1. A method for computing dummy feature density for a CMP (Chemical-Mechanical Polishing) process, wherein the dummy feature density is used to add dummy features to a layout to reduce a post-CMP topography variation, the method comprising: discretizing a layout of an integrated circuit into a plurality of panels; computing a feature density and a slack density for the plurality of panels, wherein the feature density d(i, j) for panel (i, j) specifies the density of features in the panel and the slack density S(i, j) for the panel specifies the maximum density of dummy features that can be added to the panel; and computing a dummy feature density for the plurality of panels by, iteratively, calculating an effective feature density ρ 0 (i, j) for the plurality of panels using the feature density and a function f(i, j) that models the CMP process, wherein decreasing the effective feature density variation decreases the post-CMP topography variation; calculating a filling amount Δ(i, j) for a set of panels in the plurality of panels using a target effective feature density t(i, j), the effective feature density, and the slack density; and updating the feature density, the slack density, and the dummy feature density for the set of panels using the filling amount; wherein a convergence rate of the dummy feature density computation is increased because, in each iteration, the method updates the dummy feature density for the set of panels, instead of updating the dummy feature density for only a single panel.