[Xen-devel] Re: [PATCH] Add hypercall to mark superpages to improve performance

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 30/04/2010 12:43, "Dave McCracken" <dcm@xxxxxxxx> wrote:

>> Finally, does this really require new hypercalls? Could there not instead
>>  be an always-enabled robust method for Xen to do superpage tracking?
> 
> I'm open to alternative suggestions on how to lock superpages into writable
> state once they're mapped without having to touch each individual page, even
> on the first map/unmap.  We could refcount superpage mappings in the base page
> of each superpage and then whenever a small page is mapped check its base
> page, but that would require an additional refcounted field in struct
> page_info.  I figured that would not be considered acceptable.

One option would be an array of reference counts indexed by superpage number
(i.e, mfn>>9). So kind of a separate array to page_info, and a non-zero
superpage refcount would arrange to hold a reference on every relevant page
in page_info.

That could be implemented with no extra hypercalls, and I reckon it's
probably easier to make this race-free too. Obviously it does have extra
code complexity to construct this array (which I suppose needs to be sparse,
just like page_info array, in the face of very sparse memory maps). The
space overhead (about 8 bytes per 2MB, or 0.0004% of total system memory)
would be trivial. Compared with an extra reference count in every page_info,
which would have a much higher 0.2% overhead.

 -- Keir



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