The Interaction of Parallel and Sequential Workloads 

on a Network of Workstations

Remzi H. Arpaci, Andrea C. Dusseau, Amin M. Vahdat, Lok T. Liu, Thomas E. Anderson, David A. Patterson.  The Interaction of Parallel and Sequential Workloads on a Network of Workstations.  Proc. 1995 ACM SIGMETRICS Conference, May 1995, pages 267 - 278.



This paper examines the plausibility of using a network of workstations (NOW) for a mixture of parallel and sequential jobs. Through simulations, our study examines issues that arise when combining these two workloads on a single platform. Starting from a dedicated NOW just for parallel programs, we incrementally relax uni-programming restrictions until we have a multi-programmed, multi-user NOW for both interactive sequential users and parallel programs. We show that a number of issues associated with the distributed NOW environment (e.g., daemon activity, co-scheduling skew) can have a small but noticeable effect on parallel program performance. We also find that efficient migration to idle workstations is necessary to maintain acceptable parallel application performance. Furthermore, we present a methodology for deriving an optimal delay time for recruiting idle machines for use by parallel programs; this recruitment threshold was just 3 minutes for the research cluster we measured. Finally, we quantify the effects of the additional parallel load upon interactive users by keeping track of the potential number of user delays in our simulations. When we limit the maximum number of delays per user, we can still maintain acceptable parallel program performance. In summary, we find that for our workloads a 2:1 rule applies: a NOW cluster of approximately 60 machines can sustain a 32-node parallel workload in addition to the sequential load placed upon it by interactive users.