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From: Douglas Thomas Crosher  <dtc@scrooge.ee.swin.oz.au>
Message-Id: <199610290124.MAA25150@scrooge.ee.swin.oz.au>
Subject: Re: Request to add this to FAQ re: swap space
To: current@freebsd.org
Date: Tue, 29 Oct 1996 12:24:25 +1100 (EST)
In-Reply-To: <199610290016.TAA00994@dyson.iquest.net> from "John S. Dyson" at Oct 28, 96 07:16:00 pm
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> How much disk should I allocate for swap?
> 
> Each OS has different algorithms for swap memory allocation.  FreeBSD
> takes maximum advantage of available RAM by fairly agressively pushing
> seldom used pages out to disk.  The algorithm that is used by FreeBSD
> also requires that every anonymous page in a process be backed by an
> equivalent amount of swap space.  Additionally, due to the way that
> the algorithm works, the swap space needs to be at least as a very
> minimum, the size of physical ram.  But, ignore that statement, because
> in practical terms, the minimum amount of swap space should be MUCH larger...
...
> The best formula to use is to calculate the size of all of the processes
> that you plan to run, and use that value (with a practical minimum of
> 2 times RAM.)  Figure that a heavily used X windows workstation can easily
...
> comfortable.  And remember, you need swap space for every anonymous page
> in a process. 
...
> So, to make it simple:
> 
> 	swap-space = min(2*RAM, sum of the sizes of all processes);
> 
> as usual, add some space in for expansion and for process growth.

My experience is that freebsd can use significantly more swap space
than the maximum virtual size of processes. I see this when a process
is continuously mapping and unmapping anonymous pages from a large
mmaped region. E.g. A single process using 140M of virtual memory + a
few other small processes can use up 192M.

Perhaps:
 	swap-space = min(2*RAM, 2*sum of the sizes of all processes);

And:

... The algorithm that is used by FreeBSD also requires that every
anonymous page in a process be backed by at least an equivalent amount
of swap space, and perhaps significantly more. ...

Regards
Douglas Crosher