In this paper , we describe the implementation and performance of GreeM , a massively parallel TreePM code for large-scale cosmological N -body simulations .
GreeM uses a recursive multi-section algorithm for domain decomposition .
The size of the domains are adjusted so that the total calculation time of the force becomes the same for all processes .
The loss of performance due to non-optimal load balancing is around 4 % , even for more than 10 ^ { 3 } CPU cores .
GreeM runs efficiently on PC clusters and massively-parallel computers , such as a Cray XT4 .
The measured calculation speed on Cray XT4 is 5 \times 10 ^ { 4 } particles per second per CPU core , for the case of an opening angle of \theta = 0.5 , if the number of particles per CPU core is larger than 10 ^ { 6 } .