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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: Thu, 26 Mar 2020 08:54:04 +0100

> Am 26.03.2020 um 08:21 schrieb Michael S. Tsirkin <address@hidden>:
> 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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