Optimal Reissue Policies for Reducing Tail Latency.

Interactive services send redundant requests to multiple different replicas to meet stringent tail latency requirements. These additional (reissue) requests mitigate the impact of non-deterministic delays within the system and thus increase the probability of receiving an on-time response. There are two existing approaches of using reissue requests to reduce tail latency. (1) Reissue requests immediately to one or more replicas, which multiplies the load and runs the risk of overloading the system. (2) Reissue requests if not completed after a fixed delay. The delay helps to bound the number of extra reissue requests, but it also reduces the chance for those requests to respond before a tail latency target. We introduce a new family of reissue policies, Single-Time / Random (SingleR), that reissue requests after a delay d with probability q. SingleR employs randomness to bound the reissue rate, while allowing requests to be reissued early enough so they have sufficient time to respond, exploiting the benefits of both immediate and delayed reissue of prior work. We formally prove, within a simplified analytical model, that SingleR is optimal even when compared to more complex policies that reissue multiple times. To use SingleR for interactive services, we provide efficient algorithms for calculating optimal reissue delay and probability from response time logs through data-driven approach. We apply iterative adaptation for systems with load-dependent queuing delays. The key advantage of this data-driven approach is its wide applicability and effectiveness to systems with various design choices and workload properties. We evaluated SingleR policies thoroughly. We use simulation to illustrate its internals and demonstrate its robustness to a wide range of workloads. We conduct system experiments on the Redis key-value store and Lucene search server. The results show that for utilizations ranging from 40-60%, SingleR reduces the 99th-percentile latency of Redis by 30-$70% by reissuing only 2% of requests, and the 99th-percentile latency of Lucene by 15-25% by reissuing 1% only.