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Subject: Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble
 hinting"
To:     Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Alexander Duyck <alexander.duyck@gmail.com>, kvm@vger.kernel.org,
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        akpm@linux-foundation.org, yang.zhang.wz@gmail.com,
        pagupta@redhat.com, riel@surriel.com, konrad.wilk@oracle.com,
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From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Date:   Thu, 25 Jul 2019 14:25:52 -0400
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On 7/25/19 12:16 PM, Alexander Duyck wrote:
> On Thu, 2019-07-25 at 11:16 -0400, Michael S. Tsirkin wrote:
>> On Thu, Jul 25, 2019 at 08:05:30AM -0700, Alexander Duyck wrote:
>>> On Thu, 2019-07-25 at 07:35 -0400, Nitesh Narayan Lal wrote:
>>>> 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@linux.intel.com>
>>>>>>>
>>>>>>> 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@linux.intel.com>
>>>>>> 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 suspect that the hinting/reporting would probably improve migration
>>> times based on the fact that from the sound of things it would just be
>>> migrated as a zero page.
>>>
>>> I don't have a good setup for testing migration though and I am not that
>>> familiar with trying to do a live migration. That is one of the reasons
>>> why I didn't want to stray too far from the existing balloon code as that
>>> has already been tested with migration so I would assume as long as I am
>>> doing almost the exact same thing to hint the pages away it should behave
>>> exactly the same.
>>>
>>>>>> 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.
>>> Us hinting on pages that are freed up via deflate wouldn't be too big of a
>>> deal. I would think that is something we could look at addressing as more
>>> of a follow-on if we ever needed to since it would just add more
>>> complexity.
>>>
>>> Really what I would like to see is the balloon itself get updated first to
>>> perhaps work with variable sized pages first so that we could then have
>>> pages come directly out of the balloon and go back into the freelist as
>>> hinted, or visa-versa where hinted pages could be pulled directly into the
>>> balloon without needing to notify the host.
>> Right, I agree. At this point the main thing I worry about is that
>> the interfaces only support one reporter, since a page flag is used.
>> So if we ever rewrite existing hinting to use the new mm
>> infrastructure then we can't e.g. enable both types of hinting.
> Does it make sense to have multiple types of hinting active at the same
> time though? That kind of seems wasteful to me. 


I agree.


> Ideally we should be able
> to provide the hints and have them feed whatever is supposed to be using
> them. So for example I could probably look at also clearing the bitmaps
> when migration is in process.
>
> Also, I am wonder if the free page hints would be redundant with the form
> of page hinting/reporting that I have since we should be migrating a much
> smaller footprint anyway if the pages have been madvised away before we
> even start the migration.
>
>> FWIW Nitesh's RFC does not have this limitation.
> Yes, but there are also limitations to his approach. For example the fact
> that the bitmap it maintains is back to being a hint rather then being
> very exact.

True.


>  As a result you could end up walking the bitmap for a while
> clearing bits without ever finding a free page.


Are referring to the overhead which will be introduced due to bitmap scanning on
very large guests?


>
>> I intend to think about this over the weekend.
> Sounds good. I'll try to get the stuff you have pointed out so far
> addressed and hopefully have v3 ready to go next week.
>
> Thanks.
>
> - Alex
>
-- 
Thanks
Nitesh