  It has been known for a long time that the large experimental singlet-octet
mass gap in the pseudoscalar meson mass spectrum originates from the anomaly of
the axial vector current, i.e. from nonperturbative effects and the nontrivial
topological structure of the QCD vacuum. In the N_colour -> infinity limit of
the theory, this connection elucidates in the famous Witten-Veneziano relation
between the eta'-mass and the topological susceptibility of the quenched QCD
vacuum.While lattice QCD has by now produced impressive high precision results
on the flavour nonsinglet hadron spectrum, the determination of the
pseudoscalar singlet mesons from direct correlator studies is markedly lagging
behind, due to the computational complexity in handling observables that
include OZI-rule violating diagrams, like the eta' propagator. In this article,
we report on some recent progress in dealing with the numerical bottleneck
problem.
