The Cray Inc. XT3.

Machine type	Distributed-memory multi-vector processor.
Models	XT3.
Operating system	UNICOS/lc, Cray's microkernel Unix.
Connection structure	3-D Torus.
Compilers	Fortran 95, C, C++.
Vendors information Web page	www.cray.com/products/xt3/
Year of introduction	2004.

System parameters:

Model	Cray XT3
Clock cycle	2.4 GHz
Theor. peak performance
Per Processor	4.8 Gflop/s
Per Cabinet	460.8 Gflop/s
Max. Configuration	147 Tflop/s
Memory
Per Cabinet	≤ 768 GB
Max. Configuration	196 TB
No. of processors
Per Cabinet	96
Max. Configuration	30,508
Communication bandwidth
Bisectional/cabinet	333 GB/s

Remarks:

The Cray XT3 is the commercial spinoff of the 10,000+ processor Red Storm machine, built by Cray for Sandia Laboratories. The structure is similar, be it that there are no provisions are made to have a &lquo;classified&rquo; and an &lquo;unclassified&rquo; part in the machine. The basic processor in a node, called PE (Processing Element) in Cray jargon, is the AMD Opteron 100, at 2.4 GHz. Cray has chosen for this uniprocessor version of the chip because of the lower memory latency (about 60 ns) in contrast to the SMP-enabled versions that have a memory latency that can be up to 2 times higher. Per PE up to 8 GB of memory can be configured, connected by a 6.4 HyperTransport to the processor. For connection to the outside world a PE harbours 2 PCI-X busses, a dual-ported FiberChannel Host Bus Adaptor for connecting to disk, and a 10 GB Ethernet card.

The Opteron was also chosen because of the high bandwidth the relatively ease of connecting the processor of to the network processor, Cray's SeaStar chip. For the physical connection another HyperTransport channel at 6.4 GB/s is used. The SeaStar has 6 ports with a bandwidth of 7.6 GB/s each (3.8 GB/s, incoming and outgoing). Because of its 6 ports the natural interconnection mode is therefore a 3-D torus.

Like for the earlier Cray T3E (see \ref{gone}), Cray has chosen to use a microkernel approach for the compute PEs. These are dedicated to computation and communication and are not disturbed by other OS tasks that can seriously influence the scalability (see [31]). For tasks like communicating with users, networking, and I/O special PEs are added that have versions of the OS that can handle these tasks.

The XT3 is obviously designed for a distributed memory parallel model, supporting Cray's MPI 2.0 and its one-way communication shmem library that date back to the Cray T3D/T3E systems but is still popular because of its simplicity and efficiency. The system comes in cabinets of 96 PEs, including service PEs. For larger configurations the ratio of service PEs to compute PEs (generally) can be lowered. So, a hypothetical maximal configuration of 30,508 PEs would need only 106 service PEs.

Measured Performances: The Cray XT3 is quite new and as yet no independent performance results are available.