Locality: The 3rd Wall And The Need For Innovation In Parallel Architectures

In the past we have seen two major "walls" (memory and power) whose vanquishing required significant advances in architecture. This paper discusses evidence of a third wall dealing with data locality, which is prevalent in data intensive applications where computation is dominated by memory access and movement - not flops, Such apps exhibit large sets of often persistent data, with little reuse during computation, no predictable regularity, significantly different scaling characteristics, and where streaming is becoming important. Further, as we move to highly parallel algorithms (as in running in the cloud), these issues will get even worse. Solving such problems will take a new set of innovations in architecture. In addition to data on the new wall, this paper will look at one possible technique: the concept of migrating threads, and give evidence of its potential value based on several benchmarks that have scaling difficulties on conventional architectures.