I am trying to understand the relationship between Page Outs and Swap Used as displayed in Activity Monitor. Currently, Activity Monitor shows:

Page outs: 1.6 MB Swap used: 8.1 MB

How can there be more swap used than the amount paged out?

  • Does not directly address your question (only explains why page out can be much higher than swap), but the two numbers are not comparable, Swap is a snapshot (the current amount), Page outs is cumulative (page the same page out five times and it gets counted five times)
    – Thilo
    Dec 15, 2010 at 5:08

1 Answer 1


My understanding is this:

  • Program needs access to some data. Attempts to do so, and fails, due to lack of RAM.
  • OS takes control. There is a page-out operation, where physical RAM is copied to swap.
  • There is a page-in operation, where the required data is copied to RAM.
  • OS gives control back to program.
  • Program tries again, finds its data.

OS's are getting smart about how they choose what and when to to page in/out. So the operation counts may not be the same, and the swap space may perhaps be in use without there being page outs.

Unless of course I'm backwards on what a page-in/page-out operation is. In which case your question makes perfect sense.

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