From owner-freebsd-stable  Sun Apr  5 19:24:18 1998
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To: "David E. Tweten" <tweten@frihet.com>
cc: Dan Swartzendruber <dswartz@druber.com>, dyson@FreeBSD.ORG,
        dag-erli@ifi.uio.no, stable@FreeBSD.ORG
Subject: Re: swap-leak in 2.2.5 ? 
In-reply-to: Your message of "Sun, 05 Apr 1998 18:40:39 PDT."
             <199804060140.SAA03046@ns.frihet.com> 
From: David Greenman <dg@root.com>
Reply-To: dg@root.com
Date: Sun, 05 Apr 1998 19:22:47 -0700
Sender: owner-freebsd-stable@FreeBSD.ORG
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>dswartz@druber.com said:
>>Here's an off-the-cuff idea: since the confusing usage of swap as a
>>caching mechanism is only a performance optimization, how bogus would
>>it be to not report it.  Lie.  If my workstation has 64MB of swap set
>>up, 8 of which is being used for real backing store, and 12 of which
>>is being used to cache filesystem pages, have swapinfo lie and report
>>only 8MB in use.
>
>The 4.4 BSD interaction between physical pages used for virtual memory and 
>physical pages used for file system cache doesn't work that way, and I can't 
>imagine the FreeBSD core team adding in such a botch.  It is never a good 
>idea to send a dirty file system cache page to swap.

   ...and of course we didn't. I don't know where the confusion started on
this, but what we page out to swap is the same thing we've always paged out
to swap: 'anonymous' memory that is not backed by a file. Modified file pages
(non-COW) are backed by the file and are never written to swap.

>What you see in swap under heavy I/O load, is dirty process virtual memory 
>pages moved out of real memory to make way for an expanding file system 
>cache.  There's no reason to read them back until the process faults for 
>them; it might exit first, allowing you to just abandon them.

   Right. 

-DG

David Greenman
Core-team/Principal Architect, The FreeBSD Project

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