In this work, the mobility of dislocations in f.c.c. and b.c.c. metals under deformation is quantified using a recently developed coarse-grained (CG) atomistic model. Fundamental to the CG method is an atomistic field formulation that unifies the atomistic and continuum description of materials through an Irving-Kirkwood procedure in statistical mechanics. At a fraction of the cost of full molecular dynamics (MD), the CG model is shown to be applicable for calibrating the dislocation mobility law from the bottom up. In particular, using a modest computational resource, the mobility of extremely long dislocation lines (a length up to one micron and even above) in billion-atom material samples under deformation is predicted from the atomistic to the microscale. Results show that the dislocation mobility law in f.c.c materials is insensitive to its length. In contrast, in b.c.c. materials, when the length of a dislocation is at micrometer level and above, its mobility is found to be proportional to the line length, i.e., the longer the dislocation line, the faster it moves. With the atomistic information being retained, the CG simulations also provide an explanation for the observed different line length-dependent dislocation mobility in f.c.c. and that in b.c.c. materials: both short and long dislocation lines in f.c.c. materials migrate through gliding, while dislocation motion is mainly controlled by kink pair nucleation and diffusion in b.c.c. materials, and the length will be the limiting factor for kink pair density. In parallel, in order to validate the CG model, a limited set of full MD simulations has been also performed. The one-to-one comparison between CG and MD demonstrates that the CG simulation does retain the atomistic nature of dislocation dynamics, such as (i) the dislocation velocity-dependent line energy and core structure; and (ii) the phonon wave emission from a moving dislocation. In addition, the limitation, the future development, and the potential applications of CG will be also discussed in this presentation.