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Computing in a Parallel Universe

Multicore chips could bring about the biggest change in computing since the microprocessor

Brian Hayes

Shared Memories

It is surely no coincidence that the kinds of parallelism in widest use today are the kinds that seem to be easiest for programmers to manage. Instruction-level parallelism is all but invisible to the programmer; you create a sequential series of instructions, and it's up to the hardware to find opportunities for concurrent execution.

In writing a program to run on a cluster or server farm, you can't be oblivious to parallelism, but the architecture of the system imposes a helpful discipline. Each node of the cluster is essentially an independent computer, with its own processor and private memory. The nodes are only loosely coupled; they communicate by passing messages. This protocol limits the opportunities for inter­process mischief. The software development process is not radically different; programs are often written in a conventional language such as Fortran or C, augmented by a library of routines that handle the details of message passing.

Clusters work well for tasks that readily break apart into lots of nearly independent pieces. In weather prediction, for example, each region of the atmosphere can be assigned its own CPU. The same is true of many algorithms in graphics and image synthesis. Web servers are another candidate for this treatment, since each visitor's requests can be handled independently.

In principle, multicore computer systems could be organized in the same way as clusters, with each CPU having its own private memory and with communication governed by a message-passing protocol. But with many CPUs on the same physical substrate, it's tempting to allow much closer collaboration. In particular, multicore hardware makes it easy to build shared-memory systems, where processors can exchange information simply by reading and writing the same location in memory. In software for a shared-memory machine, multiple computational processes all inhabit the same space, allowing more interesting and flexible patterns of interaction, not to mention subtler bugs.

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