Locality-aware request distribution in cluster-based network servers

Abstract

We consider cluster-based network servers in which a front-end directs incoming requests to one of a number of back-ends. Speci cally, w e consider content-based request distribution: the front-end uses the content r equested, in addition to information about the load on the back-end nodes, to choose which b a c k-end will handle this request. Content-based request distribution can improve locality in the back-ends' main memory caches, increase secondary storage scalability b y partitioning the server's database, and provide the ability to employ back-end nodes that are specialized for certain types of requests.As a speci c policy for content-based request distribution, we i n troduce a simple, practical strategy for locality-aware request distribution (LARD). With LARD, the front-end distributes incoming requests in a manner that achieves high locality in the back-ends' main memory caches as well as load balancing. Locality is increased by dynamically subdividing the server's working set over the back-ends. Trace-based simulation results and measurements on a prototype implementation demonstrate substantial performance improvements over state-of-the-art approaches that use only load information to distribute requests. On workloads with working sets that do not t in a single server node's main memory cache, the achieved throughput exceeds that of the state-of-the-art approach b y a factor of two to four.With content-based distribution, incoming requests must be handed o to a back-end in a manner transparent to the client, after the front-end has inspected the content of the request. To this end, we i n troduce an e cient TCP hando protocol that can hand o an established TCP connection in a client-transparent manner.To appear in the Proceedings of the Eighth International Conference on Architectural Support for Programming Languages and Operating Systems (ASPLOS-VIII), San Jose, CA, Oct 1998. IntroductionNetwork servers based on clusters of commodity w orkstations or PCs connected by high-speed LANs combine cutting-edge performance and low cost. A cluster-based network server consists of a front-end, responsible for request distribution, and a number of back-end nodes, responsible for request processing. The use of a front-end makes the distributed nature of the server transparent to the clients. In most current cluster servers the frontend distributes requests to back-end nodes without regard to the type of service or the content requested. That is, all back-end nodes are considered equally capable of serving a given request and the only factor guiding the request distribution is the current load of the backend nodes.With content-based r equest distribution, the frontend takes into account both the service/content r equested and the current load on the back-end nodes when deciding which back-end node should serve a given request. The potential advantages of content-based request distribution are: (1) increased performance due to improved hit rates in the back-end's main memory caches, (2) increased secon...

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