We consider the use of runtime measured workload characteristics in parallel processor scheduling. Although many researchers have considered the use of application characteristics in this domain, most of this work has assumed that such information is available apriori. In contrast, we propose and evaluate experimentally dynamic processor allocation policies that rely on determining job characteristics at runtime; in particular, we focus on measuring and using job efficiency and speedup. Our work is intended to be a first step towards the eventual development of production schedulers that use runtime measured workload characteristics in making their decisions. The experimental results we present validate the following observations: -- Despite the inherent inaccuracies of runtime measurements and the added overhead of more frequent reallocations, schedulers that use runtime measurements of workload characteristics can significantly outperform schedulers that are oblivious ..