Re: [RFC qemu 0/4] A PV solution for live migration optimization

[Michael S. Tsirkin]

On Mon, Mar 14, 2016 at 05:03:34PM +0000, Dr. David Alan Gilbert wrote:
> * Li, Liang Z (liang.z.li@xxxxxxxxx) wrote:
> > > 
> > > Hi,
> > >   I'm just catching back up on this thread; so without reference to any
> > > particular previous mail in the thread.
> > > 
> > >   1) How many of the free pages do we tell the host about?
> > >      Your main change is telling the host about all the
> > >      free pages.
> > 
> > Yes, all the guest's free pages.
> > 
> > >      If we tell the host about all the free pages, then we might
> > >      end up needing to allocate more pages and update the host
> > >      with pages we now want to use; that would have to wait for the
> > >      host to acknowledge that use of these pages, since if we don't
> > >      wait for it then it might have skipped migrating a page we
> > >      just started using (I don't understand how your series solves that).
> > >      So the guest probably needs to keep some free pages - how many?
> > 
> > Actually, there is no need to care about whether the free pages will be used by the host.
> > We only care about some of the free pages we get reused by the guest, right?
> > 
> > The dirty page logging can be used to solve this, starting the dirty page logging before getting
> > the free pages informant from guest. Even some of the free pages are modified by the guest
> > during the process of getting the free pages information, these modified pages will be traced
> > by the dirty page logging mechanism. So in the following migration_bitmap_sync() function.
> > The pages in the free pages bitmap, but latter was modified, will be reset to dirty. We won't
> > omit any dirtied pages.
> > 
> > So, guest doesn't need to keep any free pages.
> 
> OK, yes, that works; so we do:
>   * enable dirty logging
>   * ask guest for free pages
>   * initialise the migration bitmap as everything-free
>   * then later we do the normal sync-dirty bitmap stuff and it all just works.
> 
> That's nice and simple.

This works once, sure. But there's an issue is that you have
to defer migration until you get the free page list,
and this only works once. So you end up with heuristics
about how long to wait.

Instead I propose:

- mark all pages dirty as we do now.

- at start of migration, start tracking dirty
  pages in kvm, and tell guest to start tracking free pages

we can now introduce any kind of delay, for
example wait for ack from guest, or do whatever else,
or even just start migrating pages

- repeatedly:
	- get list of free pages from guest
	- clear them in migration bitmap
	- get dirty list from kvm

- at end of migration, stop tracking writes in kvm,
  and tell guest to stop tracking free pages



> > >   2) Clearing out caches
> > >      Does it make sense to clean caches?  They're apparently useful data
> > >      so if we clean them it's likely to slow the guest down; I guess
> > >      they're also likely to be fairly static data - so at least fairly
> > >      easy to migrate.
> > >      The answer here partially depends on what you want from your migration;
> > >      if you're after the fastest possible migration time it might make
> > >      sense to clean the caches and avoid migrating them; but that might
> > >      be at the cost of more disruption to the guest - there's a trade off
> > >      somewhere and it's not clear to me how you set that depending on your
> > >      guest/network/reqirements.
> > > 
> > 
> > Yes, clean the caches is an option.  Let the users decide using it or not.
> > 
> > >   3) Why is ballooning slow?
> > >      You've got a figure of 5s to balloon on an 8GB VM - but an
> > >      8GB VM isn't huge; so I worry about how long it would take
> > >      on a big VM.   We need to understand why it's slow
> > >        * is it due to the guest shuffling pages around?
> > >        * is it due to the virtio-balloon protocol sending one page
> > >          at a time?
> > >          + Do balloon pages normally clump in physical memory
> > >             - i.e. would a 'large balloon' message help
> > >             - or do we need a bitmap because it tends not to clump?
> > > 
> > 
> > I didn't do a comprehensive test. But I found most of the time spending
> > on allocating the pages and sending the PFNs to guest, I don't know that's
> > the most time consuming operation, allocating the pages or sending the PFNs.
> 
> It might be a good idea to analyse it a bit more to convince people where
> the problem is.
> 
> > >        * is it due to the madvise on the host?
> > >          If we were using the normal balloon messages, then we
> > >          could, during migration, just route those to the migration
> > >          code rather than bothering with the madvise.
> > >          If they're clumping together we could just turn that into
> > >          one big madvise; if they're not then would we benefit from
> > >          a call that lets us madvise lots of areas?
> > > 
> > 
> > My test showed madvise() is not the main reason for the long time, only taken
> > 10% of the total  inflating balloon operation time.
> > Big madvise can more or less improve the performance.
> 
> OK; 10% of the total is still pretty big even for your 8GB VM.
> 
> > >   4) Speeding up the migration of those free pages
> > >     You're using the bitmap to avoid migrating those free pages; HPe's
> > >     patchset is reconstructing a bitmap from the balloon data;  OK, so
> > >     this all makes sense to avoid migrating them - I'd also been thinking
> > >     of using pagemap to spot zero pages that would help find other zero'd
> > >     pages, but perhaps ballooned is enough?
> > > 
> > Could you describe your ideal with more details?
> 
> At the moment the migration code spends a fair amount of time checking if a page
> is zero; I was thinking perhaps the qemu could just open /proc/self/pagemap
> and check if the page was mapped; that would seem cheap if we're checking big
> ranges; and that would find all the balloon pages.
> 
> > >   5) Second-migrate
> > >     Given a VM where you've done all those tricks on, what happens when
> > >     you migrate it a second time?   I guess you're aiming for the guest
> > >     to update it's bitmap;  HPe's solution is to migrate it's balloon
> > >     bitmap along with the migration data.
> > 
> > Nothing is special in the second migration, QEMU will request the guest for free pages
> > Information, and the guest will traverse it's current free page list to construct a
> > new free page bitmap and send it to QEMU. Just like in the first migration.
> 
> Right.
> 
> Dave
> 
> > Liang
> > > 
> > > Dave
> > > 
> > > --
> > > Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
> --
> Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK