The Landscape of Seven Questions and Seven Dwarfs for Parallel Computing Research: a view from Berkeley
Parallel languages should support a rich set of data sizes and types
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- 5.3 Support of successful styles of parallelism
- 5.4 Programs must be written to work independent of the number of processors
5.2 Parallel languages should support a rich set of data sizes and typesWhile the algorithms were often the same in embedded and server benchmarks in Section 3, the data types were not. SPEC relied on single- and double-precision floating point and large integer data, while EEMBC used integer and fixed-point data that varied from 1 to 32 bits. Note that most programming languages only support the subset of data types found originally in the IBM 360 announced 40 year ago: 8-bit ASCII, 16-, and 32-bit integers, and 32-bit and 64-bit floating-point numbers. This leads to the relatively obvious observation. If the parallel research agenda inspires new languages and compilers, they should allow programmers to specify at least the following sizes (and types):
Mixed precision floating point arithmetic—separate precisions for input, internal computations, and output—has already begun to appear for BLAS routines [Demmel et al 2002]. A similar and perhaps more flexible structure will be required so that all methods can exploit it. While support for all of these types can mainly be provided entirely in software, we do not rule out additional hardware to assist efficient implementations of very wide data types. In addition to the more “primitive” data types described above, programming environments should also provide for distributed data types. These are naturally tightly coupled to the styles of parallelism that are expressed and so influence the entire design. The languages proposed in the DARPA HPLS program are currently attempting to address this issue, with a major concern being support for user-specified distributions. 5.3 Support of successful styles of parallelismProgramming languages, compilers, and architectures have often placed their bets on one style of parallel programming, usually forcing programmers to express all parallelism in that style. Now that we have a few decades of such experiments, we think that the conclusion is clear: some styles of parallelism have proven successful for some applications, and no style has proven best for all. Rather than placing all the eggs in one basket, we think programming models and architectures should support a variety of styles so that programmers can use the superior choice when the opportunity occurs. We believe that list includes at least the following:
In addition to the styles of parallelism, we also have the issue of the memory model. Because parallel systems usually contain memory models that are distributed throughout the machine, the question arises of the programmer’s view of this memory. Systems providing the illusion of a uniform shared address space have been very popular with programmers. However, scaling these to large systems remains a challenge. Memory consistency issues (relating to the visibility and ordering of local and remote memory operations) also enter the picture when locations can be updated by multiple processors, each possibly containing a cache. Explicitly partitioned systems (such as MPI) sidestep many of these issues but programmers must deal with the low level details of performing remote updates themselves. 5.4 Programs must be written to work independent of the number of processorsMPI, the current dominant programming model for parallel scientific programming, forces coders to be aware of the exact mapping of computational tasks to processors. This style has been recognized for years to increase the cognitive load on programmers, and has persisted primarily because it is expressive and delivers the best performance. [Snir et al 1998][Gursoy and Kale 2004] Because we anticipate a massive increase in exploitable concurrency, we believe that this model will break down in the near future as programmers have to explicitly deal with decomposing data, mapping tasks, and performing synchronization over many thousands of processing elements. Recent efforts in programming languages have focused on this problem and their offerings have provided models where the number of processors is not exposed [Deitz 2005][Allen et al 2006][Callahan et al 2004, Charles et al 2005].While attractive, these models have the opposite problem – delivering performance. In many cases, hints can be provided to collocate data and computation in particular memory domains. In addition, because the program is not over-specified, the system has quite a bit of freedom in mapping and scheduling that in theory can be used to optimize performance. Delivering on this promise is, however, still an open research question. Directory: images images -> Pelan pentaksiran kursus images -> Military callsign list as of dec 2010 images -> For Date: 02/01/2013 Friday Call Number Time Call Reason Action 13-768 0008 motor vehicle stop warning Issued images -> New embedded S images -> Creating Buy-In – Building Relationships images -> Deadline: Saturday, March 12th, 2016 images -> News items from Raising Our Voices images -> Last year four of the best Afrikaans Christian singers joined forces to tour Down Under with the production ‘Manne wat Glo’ images -> World-renowned vocal ensemble sweet honey in the rock images -> Peoples Voice Café History Download 232.56 Kb. Share with your friends:
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