Thursday, August 30, 2007

Parallel algorithms: A resurgence ?

My department has a fairly strong systems and hardware group, and some of the buzz I hear about involves the research opportunities in multicore architectures (I'm talking MANY MANY cores, not 2 or 4), and how parallel computation is relevant once again. In fact, I've had one request to include basic material on parallel algorithms in my graduate algorithms class.

Juxtaposed with that is the fact that the Stein-enhanced version of Introduction to Algorithms does NOT contain the chapter on parallel algorithms that the old CLR used to have. In fact, I'll probably use the chapter outline from CLR if I cover parallel algorithms at any level. I wonder what was the thinking behind removing the chapter on parallel methods, and whether this might be reversed in future editions.

Update: (that was quick! - thanks, Cliff): Tom Cormen writes in to explain the decision to remove the chapter on parallel algorithms:
Cliff Stein pointed me to this blog. Suresh asked why we removed the chapter on parallel algorithms when we wrote our second edition.

The chapter on parallel algorithms in the first edition was exclusively on PRAM algorithms. We wrote the second edition mostly during the late 1990s, into early 2001. At the time, PRAM algorithms were yesterday's news, because the PRAM model was too far from what could be built realistically. We considered including algorithms for some other parallel computing model, but there was no consensus model at the time. We felt that the best decision was to just leave out parallel algorithms and use the pages for other new material.
I'm not surprised. It *was* true that by the time the 2nd ed arrived, PRAMs were yesterday's news: in fact, streaming and external memory methods were "hotter" at the time. It'll be interesting to see if multicore actually does spur new interest in parallel algorithms, and not just parallel architectures. In recent months I've participated in discussions about algorithm design for things like the Cell and the implied architecture of nVidia's CUDA, and it's surprising how often standard parallel algorithms methods like pointer jumping and the like are being reinvented.

What makes the new platforms more interesting is that there are features (especially streaming features) that make the mapping to "standard" PRAM models not so obvious. It may not merely be a mattter of shoehorning new systems into parallel models, but of extending and modifying the models themselves.
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