A set associative cache is managed by a memory controller which places writeback instructions for modified (dirty) cache lines into a virtual write queue, determines when the number of the sets containing a modified cache line is greater than a high water mark, and elevates a priority of the writeback instructions over read operations. The controller can return the priority to normal when the number of modified sets is less than a low water mark. In an embodiment wherein the system memory device includes rank groups, the congruence classes can be mapped based on the rank groups. The number of writes pending in a rank group exceeding a different threshold can additionally be a requirement to trigger elevation of writeback priority. A dirty vector can be used to provide an indication that corresponding sets contain a modified cache line, particularly in least-recently used segments of the corresponding sets.