Re: self-paging

[ness]

Bas Wijnen wrote:
> On Wed, Nov 30, 2005 at 03:49:48PM +0100, Marcus Brinkmann wrote:
> [...]
> So strictly speaking it may not be self paging.  But it is much stronger than
> advisory.  The system is not allowed to swap out pages in a different order
> than the process indicates.
> [...]
> > It seems to me that apart from this difference, your proposal boils
> > down to: "Add noise to the system to reduce the bandwidth of covert
> > channels", which is the typical approach if all else fails.
>
>
> I wouldn't call the delays noise, but rather a low-pass filter.

I don't understand this.

> Adding noise
> is an additional option, which would be to randomly change the quota a bit
> every now and then.
>
> Thanks,
> Bas

To my view, there are several levels of (self-) paging:

- the OS completely does paging
 -> the applications don't know whether a particular page is in memory
    or not and cannot decide in what order pages are to evict
- the app indicates in what order to page out pages (but still doesn't
  know whether a particular page is in memory or not)
- the application is involved into the complete paging process
 -> the app knows what pages are in memory and what not and decides the
    order of page-out (this _can_ be realized by notizing the
    application on memory pressure and requesting it to evict a page)

I think we have a covert channel whenever an application knows whether a 
particular page is in memory or not (because it then can count the pages 
in memory and this is sth. the OS has to change on the behaviour of 
other applications).
So there still is a covert channel in your proposal (it only needs some 
time to transfer the "messages").
--
-ness-