Amdahl's Law in the Multicore Era

Mark D. Hill
Computer Sciences Department
University of Wisconsin-Madison
http://www.cs.wisc.edu/~markhill/

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.