É calculado subtraindo memória física da memória virtual:
SWAP = VIRT - RES
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para mais detalhes:
o: VIRT -- Virtual Image (kb)
The total amount of virtual memory used by the task. It includes all code, data and shared libraries
plus pages that have been swapped out and pages that have been mapped but not used.
p: SWAP -- Swapped size (kb)
Memory that is not resident but is present in a task. This is memory that has been swapped out but
could include additional non-resident memory. This column is calculated by subtracting physical mem‐
ory from virtual memory.
q: RES -- Resident size (kb)
The non-swapped physical memory a task has used.