  We study the algorithmic optimization and performance tuning of the Lattice
QCD clover-fermion solver for the K computer. We implement the L\"uscher's SAP
preconditioner with sub-blocking in which the lattice block in a node is
further divided to several sub-blocks to extract enough parallelism for the
8-core CPU SPARC64$^{\mathrm{TM}}$ VIIIfx of the K computer. To achieve a
better convergence property we use the symmetric successive over-relaxation
(SSOR) iteration with {\it locally-lexicographical} ordering for the sub-blocks
in obtaining the block inverse. The SAP preconditioner is included in the
single precision BiCGStab solver of the nested BiCGStab solver. The single
precision part of the computational kernel are solely written with the SIMD
oriented intrinsics to achieve the best performance of the \SPARC on the K
computer. We benchmark the single precision BiCGStab solver on the three
lattice sizes: $12^3\times 24$, $24^3\times 48$ and $48^3\times 96$, with
fixing the local lattice size in a node at $6^3\times 12$. We observe an ideal
weak-scaling performance from 16 nodes to 4096 nodes. The performance of a
computational kernel exceeds 50% efficiency, and the single precision BiCGstab
has $\sim26% susutained efficiency.
