Benefits and drawbacks of redundant batch requests

Most parallel computing platforms are controlled by batch schedulers that place requests for computation in a queue until access to compute nodes is granted. Queue waiting times are notoriously hard to predict, making it difficult for users not only to estimate when their applications may start, but also to pick among multiple batch-scheduled platforms the one that will produce the shortest turnaround time. As a result, an increasing number of users resort to "redundant requests": several requests are simultaneously submitted to multiple batch schedulers on behalf of a single job; once one of these requests is granted access to compute nodes, the others are canceled. Using simulation as well as experiments with a production batch scheduler we evaluate the impact of redundant requests on (1) average job performance, (2) schedule fairness, (3) system load, and (4) system predictability. We find that some of the popularly held beliefs about the harmfulness of redundant batch requests are unfounded. We also find that the two most critical issues with redundant requests are the additional load on current middleware infrastructures and unfairness towards users who do not use redundant requests. Using our experimental results we quantify both impacts in terms of the number of users who use redundant requests and of the amount of request redundancy these users employ. © Springer Science + Business Media B.V. 2007.