Glossary
Swap
Space on your startup disk that macOS uses as extra memory when RAM runs short.
What it is
When compression isn't enough, macOS writes memory pages to swap files on the startup disk and reads them back when they are needed. Apple's term in Activity Monitor is “Swap Used”.
Reading from an SSD is fast, but much slower than RAM. A little swap that stays flat is fine. Swap that keeps growing while memory pressure is yellow or red is the classic sign of too little memory for the current work.
Why it matters
Swap growth is the most direct evidence of a memory shortfall, and it adds writes to your SSD. The useful question is which apps grew while swap grew.
In Activity Monitor
“Swap Used” under the Memory Pressure graph in the Memory tab.
In iKnowMyMac
The Memory tab shows swap. Diagnosis looks for swap growth in the last hour and attributes it to the apps whose memory grew most.
sysctl vm.swapusageShows total, used and free swap.
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