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Re: self-paging

From: Bas Wijnen
Subject: Re: self-paging
Date: Wed, 7 Dec 2005 18:54:13 +0100
User-agent: Mutt/1.5.11

On Wed, Dec 07, 2005 at 11:30:44AM +0100, Emmanuel Colbus wrote:
> Jonathan S. Shapiro a ?crit :
> > If two applications are sharing a pool, they are already in direct
> > communication, and considerations of covert channels become irrelevant.
> > The thing we are sharing may be read-only. Adding the requirement for a
> > shared mutable region causes it to become read-write, and therefore a
> > communication channel.

Mind if I consider this a contradiction?  I agree with the second statement

> It uses the "accessed" bit in the page table entry : if it is 0, the page
> hasn't been accessed since the last time the keeper set it to 0, so its new
> age is "previous age + 1 keeper cycle time"; if it is 1, its new age is 0.
> At page arrival, "previous age" is set to 0 (or was set to 0 for long,
> because it is also the value for unused pages); but the keeper can't
> differentiate such a page from a recently accessed one.

This sounds like a very processor-intensive method for something which can be
notification-based.  When it's notification-based, the keeper can simply go to
sleep until something interesting happens (e.g. a page fault).  In this system
it's active all the time (well, it sleeps in between, but it will definitely
never be swapped out for long).  That is a Bad Thing IMO.


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