In this paper we study the parallel scalability of variants of additive Schwarz preconditioners for three dimensional non-overlapping domain decomposition methods. To alleviate the
computational cost, both in terms of memory and floating-point complexity, we investigate
variants based on a sparse approximation or on mixed 32- and 64-bit calculation. The robustness of the preconditioners is illustrated on a set of linear systems arising from the finite
element discretization of elliptic PDEs through extensive parallel experiments on up to 1000
processors. Their efficiency from a numerical and parallel performance view point are studied