Amdahl's Law in the Multicore Era

Over the last several decades computer architects have been phenomenally successful turning the transistor bounty provided by Moore's Law into chips with ever increasing single-threaded performance. During many of these successful years, however, many researchers paid scant attention to multiprocessor work. Now as vendors turn to multicore chips, researchers are reacting with more papers on multi-threaded systems. While this is good, we are concerned that further work on single-thread performance will be squashed.

To help understand future high-level trade-offs, we develop a corollary to Amdahl's Law for multicore chips [Hill& Marty, IEEE Computer 2008]. It models fixed chip resources for alternative designs that use symmetric cores, asymmetric cores, or dynamic techniques that allow cores to work together on sequential execution. Our results encourage multicore designers to view performance of the entire chip rather than focus on core efficiencies. Moreover, we observe that obtaining optimal multicore performance requires further research BOTH in extracting more parallelism and making sequential cores faster.

This talk is based on an HPCA 2008 keynote address.

Mark D. Hill (http://www.cs.wisc.edu/~markhill) is professor in both the Computer Sciences Department and the Electrical and Computer Engineering Department at the University of Wisconsin-Madison, where he also co-leads the Wisconsin Multifacet project with David Wood. He earned a Ph.D. from the University of California, Berkeley. He is an ACM Fellow and a Fellow of the IEEE. His past work ranges from refining multiprocessor memory consistency models to developing the 3C model of cache behavior (compulsory, capacity, and conflict misses).