The third phase of the Cray XT supercomputer (louhi.csc.fi) was installed in September 2008. It consists of the earlier XT4 part and the new XT5 part. The XT4 part consists of 1012 and the XT5 part of 1344 quad-core AMD Opteron Barcelona processors. The clock rate of these processors is 2.3 GHz. In addition, attached to Louhi is the PRACE prototype Loviatar, upgraded in June 2009, with two XT5 cabinets comprising 360 quad-core AMD Opteron Shanghai processors whose clock rate is 2.7 GHz. The facility totals 10864 compute cores. In general, the amount of memory is 1 GB per core but 384 cores of XT4 and 768 cores of XT5 have 2 GB per core. The total amount of memory is 11.7 TB.

The compute nodes of Louhi have the Compute Node Linux (CNL) operating system. Louhi has also six login nodes with the SuSE Linux operating system. Together these operating systems are called as the Cray Linux Environment (CLE), whose name was formerly UNICOS/lc.

For the development of user's own software, the programming environment consists of Portland, GNU Compiler Collection and Pathscale Fortran, C and C++ compilers, and the new Cray Compiling Environment (CCE) Fortran, C and UPC compilers. In addition, there are MPI, ACML, and LibSci libaries, TotalView debugger and some profiling tools provided by Cray. The batch processing system of Louhi is PBS Pro.

The theoretical peak performance of Louhi is 102.3 TF and the real performance 76.5 TF according to Linpack tests. It is one of the most powerful computers in Europe.

Louhi is reserved for jobs which need plenty of computing power and scale well for a large number of processors. Parallel jobs can be run with minimum 32 and maximum 2048 cores. For a specific reason, e.g., when it is necessary to solve computing problems classified as "grand challenges", it is possible to use even a greater amount of processors.