Inter-zone swapping

Inter-zone swapping

[Cesar Eduardo Barros]

(I'm a complete newbie at vm, so if I'm saying something stupid here, don't be
 so hard on me)

I've been thinking about the following scenario: by some random chance, we have
the DMA zone full of recent pages, the normal zone full of old pages, and the
highmem zone empty (I don't have that much memory). So the right place to page
out from would be the normal zone (since if we swap out from the DMA zone we'll
be swapping out a page a program will need soon). But suppose the DMA zone is
almost full, so something needs to be taken from it or we might risk not having
free memory for atomic allocations for the sound card (or other random
DMA-using driver).

Then would it be useful to "swap" a page from the DMA zone into the normal zone
(and of course after that ending up swapping from the normal zone to the disk)?

-- 
Cesar Eduardo Barros
cesarb@nitnet.com.br
cesarb@dcc.ufrj.br
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Re: Inter-zone swapping

[Stephen C. Tweedie]

Hi,

On Sat, Jul 22, 2000 at 10:27:40PM -0300, Cesar Eduardo Barros wrote:
> 
> Then would it be useful to "swap" a page from the DMA zone into the normal zone
> (and of course after that ending up swapping from the normal zone to the disk)?

Yes.  There are _lots_ of other possible applications for that sort of
non-IO-consuming relocation-style swapping, including memory
defragmentation (we really need that if we want to support things like
large page stuff on Intel boxes for user space).

Cheers,
 Stephen
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Re: Inter-zone swapping

[Mark Hahn]

> > Then would it be useful to "swap" a page from the DMA zone into the normal zone
> > (and of course after that ending up swapping from the normal zone to the disk)?
> 
> Yes.  There are _lots_ of other possible applications for that sort of
> non-IO-consuming relocation-style swapping, including memory
> defragmentation (we really need that if we want to support things like
> large page stuff on Intel boxes for user space).

doesn't good page coloring need this ability, as well, 
to move around which physical page is backing a virtual one?
guess it depends on whether the arch does cache lookups based
on p or v addresses...

thanks, mark hahn.

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Re: Inter-zone swapping

[Stephen C. Tweedie]

Hi,

On Tue, Jul 25, 2000 at 05:46:10PM -0400, Mark Hahn wrote:
> 
> doesn't good page coloring need this ability, as well, 
> to move around which physical page is backing a virtual one?
> guess it depends on whether the arch does cache lookups based
> on p or v addresses...

Yes.  Fortunately, very few archs do this the wrong way. :-)  Where
they do, page colouring is desperately important, but you can usually
do that within zones rather than having to balance between zones (it's
a similar problem but working along a different axis).

Cheers,
 Stephen
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