  The dominant cost of most lattice QCD simulations is the inversion of the
Dirac operator required to calculate the force term in the RHMC update. One way
to improve this situation is to use multiple pseudofermions, which reduces the
size and variance of this force and hence allows a larger integration step size
to be used. This means fewer force term calculations are required, but at the
cost of having to invert the Dirac operator for each pseudofermion field. This
bottleneck can be addressed: recently there has been renewed interest in the
use of block Krylov solvers, which can solve multiple right hand side vectors
with significantly fewer iterations than are required if each vector is solved
using a separate Krylov solver. We combine these two ideas, achieving a
significant speed-up of RHMC lattice QCD simulations.
