Re: Xen balloon driver improvement (version 1)

[David Vrabel <david.vrabel@citrix.com>]

On 23/10/14 12:42, Andrew Cooper wrote:
> On 23/10/14 12:00, Ian Campbell wrote:
>> On Wed, 2014-10-22 at 19:29 +0100, Wei Liu wrote:
>>
>>>>> For instance, balloon driver can maintain three queues:
>>>>>
>>>>> 1. queue for 2 MB pages
>>>>> 1. queue for 4 KB pages (delegated to core balloon driver)
>>>>> 1. queue for pages used to mapped pages from other domain
>>>> What about 1GB pages?
>>>>
>>> I wouldn't bother with 1GB pages here.
>> Guests which don't have special privileges are limited to 2M contiguous
>> allocations anyway, to stop them from consuming "precious" higher order
>> mappings.
>>
>> I'm not sure that's still worthwhile (e.g. is it valid on ARM or
>> shadow/HAP x86? I'm not sure).
>>
>>> It would require too much work to coalesce 4KB pages to 1GB pages. 
>> FWIW in practice You'd probably coalesce 4K pages into 2M  and then 2M
>> into 1G.
>>
>>
> 
> Wouldn't it be wonderful to be able to run a 1GB PVH guest on a 1GB HAP
> mapping, with the PVH guest making use of 2MB mapping where possible.
> 
> PVH (ought) to be able to do away with the MTRR caching issues, the
> legacy IO regions, and so long as the guest doesn't balloon pages out or
> map a foreign grant, it won't shatter the host superpage.
> 
> But in principle, I agree that making better use of 2MB pages is more
> important than considering 1GB pages at the moment.

IMO, if you're that concerned about eking out the last bit of
performance out of a guest you probably won't be ballooning.

David