After having investigated the regular prisms and prism tilings in the $\SLR$ space in the previous work \cite{Sz13-1} of the second author, we consider the problem of geodesic ball packings related to those tilings and their symmetry groups $\mathbf{pq2_1}$. $\SLR$ is one of the eight Thurston geometries that can be derived from the 3-dimensional Lie group of all $2\times 2$ real matrices with determinant one. In this paper we consider geodesic spheres and balls in $\SLR$ (even in $\mathbf{SL_{\mathrm{2}}R})$, if their radii $\rho\in [0, \frac{\pi}{2})$, and determine their volumes. Moreover, we consider the prisms of the above space and compute their volumes, define the notion of the geodesic ball packing and its density. We develop a procedure to determine the densities of the densest geodesic ball packings for the tilings, or in this paper more precisely, for their generating groups $\mathbf{pq2_1}$ (for integer rotational parameters $p,q$; $3\le p, \frac{2p}{p-2} Comment: 16 pages, 5 figures
