MPP vs Clusters

The top500 list uses a slightly different distinction between an MPP and a cluster, as explained inDongarra et al. paper:

[a cluster is a] parallel computer system comprising an integrated collection of independent nodes, each of which is a system in its own right, capable of independent operation and derived from products developed and marketed for other stand-alone purposes

Compared to a cluster, a modern MPP (such as the IBM Blue Gene) is more tightly-integrated: individual nodes cannot run on their own and they are connected by a custom network (like a multidimensional torus). But, similarly to a cluster, an MPP doesn’t have shared memory. I’d be thus extremely careful to use terms SIMD and MIMD in this context as they usually describe shared memory architectures (SMP).