Re: [Qemu-devel] [RFC Design Doc]Speed up live migration by skipping free pages

[Michael S. Tsirkin]

On Thu, Mar 24, 2016 at 05:49:33PM +0000, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (address@hidden) wrote:
> > On Thu, Mar 24, 2016 at 04:05:16PM +0000, Li, Liang Z wrote:
> > > 
> > > 
> > > On %D, %SN wrote:
> > > %Q
> > > 
> > > %C
> > > 
> > > Liang
> > > 
> > > 
> > > > -----Original Message-----
> > > > From: Michael S. Tsirkin [mailto:address@hidden
> > > > Sent: Thursday, March 24, 2016 11:57 PM
> > > > To: Li, Liang Z
> > > > Cc: Dr. David Alan Gilbert; Wei Yang; address@hidden;
> > > > address@hidden; address@hidden; address@hidden;
> > > > address@hidden; address@hidden; address@hidden;
> > > > address@hidden; address@hidden;
> > > > address@hidden; address@hidden; address@hidden;
> > > > address@hidden
> > > > Subject: Re: [RFC Design Doc]Speed up live migration by skipping free 
> > > > pages
> > > > 
> > > > On Thu, Mar 24, 2016 at 03:53:25PM +0000, Li, Liang Z wrote:
> > > > > > > > > Not very complex, we can implement like this:
> > > > > > > > >
> > > > > > > > > 1. Set all the bits in the migration_bitmap_rcu->bmap to 1 2.
> > > > > > > > > Clear all the bits in ram_list.
> > > > > > > > > dirty_memory[DIRTY_MEMORY_MIGRATION]
> > > > > > > > > 3. Send the get_free_page_bitmap request 4. Start to send
> > > > > > > > > pages to destination and check if the free_page_bitmap is 
> > > > > > > > > ready
> > > > > > > > >     if (is_ready) {
> > > > > > > > >           filter out the free pages from  
> > > > > > > > > migration_bitmap_rcu->bmap;
> > > > > > > > >           migration_bitmap_sync();
> > > > > > > > >     }
> > > > > > > > >      continue until live migration complete.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > Is that right?
> > > > > > > >
> > > > > > > > The order I'm trying to understand is something like:
> > > > > > > >
> > > > > > > >     a) Send the get_free_page_bitmap request
> > > > > > > >     b) Start sending pages
> > > > > > > >     c) Reach the end of memory
> > > > > > > >       [ is_ready is false - guest hasn't made free map yet ]
> > > > > > > >     d) normal migration_bitmap_sync() at end of first pass
> > > > > > > >     e) Carry on sending dirty pages
> > > > > > > >     f) is_ready is true
> > > > > > > >       f.1) filter out free pages?
> > > > > > > >       f.2) migration_bitmap_sync()
> > > > > > > >
> > > > > > > > It's f.1 I'm worried about.  If the guest started generating the
> > > > > > > > free bitmap before (d), then a page marked as 'free' in f.1
> > > > > > > > might have become dirty before (d) and so (f.2) doesn't set the
> > > > > > > > dirty again, and so we can't filter out pages in f.1.
> > > > > > > >
> > > > > > >
> > > > > > > As you described, the order is incorrect.
> > > > > > >
> > > > > > > Liang
> > > > > >
> > > > > >
> > > > > > So to make it safe, what is required is to make sure no free list us
> > > > > > outstanding before calling migration_bitmap_sync.
> > > > > >
> > > > > > If one is outstanding, filter out pages before calling
> > > > migration_bitmap_sync.
> > > > > >
> > > > > > Of course, if we just do it like we normally do with migration, then
> > > > > > by the time we call migration_bitmap_sync dirty bitmap is completely
> > > > > > empty, so there won't be anything to filter out.
> > > > > >
> > > > > > One way to address this is call migration_bitmap_sync in the IO
> > > > > > handler, while VCPU is stopped, then make sure to filter out pages
> > > > > > before the next migration_bitmap_sync.
> > > > > >
> > > > > > Another is to start filtering out pages upon IO handler, but make
> > > > > > sure to flush the queue before calling migration_bitmap_sync.
> > > > > >
> > > > >
> > > > > It's really complex, maybe we should switch to a simple start,  just
> > > > > skip the free page in the ram bulk stage and make it asynchronous?
> > > > >
> > > > > Liang
> > > > 
> > > > You mean like your patches do? No, blocking bulk migration until guest
> > > > response is basically a non-starter.
> > > > 
> > > 
> > > No, don't wait anymore. Like below (copy from previous thread)
> > > --------------------------------------------------------------
> > > 1. Set all the bits in the migration_bitmap_rcu->bmap to 1 
> > > 2. Clear all the bits in ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION]
> > > 3. Send the get_free_page_bitmap request 
> > > 4. Start to send  pages to destination and check if the free_page_bitmap 
> > > is ready
> > >    if (is_ready) {
> > >      filter out the free pages from  migration_bitmap_rcu->bmap;
> > >      migration_bitmap_sync();
> > >  }
> > > continue until live migration complete.
> > > ---------------------------------------------------------------
> > > Can this work?
> > > 
> > > Liang
> > 
> > Not if you get the ready bit asynchronously like you wrote here
> > since is_ready can get set while you called migration_bitmap_sync.
> > 
> > As I said previously,
> > to make this work you need to filter out synchronously while VCPU is
> > stopped and while free pages from list are not being used.
> > 
> > Alternatively prevent getting free page list
> > and filtering them out from
> > guest from racing with migration_bitmap_sync.
> > 
> > For example, flush the VQ after migration_bitmap_sync.
> > So:
> > 
> >     lock
> >     migration_bitmap_sync();
> >     while (elem = virtqueue_pop) {
> >         virtqueue_push(elem)
> >         g_free(elem)
> >     }
> >     unlock
> > 
> > 
> > while in handle_output
> > 
> >     lock
> >     while (elem = virtqueue_pop) {
> >         list = get_free_list(elem)
> >         filter_out_free(list)
> >         virtqueue_push(elem)
> >         free(elem)
> >     }
> >     unlock
> > 
> > 
> > lock prevents migration_bitmap_sync from racing
> > against  handle_output
> 
> I think the easier way is just to ignore the guests
> free list response if it comes back after the first
> pass.
> 
> Dave

That's a subset of course - after the first pass ==
after migration_bitmap_sync.

But it's really nasty - for example, how do you know
it's the response from this migration round and
not a previous one?

It is really better to just keep things orthogonal
and not introduce arbitrary limitations.


For example, with post-copy there's no first pass,
and it can still benefit from this optimization.



> > 
> > 
> > This way you can actually use ioeventfd
> > for this VQ so VCPU won't be blocked.
> > 
> > I do not think this is so complex, and
> > this way you can add requests for guest
> > free bitmap at an arbitary interval
> > either in host or in guest.
> > 
> > For example, add a value that says how often
> > should guest update the bitmap, set it to 0
> > to disable updates after migration done.
> > 
> > Or, make guest resubmit a new one when we consume
> > the old one, run handle_output about through
> > a periodic timer on host.
> > 
> > 
> > > > --
> > > > MST
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK