A Better Model for Job Redundancy: Decoupling Server Slowdown and Job Size
IEEE Modeling, Analysis and Simulation of Computer and Telecommunication Systems (MASCOTS 2016), London, UK, September 2016.
Kristen Gardner, Mor Harchol-Balter, Alan Scheller-Wolf
Carnegie Mellon University
Recent computer systems research has proposed using redundant requests to reduce latency. The idea is to replicate a request so that it joins the queue at multiple servers. The request is considered complete as soon as any one copy of the request completes. Redundancy is beneficial because it allows us to overcome server-side variability – the fact that the server we choose might be temporarily slow due to factors such as background load, network interrupts, and garbage collection. When there is significant server-side variability, replicating requests can greatly reduce response times.
In the past few years, queueing theorists have begun to study redundancy, first via approximations, and, more recently, via exact analysis. Unfortunately, for analytical tractability, most existing theoretical analysis has assumed an Independent Runtimes (IR) model, wherein the replicas of a job each experience independent runtimes (service times) at different servers. The IR model is unrealistic and has led to theoretical results which can be at odds with computer systems implementation results.
This paper introduces a much more realistic model of redundancy. Our model allows us to decouple the inherent job size (X) from the server-side slowdown (S), where we track both S and X for each job. Analysis within the S&X model is, of course, much more difficult. Nevertheless, we design a policy, Redundant-to- Idle-Queue (RIQ) which is both analytically tractable within the S&X model and has provably excellent performance.
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