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

From: Nitesh Narayan Lal <nitesh@redhat.com>
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, david@redhat.com, dave.hansen@intel.com,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	akpm@linux-foundation.org, yang.zhang.wz@gmail.com,
	pagupta@redhat.com, riel@surriel.com, konrad.wilk@oracle.com,
	lcapitulino@redhat.com, wei.w.wang@intel.com,
	aarcange@redhat.com, pbonzini@redhat.com,
	dan.j.williams@intel.com
Subject: Re: [PATCH v2 QEMU] virtio-balloon: Provide a interface for "bubble hinting"
Date: Thu, 25 Jul 2019 16:14:02 -0400	[thread overview]
Message-ID: <3d2bd6cb-5cca-17ec-5ebe-73cbf39f43dd@redhat.com> (raw)
In-Reply-To: <6bee80b95885e74a5e46e3bd3e708d092b4a666f.camel@linux.intel.com>

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@linux.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