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

Mark S. Miller markm at cs.jhu.edu
Wed Nov 29 10:41:23 CST 2006


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?

Yes thanks!

In addition, once such processes are deafened, this can contribute towards 
alleviating wall banging in an indirect and imperfect way:

The main wall banging channel is variable resource use. The main defense 
against such wall banging is preallocation of worst-case resources, and then 
wasting the slack -- the difference between actual and worst-case resource 
needs. Since typical resource needs can often be orders of magnitude less than 
worst case, this waste can be quite costly.

However, if one has a bunch of deafened processes that could do useful batch 
work, one can run these in the slack. As deafened processes, they can't sense 
how much slack they're getting, or how this slack is spread over real time.

By "batch" above, the key feature is that no one needs an answer from these 
promptly, so these answers can be delayed and aggregated. Revealing which 
answers are available at some future time does reveal some covert info, but 
hey, I said it was imperfect.

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

     Cheers,
     --MarkM


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