Swapping when memory is idle??

[RW]

On Thu, 20 Aug 2020 14:16:08 +0100
Steve O'Hara-Smith wrote:

> On Thu, 20 Aug 2020 10:53:27 +0300
> Odhiambo Washington <odhiambo at gmail.com> wrote:
> 
> > I have a machine with 16GB RAM and not heavily used.
> > I see from `top` some things that I do not understand well.
> > Why would a system use swap when memory is idle?  
> 
> 	The system will push pages out to swap in response to memory
> pressure, trying to choose the pages least likely to be used soon.
> Once the memory pressure recovers there will be free memory again -
> and the pressure may only last a tiny amount of time. However once a
> page is pushed out to swap it will stay there until something tries
> to use it ...

Also it's not an either/or. Pages that are read back in don't free-up
swap space, instead they become "clean pages" which can reallocated
instantaneously. 

Writing to swap in the first place is mainly done to create clean
pages rather than free pages.