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Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to ha

From: David Hildenbrand
Subject: Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to handle THP spilt issue
Date: Tue, 31 Mar 2020 16:34:48 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0

On 31.03.20 16:29, David Hildenbrand wrote:
> On 31.03.20 16:18, Michael S. Tsirkin wrote:
>> On Tue, Mar 31, 2020 at 04:09:59PM +0200, David Hildenbrand wrote:
>> ...
>>>>>>>>>>>>>> So if we want to address this, IMHO this calls for a new API.
>>>>>>>>>>>>>> Along the lines of
>>>>>>>>>>>>>>    struct page *alloc_page_range(gfp_t gfp, unsigned int 
>>>>>>>>>>>>>> min_order,
>>>>>>>>>>>>>>                    unsigned int max_order, unsigned int *order)
>>>>>>>>>>>>>> the idea would then be to return at a number of pages in the 
>>>>>>>>>>>>>> given
>>>>>>>>>>>>>> range.
>>>>>>>>>>>>>> What do you think? Want to try implementing that?
>> ..
>>> I expect the whole "steal huge pages from your guest" to be problematic,
>>> as I already mentioned to Alex. This needs a performance evaluation.
>>> This all smells like a lot of workload dependent fine-tuning. :)
>> So that's why I proposed the API above.
>> The idea is that *if we are allocating a huge page anyway*,
>> rather than break it up let's send it whole to the device.
>> If we have smaller pages, return smaller pages.
> Sorry, I still fail to see why you cannot do that with my version of
> balloon_pages_alloc(). But maybe I haven't understood the magic you
> expect to happen in alloc_page_range() :)
> It's just going via a different inflate queue once we have that page, as
> I stated in front of my draft patch "but with an
> optimized reporting interface".
>> That seems like it would always be an improvement, whatever the
>> workload.
> Don't think so. Assume there are plenty of 4k pages lying around. It
> might actually be *bad* for guest performance if you take a huge page
> instead of all the leftover 4k pages that cannot be merged. Only at the
> point where you would want to break a bigger page up and report it in
> pieces, where it would definitely make no difference.

I just understood what you mean :) and now it makes sense - it avoids
exactly that. Basically

1. Try to allocate order-0. No split necessary? return the page
2. Try to allocate order-1. No split necessary? return the page

up to MAX_ORDER - 1.

Yeah, I guess this will need a new kernel API.


David / dhildenb

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