Re: [RFC for Linux] virtio_balloon: Add VIRTIO_BALLOON_F_THP_ORDER to handle THP spilt issue

[Michael S. Tsirkin]

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?

-- 
MST