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

[Li, Liang Z]

> > > > 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