From owner-freebsd-questions  Mon Jan 22 10:26:38 2001
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Date: Mon, 22 Jan 2001 10:25:54 -0800
From: "Crist J. Clark" <cjclark@reflexnet.net>
To: Kris Kennaway <kris@FreeBSD.org>
Cc: questions@FreeBSD.org
Subject: Re: small program eats lot of memory
Message-ID: <20010122102554.A77938@rfx-216-196-73-168.users.reflex>
Reply-To: cjclark@alum.mit.edu
References: <14955.1209.195848.394006@guru.mired.org> <01012121054701.03293@buffy> <20010121152246.X10761@rfx-216-196-73-168.users.reflex> <20010121165041.A76170@citusc17.usc.edu> <20010121203207.F10761@rfx-216-196-73-168.users.reflex> <20010122025541.B84078@citusc17.usc.edu>
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In-Reply-To: <20010122025541.B84078@citusc17.usc.edu>; from kris@FreeBSD.org on Mon, Jan 22, 2001 at 02:55:41AM -0800
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On Mon, Jan 22, 2001 at 02:55:41AM -0800, Kris Kennaway wrote:
> On Sun, Jan 21, 2001 at 08:32:07PM -0800, Crist J. Clark wrote:
> 
> > > No, I believe he is correct. If you have two shared programs in
> > > memory, the VM system will only have one copy of the libc code shared
> > > between them (similarly with any other common libraries).
> > 
> > That's really something if it's true. I don't see any mention of
> > features like that on rtld(1).
> 
> It's a generic feature of the VM system which shared libraries happen
> to incidentally benefit from. If you map the same object more than
> once, it only maintains one copy of it in VM, until one of the users
> dirties a page (changes something), at which point the page is copied
> ("copy on write").

Thanks for the followup. I did a little research and had found that
answer. As you mention, once a page is 'dirtied' it must be copied. I
realized that needed to happen, but it never occured to me you could
wait for write before you actually do the copy. I see how that can
save memory.

Shows what I know about the VM. Thanks.
-- 
Crist J. Clark                           cjclark@alum.mit.edu


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