  We discuss a detailed weak scaling analysis of GEM, a 3D MPI-parallelised
gyrofluid code used in theoretical plasma physics at the Max Planck Institute
of Plasma Physics, IPP at Garching b. M\"unchen, Germany. Within a PRACE
Preparatory Access Project various versions of the code have been analysed on
the HPC systems SuperMUC at LRZ and JUQUEEN at J\"ulich Supercomputing Centre
(JSC) to improve the parallel scalability of the application. The diagnostic
tool Scalasca has been used to filter out suboptimal routines. The code uses
the electromagnetic gyrofluid model which is a superset of magnetohydrodynamic
and drift-Alfv\'en microturbulance and also includes several relevant kinetic
processes. GEM can be used with different geometries depending on the targeted
use case, and has been proven to show good scalability when the computational
domain is distributed amongst two dimensions. Such a distribution allows grids
with sufficient size to describe small scale tokamak devices. In order to
enable simulation of very large tokamaks (such as the next generation nuclear
fusion device ITER in Cadarache, France) the third dimension has been
parallelised and weak scaling has been achieved for significantly larger grids.
