We owe thanks to Bill Hartner and Cliff White for running many benchmarks, to Rusty Russell, Anton Blanchard, Paul Mackerras, and the rest of the IBM OzLabs group for many valuable discussions, to Andrew Morton, Hugh Dickens, Davide Libenzi, and Rusty Russell again for their careful review of much code, and to Matt Dobson, the anonymous referees, and shepherd Ray Bryant for their substantial help in rendering this paper human-readable. We are indebted to Dan Frye, Hans Tannenberg, Chris Maher, Randy Kalmeta, Vijay Sukthankar, and Hugh Blemings for their support of this effort.