![]() ![]() The program may use THREADS as a compile-time constant.Threads is specified at compile time by the user.There is a form of parallel loop that we will see later.upc_barrier is a global synchronization: all wait.MYTHREAD specifies thread index (0.THREADS-1).Number of threads specified at compile-time or run-time available as program variable THREADS.A number of threads working independently in a SPMD fashion.Common and familiar syntax and semantics for parallel C with simple extensions to ANSI C.Programmers are clever and careful, and may need to get close to hardware.A partitioned global address space language.An explicit parallel extension of ANSI C.ARMCI, GASNet for distributed memory implementations.Open64 open source research compiler infrastructure.Open source compiler from UCB runs on most machines.Open source compiler available from Rice.Commercial compiler available on Cray machines.Open source compiler from LBNL/UCB (and another from MTU).Commercial compilers available on Cray, SGI, HP machines.A successful language/library must run everywhere.Potential productivity advantage for irregular applications.Potential performance advantage for fine-grained, one-sided programs.One-Sided Communication May Improve Performance Distributed array generality and how they are constructed X X X Shared Global address space ptr: ptr: ptr: Private.Some models have a separate private memory area.Both individual and bulk memory copies.One-sided communication through reads/writes of shared variables.Remote memory may stay remote: no automatic caching implied.Shared memory is partitioned by processors.The languages share the global address space abstraction.Global Address Space Eases Programming Thread0 Thread1 Threadn Base languages differ: UPC (C), CAF (Fortran), Titanium (Java).Initial implementation can use fine-grained shared memory.Performance transparency and tunability are goals.Programmer control over performance critical decisions.Address space is logically partitioned.Allows programmer to directly represent distributed data structures.Fixed at program start-up, typically 1 thread per processor.Explicitly-parallel programming model with SPMD parallelism.Partitioned Global Address Space Languages local/global distinction, i.e., layout matters (performance).SPMD parallelism like MPI (performance).global address space like threads (programmability). ![]() Global Address Space (GAS) Languages take the best of both.Easier to program, but less scalable performance.Usually for non-scientific applications.Shared memory with threads in OpenMP, Threads+C/C++/F or Java.Usually for scientific applications with C++/Fortran.Most parallel programs are written using either:.Unified Parallel C (UPC) Parry Husbands Lawrence Berkeley National Laboratory Borrowed from Kathy Yelickâs (UCB) CS267 Lecture Slides adapted from some by Tarek El-Ghazawi (GWU)
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