Louhi is a Cray XT4/XT5 Massively Parallel Processor (MPP) supercomputer.  Louhi has 1012 XT4 compute nodes, each containing a quad-core 2.3-GHz AMD Opteron 64‑bit processors (the code name is Barcelona),  672 XT5 nodes each containing two quad-core 2.3 GHz AMD Opteron 64‑bit  Barcelona processors, and 180 XT5 nodes belonging to the PRACE (Partnership for Advanced Computing in Europe) project each containing two quad-core 2.7 GHz AMD Opteron 64‑bit  Shanghai processors.  Thus the Cray XT4 part has 4048 cores and the Cray XT5 part has 6816 compute cores, altogether 10864 cores and theoretical computing speed 102,2 Tflop/s.  There is 1 GB or 2 GB of memory per core.

The Cray SeaStar2 communication system interconects the processors in a 3D torus.

In addition there are 22 service nodes for login, I/O, boot, database and other service usage. The service nodes run SuSE Linux.

The service nodes and compute nodes are set up physically in 18 cabinets. See Chapter Cray XT system.

The operting system of Louhi is called Cray Linux Environment (CLE, the present version is 2.2). It consists of the 64-bit SuSE Linux Enterpise server (the present version is 10.1), running on login and other service nodes, and the 64-bit Compute Node Linux (CNL) running on compute nodes. See, for example, Chapters Compute Node Linux (CNL) and Porting codes for more information.