Re: Design question for PV superpage support

[Mick Jordan <Mick.Jordan@sun.com>]

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On 03/02/09 10:30, Keir Fraser wrote:
> On 02/03/2009 18:03, "Dan Magenheimer" <dan.magenheimer@oracle.com> wrote:
>
>   
>>> From: Keir Fraser [mailto:keir.fraser@eu.citrix.com]
>>>
>>> It'd be nice to fall back to the case of not being able to
>>> guarantee all 2MB
>>> extents are aligned and contiguous. So for example being able
>>> to migrate to
>>> or restore on a system that currently doesn't have enough
>>> contiguous memory.
>>>       
>> Well, yes and no.  I believe the ONLY reason to use 2MB
>> pages is to achieve a significant performance advantage.
>> And I suspect emulating 2MB "virtual pages" on 4KB physical
>> pages will perform at least slightly worse than just
>> 4KB-on-4KB, true?
>>     
>
> If you make this constraint then you risk creating domains that you cannot
> always conveniently restore. Obviously you would allocate 2MB extents
> wherever possible, since that is the whole point of this drawn out exercise.
>   
Indeed, performance is the issue, less TLB misses. I'm happy to use 2MB 
pages when I can and fall back on 4K when I can't. I just want Xen not 
to fall over and save/restore to work.

Mick




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