Electronic Design

Parallel Programming And Multicore Environments

Multicore chip designs, large symmetrical multiprocessing (SMP) systems, and clustering can bring many processors to bear on an application. But without proper software, they're simply large collections of processor cores and memory. And conventional serial programming languages don't make handling an expansive suite of computing elements any easier.

The current approach for tackling transaction-oriented Web traffic is to distribute a large number of conventional sequential applications across multiple cores. Azul does this for Java applets with its 48-core Vega 2 chip (see "Multicore My Way,"). This approach works well for clustering current applications that don't use a large part of the whole set of processors, but it doesn't work as well when trying to scale an application that has minimal multithreading.

The problem is that conventional programming languages assume a sequential programming model. This wasn't a problem when most systems employed a single processor. Though it's an advantage in a complex environment, multithreading normally is used sparingly so it doesn't overload the processor or operating system. Operating-system support of lightweight threads has increased the use of threads, but applications with more than a dozen threads are unusual.

One alternative is to use a system like the message passing interface (MPI) currently used by applications running on supercomputers and large clusters. MPI still requires explicit thread definition, yet it lets threads communicate easily with each other. The approach works, but it doesn't scale well unless the applications are carefully designed and deployed.

Another alternative is specialized runtime systems like Intel's Thread Building Blocks (see "Multiple Threads Make Chunk Change" ). It targets applications that manipulate large chunks of data such as arrays. In this case, the number of threads typically matches the number of cores available, with each pulling from a common work queue. This keeps everything running, but it targets a limited area of parallel programming.

Parallel programming languages turn conventional sequential programming semantics on end so the default is parallel execution rather than serial. The compiler and runtime environment must optimize the parallel execution, relieving the programmer of this chore. Sequential execution must be specified explicitly since it is now the exception rather than the rule.

Parallel programming languages tend to still be in the research side of development. Sun's Fortress addresses a range of applications and programming issues, but it assumes a system with many cores and good communication or shared memory between cores. Threads are synchronized with atomic blocks of code.

Some approaches attempt to extend an existing language like C. Cilk, a multithreaded parallel programming language based on C, retains C's serial semantics. It also adds a handful of keywords and parallel semantics. And, it supports speculative parallelism.

Parallel programming languages face a number of challenges, from technical issues and optimizations to social issues. It's unusual for most programmers to switch programming languages, and learning a new programming paradigm typically requires a significant investment in time and effort.

Parallel programming languages work well in many environments, but they may not be as useful as they are in heterogeneous configurations of cooperative cores. These application-specific core combinations are growing more common as cores become more numerous. In this case, the interconnection of systems is a major part of a designer's job.

Taking advantage of parallel hardware environments will require a major shift in programming. But software will continue playing catchup with hardware.

MPI Forum • www-unix.mcs.anl.gov/mpi
Sun Microsystems • www.sun.com
The Cilk Project supertech.lcs.mit.edu/cilk

TAGS: Intel
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