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

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Message-ID: <3d2bd6cb-5cca-17ec-5ebe-73cbf39f43dd@redhat.com>
Date:   Thu, 25 Jul 2019 16:14:02 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>, kvm@...r.kernel.org,
        david@...hat.com, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, yang.zhang.wz@...il.com,
        pagupta@...hat.com, riel@...riel.com, konrad.wilk@...cle.com,
        lcapitulino@...hat.com, wei.w.wang@...el.com, aarcange@...hat.com,
        pbonzini@...hat.com, dan.j.williams@...el.com
Subject: Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble
 hinting"


On 7/25/19 4:00 PM, Alexander Duyck wrote:
> On Thu, 2019-07-25 at 14:25 -0400, Nitesh Narayan Lal wrote:
>> 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@...ux.intel.com>
>>>>>>>>>
> <snip>
>
>
>>> 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?
> Yes. One concern I have had is that for large memory footprints the RFC
> would end up having a large number of false positives on an highly active
> system. I am worried it will result in a feedback loop where having more
> false hits slows down your processing speed, and the slower your
> processing speed the more likely you are to encounter more false hits.
>
>

It is definitely an interesting thing to see, I intend to test such a scenario
with large guest before my next posting.

-- 
Thanks
Nitesh