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Date:   Thu, 25 Jul 2019 13:19:45 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc:     Nitesh Narayan Lal <nitesh@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>,
        kvm@vger.kernel.org, david@redhat.com, dave.hansen@intel.com,
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        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"
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On Thu, Jul 25, 2019 at 09:16:21AM -0700, 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. 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.

Good points.

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

For sure.

> > 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