as a single shared memory (global address space). Generically, this approach is referred to as "virtual shared memory". Note: although KSR is no longer in business, there is no reason to suggest that a similar implementation will not be made available by another vendor in the future. The SGI Origin employed the CC-NUMA type of shared memory architecture, where every task has direct access to global memory. However, the ability to send and receive messages with MPI, as is commonly done over a network of distributed memory machines, is not only implemented but is very commonly used.

  4. Shared Memory Model In the shared-memory programming model, tasks share a common address space, which they read and write asynchronously. Various mechanisms such as locks/semaphores may be used to control access to the shared memory. An advantage of this model from the programmer's point of view is that the notion of data ownership is lacking, so there is no need to specify explicitly the communication of data between tasks. Program development can often be simplified. An important disadvantage in terms of performance is that it becomes more difficult to understand and manage data locality. Keeping data local to the processor that works on it conserves memory accesses, cache refreshes and bus traffic that occurs when multiple processors use the same data. Unfortunately, controlling data locality is hard to understand and beyond the control of the average user.

  5. Threads In the threads model of parallel programming, a single process can have multiple, concurrent execution paths. Perhaps the most simple analogy that can be used to describe threads is the concept of a single program that includes a number of subroutines: Threads Model Threads are commonly associated with shared memory architectures and operating systems.

  6. Two Types of Threads Unrelated standardization efforts have resulted in two very different implementations of threads: POSIX Threads and OpenMP. POSIX Threads * Library based; requires parallel coding * Specified by the IEEE POSIX 1003.1c standard (1995). * C Language only * Commonly referred to as Pthreads. * Most HW vendors offer Pthreads in addition to proprietary implementations. * Very explicit parallelism; requires significant programmer attention to detail. OpenMP * Compiler directive based; can use serial code * Endorsed by HW & SW vendors. * Portable/multi-platform, including Unix and Windows NT platforms * Available in C/C++ and Fortran implementations * Can be very easy and simple to use - provides for "incremental parallelism"

  7. Message Passing Model The message passing model demonstrates the following characteristics: A set of tasks that use their own local memory during computation. Multiple tasks can reside on the same physical machine and/or across an arbitrary number of machines. Tasks exchange data through communications by sending and receiving messages. Data transfer usually requires cooperative operations to be performed by each process. For example, a send operation must have a matching receive operation.

  8. Data Parallel Model The data parallel model demonstrates the following characteristics: Most parallel work focuses on performing operations on a data set. The data set is typically organized into a common structure, such as an array or cube. A set of tasks work collectively on the same data structure, however, each task works on a different partition of the same data structure. Tasks perform the same operation on their partition of work, for example, "add 4 to every array element". On shared memory architectures, all tasks may have access to the data structure through global memory. On distributed memory architectures the data structure is split up and resides as chunks in the local memory of each task.

  9. Hybrid Parallel Model In this model, any two or more parallel programming models are combined. Currently, a common example of a hybrid model is the combination of the message passing model (MPI) with either the threads model (POSIX threads) or the shared memory model (OpenMP). This hybrid model lends itself well to the increasingly common hardware environment of networked SMP machines. Another common example of a hybrid model is combining data parallel with message passing. Distributed memory architectures use message passing to transmit data between tasks, transparently to the programmer.

  10. Single Program Multiple Data (SPMD) SPMD is actually a "high level" programming model that can be built upon any combination of the previously mentioned parallel programming models. SPMD Model A single program is executed by all tasks simultaneously. At any moment in time, tasks can be executing the same or different instructions within the same program. SPMD programs usually have logic programmed into them to allow different tasks to branch or conditionally execute only those parts of the program they are designed to execute. That is, tasks do not necessarily have to execute the entire program - perhaps only a portion of it. All tasks may use different data

  11. Multiple Program Multiple Data (MPMD) Like SPMD, MPMD is actually a "high level" programming model that can be built upon any combination of the previously mentioned parallel programming models. MPMD Model MPMD applications typically have multiple executable object files (programs). While the application is being run in parallel, each task can be executing the same or different program as other tasks. All tasks may use different data

  12. Automatic vs. Manual Parallelization Designing and developing parallel programs has characteristically been a very manual process. The programmer is typically responsible for both identifying and actually implementing parallelism. Very often, manually developing parallel codes is a time consuming, complex, error-prone and iterative process. For a number of years now, various tools have been available to assist the programmer with converting serial programs into parallel programs. The most common type of tool used to automatically parallelize a serial program is a parallelizing compiler or pre-processor.

  13. Parallelizing Compilers • A parallelizing compiler generally works in two different ways: • Fully Automatic • The compiler analyzes the source code and identifies opportunities for parallelism. • The analysis includes identifying inhibitors to parallelism and possibly a cost weighting on whether or not the parallelism would actually improve performance. • Loops (do, for) loops are the most frequent target for automatic parallelization. • Programmer Directed • Using "compiler directives" or possibly compiler flags, the programmer explicitly tells the compiler how to parallelize the code. • May be able to be used in conjunction with some degree of automatic parallelization also.

  14. Issues with Automatic Parallelization If you are beginning with an existing serial code and have time or budget constraints, then automatic parallelization may be the answer. However, there are several important caveats that apply to automatic parallelization: Wrong results may be produced Performance may actually degrade Much less flexible than manual parallelization Limited to a subset (mostly loops) of code May actually not parallelize code if the analysis suggests there are inhibitors or the code is too complex

  15. Understanding the Problem Undoubtedly, the first step in developing parallel software is to first understand the problem that you wish to solve in parallel. If you are starting with a serial program, this necessitates understanding the existing code also. Before spending time in an attempt to develop a parallel solution for a problem, determine whether or not the problem is one that can actually be parallelized.

  16. Example of Parallelizable Problem: This problem is able to be solved in parallel. Each of the molecular conformations is independently determinable. The calculation of the minimum energy conformation is also a parallelizable problem. Calculate the potential energy for each of several thousand independent conformations of a molecule. When done, find the minimum energy conformation. Example of a Non-parallelizable Problem: This is a non-parallelizable problem because the calculation of the Fibonacci sequence as shown would entail dependent calculations rather than independent ones. The calculation of the F(n) value uses those of both F(n-1) and F(n-2). These three terms cannot be calculated independently and therefore, not in parallel. Calculation of the Fibonacci series (1, 1, 2, 3,5, 8, 13, 21,. . .) by use of the formula: F(n) = F(n-1) + F(n-2)

  17. Methods to Improve Parallel Algorithm Performance 1. Identify the program's hotspots: 2. Know where most of the real work is being done. The majority of scientific and technical programs usually accomplish most of their work in a few places. Profilers and performance analysis tools can help here Focus on parallelizing hotspots, ignore sections comprising little CPU usage. 3. Identify bottlenecks in the program Are there areas that are disproportionately slow May be possible to restructure the program or use a different algorithm 4. Identify inhibitors to parallelism. One common class of inhibitor is data dependence, as demonstrated by the Fibonacci sequence above. 5. Investigate other algorithms if possible. This may be the single most important consideration when designing a parallel application.

  18. Partitioning • One of the first steps in designing a parallel program is to break the problem into discrete chunks of work that can be distributed to multiple tasks. This is known as decomposition or partitioning. • There are two basic ways to partition computational work among parallel tasks: • domain decomposition • and • functional decomposition.

  19. Domain Decomposition In this type of partitioning, the data associated with a problem is decomposed. Each parallel task then works on a portion of of the data. There are different ways to partition data: BLOCK CYCLIC

  20. Functional Decomposition In this approach, the focus is on the computation that is to be performed rather than on the data manipulated by the computation. The problem is decomposed according to the work that must be done. Each task then performs a portion of the overall work.

  21. Ecosystem Modeling Example of Functional Decomposition Each program calculates the population of a given group, where each group's growth depends on that of its neighbors. As time progresses, each process calculates its current state, then exchanges information with the neighbor populations. All tasks then progress to calculate the state at the next time step.

  22. Signal Processing An audio signal data set is passed through four distinct computational filters. Each filter is a separate process. The first segment of data must pass through the first filter before progressing to the second. When it does, the second segment of data passes through the first filter. By the time the fourth segment of data is in the first filter, all four tasks are busy.

  23. IMADS a tool for functional decomposition Intelligent Multiple Agent Development System Input Output Function Module Rule-Based Module Neural Network Module

  24. Parallel Virtual Machine

  25. Parallel Virtual Machine PVM (Parallel Virtual Machine) is a portable message-passing programming system, designed to link separate host machines to form a ``virtual machine'' which is a single, manageable computing resource. The virtual machine can be composed of hosts of varying types, in physically remote locations. PVM applications can be composed of any number of separate processes, or components, written in a mixture of C, C++ and Fortran. The system is portable to a wide variety of architectures, including workstations, multiprocessors, supercomputers and PCs. PVM is a by-product of ongoing research at several institutions, and is made available to the public free of charge. NetLib Bob Manchek

  26. PVM Modes of Operation Parallel computing using a system such as PVM may be approached from three fundamental viewpoints, based on the organization of the computing tasks. Crowd Computing Tree Computation Hybrid

  27. Crowd Computing Crowd Computing is a collection of closely related processes, typically executing the same code, perform computations on different portions of the workload, usually involving the periodic exchange of intermediate results. This paradigm can be further subdivided into two categories: The master-slave (or host-node ) model in which a separate ``control'' program termed the master is responsible for process spawning, initialization, collection and display of results, and perhaps timing of functions. The slave programs perform the actual computation involved; they either are allocated their workloads by the master (statically or dynamically) or perform the allocations themselves. The node-only model where multiple instances of a single program execute, with one process (typically the one initiated manually) taking over the non-computational responsibilities in addition to contributing to the computation itself.

  28. PVM Mandelbrot example of Crowd Computing {Master Mandelbrot algorithm.} {Initial placement} for i := 0 to NumWorkers - 1 pvm_spawn(<worker name>) {Start up worker i} pvm_send(<worker tid>,999) {Send task to worker i} endfor {Receive-send} while (WorkToDo) pvm_recv(888) {Receive result} pvm_send(<available worker tid>,999) {Send next task to available worker} display result endwhile {Gather remaining results.} for i := 0 to NumWorkers - 1 pvm_recv(888) {Receive result} pvm_kill(<worker tid i>) {Terminate worker i} display result endfor {Worker Mandelbrot algorithm.} while (true) pvm_recv(999) {Receive task} result := MandelbrotCalculations(task) {Compute result} pvm_send(<master tid>,888) {Send result to master} endwhile

  29. Tree Computation The second model supported by PVM is termed a tree computation . In this scenario, processes are spawned in a tree-like manner, thereby establishing a tree-like, parent-child relationship (as opposed to crowd computations where a star-like relationship exists). This paradigm, although less commonly used, is an extremely natural fit to applications where the total workload is not known a priori, for example, in branch-and-bound algorithms, alpha-beta search, and recursive divide-and-conquer algorithms.

  30. PVM Split-Sort-Merge example of Tree Computation { Spawn and partition list based on a broadcast tree pattern. } for i := 1 to N, such that 2^N = NumProcs forallprocessors P such that P < 2^i pvm_spawn(...) {process id P XOR 2^i} if P < 2^(i-1) then midpt: = PartitionList(list); {Send list[0..midpt] to P XOR 2^i} pvm_send((P XOR 2^i),999) list := list[midpt+1..MAXSIZE] else pvm_recv(999) {receive the list} endif endfor endfor { Sort remaining list. } Quicksort(list[midpt+1..MAXSIZE]) { Gather/merge sorted sub-lists. } for i := N downto 1, such that 2^N = NumProcs forall processors P such that P < 2^i if P > 2^(i-1) then pvm_send((P XOR 2^i),888) {Send list to P XOR 2^i} else pvm_recv(888) {receive temp list} merge templist into list endif endfor endfor code written for a 4-node HyperCube -

  31. Hybrid The third model, which we term hybrid, can be thought of as a combination of the tree model and crowd model. Essentially, this paradigm possesses an arbitrary spawning structure: that is, at any point during application execution, the process relationship structure may resemble an arbitrary and changing graph.

  32. Message Passing Interface

  33. MPI Hello World #include <stdio.h> #include <mpi.h> int main(int argc, char ** argv) { int size,rank; int length; char name[80]; MPI_Init(&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD,&rank); MPI_Comm_size(MPI_COMM_WORLD,&size); MPI_Get_processor_name(name,&length); printf("Hello MPI! Process %d of %d on %s\n",size,rank,name); MPI_Finalize(); return 0; }

  34. Compute Unified Device Architecture (CUDA)

  35. to be continued...