We have alluded several times during this book to the fact that computing workloads have highly variable job sizes (service requirements), that are not well described by an Exponential distribution. This chapter is a story of my own experience in studying UNIX jobs in the mid-1990s, as a PhD student at U.C. Berkeley. Results of this research are detailed in [84, 85]. The story serves as both an introduction to empirical measurements of computer workloads and as a case study of how a deeper understanding of computer workloads can lead to improved computer system designs. The remaining chapters in the book address modeling and performance evaluation of systems with high-variability workloads.

Grad School Tales … Process Migration

In the mid-1990s, an important research area was CPU load balancing in a Network of Workstations (at U.C. Berkeley it was coined the “N.O.W. project”). The idea in CPU load balancing is that CPU-bound jobs might benefit from being migrated from a heavily loaded workstation to a more lightly loaded workstation in the network. CPU load balancing is still important in today's networks of servers. It is not free, however: Migration can be expensive if the job has a lot of “state” that has to be migrated with the job (e.g., lots of open files associated with the job), as is common for jobs that have been running for a while. When the state associated with the job is great, then the time to migrate the job to another machine is high, and hence it might not be worth migrating that job.