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

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Date:   Thu, 25 Jul 2019 13:00:07 -0700
From:   Alexander Duyck <alexander.h.duyck@...ux.intel.com>
To:     Nitesh Narayan Lal <nitesh@...hat.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 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.