Introducing the Cray XMT Petr Konecny Cray Inc. 411 First Avenue South Seattle, Washington 98104
[email protected] May 5th 2007 Abstract Cray recently introduced the Cray XMT system, which builds on the parallel architecture of the Cray XT3 system and the multithreaded architecture of the Cray MTA systems with a Cray-designed multithreaded processor. This paper highlights not only the details of the architecture, but also the application areas it is best suited for. 1 Introduction past two decades the processor speeds have increased several orders of magnitude while memory speeds Cray XMT is the third generation multithreaded ar- increased only slightly. Despite this disparity the chitecture built by Cray Inc. The two previous gen- performance of most applications continued to grow. erations, the MTA-1 and MTA-2 systems [2], were This has been achieved in large part by exploiting lo- fully custom systems that were expensive to man- cality in memory access patterns and by introducing ufacture and support. Developed under code name hierarchical memory systems. The benefit of these Eldorado [4], the Cray XMT uses many parts built multilevel caches is twofold: they lower average la- for other commercial system, thereby, significantly tency of memory operations and they amortize com- lowering system costs while maintaining the sim- munication overheads by transferring larger blocks ple programming model of the two previous genera- of data. tions. Reusing commodity components significantly The presence of multiple levels of caches forces improves price/performance ratio over the two pre- programmers to write code that exploits them, if vious generations. they want to achieve good performance of their ap- Cray XMT leverages the development of the Red plication.