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    SubjectRe: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to handle THP spilt issue
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    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 guess Hui Zhu now has something to look into/work on :)

    --
    Thanks,

    David / dhildenb

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