PDL Abstract

Reducing Latency via Redundant Requests: Exact Analysis

Proceedings of ACM Sigmetrics/Performance 2015 Conference on Measurement and Modeling of Computer Systems (SIGMETRICS 15), Portland, OR. June 2015.

Kristen Gardner, Sam Zbarsky, Sherwin Doroudi, Mor Harchol-Balter, Esa Hyytia*,
Alan Scheller-Wolf

Carnegie Mellon University
*Dept. of Communications and Networking, Aalto University


Recent computer systems research has proposed using redundant requests to reduce latency. The idea is to run a request on multiple servers and wait for the rst completion (discarding all remaining copies of the request). However there is no exact analysis of systems with redundancy.

This paper presents the first exact analysis of systems with redundancy. We allow for any number of classes of redundant requests, any number of classes of non-redundant requests, any degree of redundancy, and any number of heterogeneous servers. In all cases we derive the limiting distribution on the state of the system.

In small (two or three server) systems, we derive simple forms for the distribution of response time of both the redundant classes and non-redundant classes, and we quantify the "gain" to redundant classes and "pain" to non-redundant classes caused by redundancy. We nd some surprising results. First, the response time of a fully redundant class follows a simple Exponential distribution and that of the non-redundant class follows a Generalized Hyperexponential. Second, fully redundant classes are "immune" to any pain caused by other classes becoming redundant.

We also compare redundancy with other approaches for reducing latency, such as optimal probabilistic splitting of a class among servers (Opt-Split) and Join-the-Shortest-Queue (JSQ) routing of a class. We nd that, in many cases, redundancy outperforms JSQ and Opt-Split with respect to overall response time, making it an attractive solution.