Cray XMT

Cray XMT (Cray eXtreme MultiThreading, codenamed Eldorado) is a scalable multithreaded shared memory supercomputer architecture by Cray, based on the third generation of the Tera MTA architecture, targeted at large graph problems (e.g. semantic databases, big data, pattern matching). Presented in 2005, it supersedes the earlier unsuccessful Cray MTA-2. It uses the Threadstorm3 CPUs inside Cray XT3 blades. Designed to make use of commodity parts and existing subsystems for other commercial systems, it alleviated the shortcomings of Cray MTA-2's high cost of fully custom manufacture and support. It brought various substantial improvements over Cray MTA-2, most notably nearly tripling the peak performance, and vastly increased maximum CPU count to 8,192 and maximum memory to 128 TB, with a data TLB of maximal 512 TB.

Cray XMT uses a scrambled content-addressable memory model on DDR1 ECC modules to implicitly load-balance memory access across the whole shared global address space of the system. Use of 4 additional Extended Memory Semantics bits (full/empty, forwarding and 2 trap bits) per 64-bit memory word enables lightweight, fine-grained synchronization on all memory. There are no hardware interrupts and hardware threads are allocated by an instruction, not the OS.

Front-end (login, I/O, and other service nodes, utilizing AMD Opteron processors and running SLES Linux) and back-end (compute nodes, utilizing Threadstorm3 processors and running MTK, a simple BSD Unix-based microkernel) communicate through the LUC (Lightweight User Communication) interface, a RPC-style bidirectional client/server interface.