[cap-talk] - Bellizzomi - Capabilities and Shapiro's focus, Coyotos, etc.

Mark Miller erights at gmail.com
Wed Nov 29 16:17:58 CST 2006


On 11/29/06, Jed at Webstart <donnelley1 at webstart.com> wrote:
> At 08:41 AM 11/29/2006, Mark S. Miller wrote:
> >Karp, Alan H wrote:
> > > While you can't prevent wall banging, you can prevent wall listening by
> > > removing all forms of indeterminacy, such as access to the system clock.
> > > Any process that is deterministically replayable meets this criterion.
> > > Did I get that right, MarkM?
>
> "prevent" is a strong word.  Let's see how this works.

I think I confused matters by introducing "deafening" without defining
it, and then talking about both wall listening and wall banging
together. Sorry.

The unpleasant "wall banging" metaphor: Even after you've put
prisoners in separate supposedly isolated cells, you can't prevent
prisoners from banging -- from making noises that other
hearing-enabled prisoners can listen to. Our even more unpleasant
extension of this metaphor: Yes, you can't prevent prisoners from
making noise. But for those prisoners whose hearing abilities you
don't need, you can deafen them so that they can't listen to the
noises made by the other prisoners. Alan and I are only claiming that
we can *prevent* wall listening, and only for those jobs that don't
need dangerous sources of non-determinism, such as
*) any interactive access to the outside world, or
*) the ability to read the clock, or
*) the ability to spawn internal process which can run internal races.

A word processor cannot be deafened, because it need to interact with
a user. A compiler can be deafened. I suspect most programs are in the
*cannot be deafened* category, but perhaps enough programs can be
deafened that they can be used to take up the resource utilization
slack I explained before -- if one wants to try to imperfectly inhibit
wall banging.

So, I agree that "prevent" is too strong a word regarding wall
banging. I claim it is reasonable for wall listening, but only for a
restricted category of applications.

-- 
Text by me above is hereby placed in the public domain

    Cheers,
    --MarkM


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