Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble hinting"

[Nitesh Narayan Lal]

On 7/24/19 6:03 PM, Alexander Duyck wrote:
> On Wed, 2019-07-24 at 17:38 -0400, Michael S. Tsirkin wrote:
>> On Wed, Jul 24, 2019 at 10:12:10AM -0700, Alexander Duyck wrote:
>>> From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>>>
>>> Add support for what I am referring to as "bubble hinting". Basically the
>>> idea is to function very similar to how the balloon works in that we
>>> basically end up madvising the page as not being used. However we don't
>>> really need to bother with any deflate type logic since the page will be
>>> faulted back into the guest when it is read or written to.
>>>
>>> This is meant to be a simplification of the existing balloon interface
>>> to use for providing hints to what memory needs to be freed. I am assuming
>>> this is safe to do as the deflate logic does not actually appear to do very
>>> much other than tracking what subpages have been released and which ones
>>> haven't.
>>>
>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>> BTW I wonder about migration here.  When we migrate we lose all hints
>> right?  Well destination could be smarter, detect that page is full of
>> 0s and just map a zero page. Then we don't need a hint as such - but I
>> don't think it's done like that ATM.
> I was wondering about that a bit myself. If you migrate with a balloon
> active what currently happens with the pages in the balloon? Do you
> actually migrate them, or do you ignore them and just assume a zero page?
> I'm just reusing the ram_block_discard_range logic that was being used for
> the balloon inflation so I would assume the behavior would be the same.
I agree, however, I think it is worth investigating to see if enabling hinting
adds some sort of overhead specifically in this kind of scenarios. What do you
think?
>> I also wonder about interaction with deflate.  ATM deflate will add
>> pages to the free list, then balloon will come right back and report
>> them as free.
> I don't know how likely it is that somebody who is getting the free page
> reporting is likely to want to also use the balloon to take up memory.
I think it is possible. There are two possibilities:
1. User has a workload running, which is allocating and freeing the pages and at
the same time, user deflates.
If these new pages get used by this workload, we don't have to worry as you are
already handling that by not hinting the free pages immediately.
2. Guest is idle and the user adds up some memory, for this situation what you
have explained below does seems reasonable.
> However hinting on a page that came out of deflate might make sense when
> you consider that the balloon operates on 4K pages and the hints are on 2M
> pages. You are likely going to lose track of it all anyway as you have to
> work to merge the 4K pages up to the higher order page.
>
-- 
Thanks
Nitesh