What to Do with All this Hardware? (Invited Lecture)

Abstract

The upcoming so-called “on-chip Billion transistor” era raises the question: What to do with all the on-chip hardware once the returns on adding more on-chip memory start to diminish?

Parallel computing has been a strategic area of growth for computer science since the 1940s. So far, parallel computing affected main stream computer science only in a limited way. The key problem with parallel computers has been their programmability.

The parallel algorithms research community has developed a theory of parallel algorithms for a very simple parallel computation model, the so-called PRAM (for parallel random-access machine, or model). That theory appears to be second in magnitude only to serial algorithmics.

However, the evolution of parallel computers never reached a situation where the PRAM algorithmic computation model offered effective abstraction for them. So, this elegant algorithmic theory remained in the ivory towers of theorists. Not only that it has not been matched with a real computer system, there has hardly been an experimental study of what works better, more refined performance measurements, and a broad study of applications. For example, the general question “how good par- allel algorithms can really be” has remained generally open.

Explicit Multi-Threading (XMT) is a new fine-grained computation framework which tries to address the hardware opportunity using the PRAM parallel algorithmic knowledge base. XMT aims at faster single- task completion time by way of executing in parallel many instruction all within a single chip. Building on some key ideas of parallel computing, XMT covers the spectrum from algorithms through architecture to im- plementation; the main implementation related innovation in XMT was through the incorporation of low-overhead hardware mechanisms (for more effective fine-grained parallelism).

The two key research questions facing our “PRAM-on-chip vision” are: (i) “how to build?” an XMT computer, and (ii) “who cares?”; that is, what will be the key applications?