Dirty vs resident vs virtual memory?

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Four metrics

Virtual — the whole range of virtual addresses the process has reserved: every segment of the binary and dylibs, the shared cache, stacks, malloc regions, mmap'd files. On a 64-bit system that is gigabytes "on paper" (addresses are cheap) — the number tells you almost nothing.
Resident (RSS) — how much of that is physically in RAM right now. It includes pages that are not "yours": system framework code shared by every process, so RSS overstates the app's guilt.
Dirty — pages the process has modified: heap, stacks, decoded images, __DATA after a write. They cannot be evicted because there is nothing to restore them from — only kept or compressed.
Compressed — dirty pages the memory compressor has squeezed and keeps in RAM in compressed form.

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