1 Stable Performance Forcc - Numa Using First Touch

A key problem for shared-memory systems is unpredictable performance. A critical innuence on performance is page placement: a poor choice of home node can severely degrade application performance because of the increased latency of accessing remote rather than local data. Two approaches to page placement are the simple policies \\rst-touch" and \round-robin", but neither of these policies suits all applications. We examine the advantages of each strategy, the problems that can result from a poor choice of placement policy, and how these problems can be alleviated by using proxies. Proxies route remote read requests via intermediate nodes, where combining is used to reduce contention at the home node. Our simulation results indicate that by using reactive proxies with rst-touch page placement, performance is always better than using either page placement policy without proxies. These results suggest that the application programmer can obtain stable performance without knowing the underlying implementation of cc-numa, and can avoid time-consuming performance tuning. 1.1 INTRODUCTION Unpredictable performance anomalies have hampered the acceptance of coherent cache non-uniform memory access (cc-numa) shared-memory architectures. One source of performance problems is the location of shared data: each page of shared data is allocated in distributed memory at a home node, by the 1 2 operating system, when it is rst accessed. The choice of home node for a page is commonly on a rst-touch or round-robin basis, but neither of these policies is suited to all applications, and a poor choice of page placement policy can have a marked eeect on the performance of an application. In this paper we examine the eeects of simple page placement, and describe how a technique for reducing read contention in cc-numa machines can alleviate problems with inappropriate page placement. In the proxy protocol, we associate a small set of nodes with each location in shared memory, which act as intermediaries for remote read requests. Using execution-driven simulations, we show that using the reactive variant of proxies, in conjunction with rst-touch page placement, yields performance which is always better than using either of the simple page placement strategies without proxies. This suggests that, by using rst-touch page placement with reactive proxies, application programmers can be conndent that they will obtain stable performance. The rest of the paper is structured as follows: page placement is discussed in Section 1.2, and the proxy protocol is explained in Section 1.3. We describe our simulated architecture and experimental …