1. A computing method, comprising: running, on a plurality of compute nodes, multiple workloads that access respective sets of memory pages; calculating respective bitmaps for at least some of the workloads, wherein: (i) a bitmap of a workload is statistically indicative of a cardinality of the set of memory pages used by the workload; (ii) a union of two or more bitmaps is statistically indicative of the cardinality of a union of the sets of memory pages used by the two or more corresponding workloads; and (iii) an intersection of first and second bitmaps is statistically indicative of an overlap between respective first and second sets of memory pages used by the corresponding workloads; and deciding to migrate a selected workload from a source compute node to a destination compute node, based on one or more of the bitmaps, wherein deciding to migrate the selected workload comprises choosing one or both of the selected workload and the destination compute node, based on a selection criterion that is defined over one or more of the bitmaps and aims to maximize the overlap between the memory pages used by the selected workload and the memory pages used by existing workloads on the destination compute node.