Dan Reed on Little’s Law in the Age of Exascale

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Dan Reed from the University of Iowa writes that the design of an Exascale systems needs to take into account Little’s Law, a simple, yet subtle forumula that that relates performance (throughput), delay (response time) and number of interacting units (customers).

Little’s Law also says there is no free lunch. Optimizing for high system utilization leads to large wait times for customers, as anyone who has waited on a busy telephone reservation line knows all too well. Conversely, ensuring short waiting lines requires system utilization to be low. Simply put, one must choose between high system efficiency and short customer wait times. One cannot (in general) have both.

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