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Date:      Thu, 20 Aug 2020 19:15:42 +0100
From:      RW <rwmaillists@googlemail.com>
To:        freebsd-questions@freebsd.org
Subject:   Re: Swapping when memory is idle??
Message-ID:  <20200820191542.1ea410d8@gumby.homeunix.com>
In-Reply-To: <20200820141608.a8bb35f9dd089d9bc190253b@sohara.org>
References:  <CAAdA2WP8cr-uRgDRT_GX0faCSAFWORn9UMwqLZOWtLgXd0Hc9w@mail.gmail.com> <20200820141608.a8bb35f9dd089d9bc190253b@sohara.org>

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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@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. 






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