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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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    > Am 26.03.2020 um 08:21 schrieb Michael S. Tsirkin <mst@redhat.com>:
    >
    > On Thu, Mar 12, 2020 at 09:51:25AM +0100, David Hildenbrand wrote:
    >>> On 12.03.20 09:47, Michael S. Tsirkin wrote:
    >>> On Thu, Mar 12, 2020 at 09:37:32AM +0100, David Hildenbrand wrote:
    >>>> 2. You are essentially stealing THPs in the guest. So the fastest
    >>>> mapping (THP in guest and host) is gone. The guest won't be able to make
    >>>> use of THP where it previously was able to. I can imagine this implies a
    >>>> performance degradation for some workloads. This needs a proper
    >>>> performance evaluation.
    >>>
    >>> I think the problem is more with the alloc_pages API.
    >>> That gives you exactly the given order, and if there's
    >>> a larger chunk available, it will split it up.
    >>>
    >>> But for balloon - I suspect lots of other users,
    >>> we do not want to stress the system but if a large
    >>> chunk is available anyway, then we could handle
    >>> that more optimally by getting it all in one go.
    >>>
    >>>
    >>> 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?
    >>
    >> You can just start with the highest order and decrement the order until
    >> your allocation succeeds using alloc_pages(), which would be enough for
    >> a first version. At least I don't see the immediate need for a new
    >> kernel API.
    >
    > OK I remember now. The problem is with reclaim. Unless reclaim is
    > completely disabled, any of these calls can sleep. After it wakes up,
    > we would like to get the larger order that has become available
    > meanwhile.
    >

    Yes, but that‘s a pure optimization IMHO.

    So I think we should do a trivial implementation first and then see what we gain from a new allocator API. Then we might also be able to justify it using real numbers.

    >
    >> --
    >> Thanks,
    >>
    >> David / dhildenb
    >

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