[Xen-devel] Re: Memory fragmentation, order>0 allocation, and 4.0 dynamic RAM optimization features

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 08/02/2010 18:13, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> In a recent thread:
> 
> http://lists.xensource.com/archives/html/xen-devel/2010-02/msg00295.html
> 
> Jan Beulich points out that the memory fragmentation that results
> from Transcendent Memory ("tmem") sometimes causes problems for
> domain creation and PV migration because the shadow code requires
> order=2 allocations and the domain struct is order=4.
> 
> Though tmem accelerates fragmentation, I *think* this fragmentation
> can occur with page sharing/swapping, and possibly PoD.  In fact,
> I think it can occur even with just ballooning.
> 
> I think the domain struct issue should be relatively easy to
> resolve (though maybe with a large patch), but the shadow code
> may be much harder.

I think everything but the shadow use of order-2 allocations is pretty easy
to fix; just a case of logically carving up the multi-page structures.

I'm not sure, but suspect that fragmenting the shadow allocations may
require extra book-keeping space in the page_info structure, for example.

 -- Keir



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