From owner-freebsd-hackers  Wed Feb 17 10:59:34 1999
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From: Terry Lambert <tlambert@primenet.com>
Message-Id: <199902171858.LAA21748@usr07.primenet.com>
Subject: Re: vm_page_zero_fill
To: dyson@iquest.net
Date: Wed, 17 Feb 1999 18:58:38 +0000 (GMT)
Cc: tlambert@primenet.com, toasty@home.dragondata.com,
	mike@smith.net.au, hackers@FreeBSD.ORG
In-Reply-To: <199902171844.NAA69882@y.dyson.net> from "John S. Dyson" at Feb 17, 99 01:44:19 pm
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> > This is robbing Peter to pay Paul; in a way.  The base assumption
> > that you are hiding is that you aren't constrained by memory
> > bandwidth.  This isn't true if you are nearly saturating a PCI
> > bus with 4 BT848's (to pick the highest memory bandwidth application
> > I know about).
>
> Prezeroing doesn't take any significant CPU if there are no cycles
> available.  It does increase latency slightly, if zeroing is allowed
> to happen.

He's strapped on memory bandwidth, not CPU cycles.  He's willing to
eat zeroing on in those cases where he has no choice because it
impacts base functionality.


> > Maybe we need to go back to first principles, and examine the
> > assumptions about what constraints are in effect under various
> > usage models, and make trades like these optional instead of
> > mandatory.  I think that's all he wants, anyway.
>
> The prezeroing isn't adding any cost to him, the ability to support
> returning non-initialized data from the kernel would be useful.  In
> that case, turning off prezeroing *might* help (but probably won't.)

Again, he's wanting to reclaim memory bandwidth from the prezeroing
of pages that are prezeroed not because they need to be, but for
security reasons that he doesn't care about.


					Terry Lambert
					terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.


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