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

[Li, Liang Z]

> * Wei Yang (address@hidden) wrote:
> > On Wed, Mar 23, 2016 at 06:48:22AM +0000, Li, Liang Z wrote:
> > [...]
> > >> > 8. Pseudo code
> > >> > Dirty page logging should be enabled before getting the free page
> > >> > information from guest, this is important because during the
> > >> > process of getting free pages, some free pages may be used and
> > >> > written by the guest, dirty page logging can trace these pages.
> > >> > The pseudo code is like below:
> > >> >
> > >> >     -----------------------------------------------
> > >> >     MigrationState *s = migrate_get_current();
> > >> >     ...
> > >> >
> > >> >     memory_global_dirty_log_start();
> > >> >
> > >> >     if (get_guest_mem_info(&info)) {
> > >> >         while (!get_free_page_bmap(free_page_bitmap,
> > >> > drop_page_cache)
> > >> &&
> > >> >                s->state != MIGRATION_STATUS_CANCELLING) {
> > >> >             usleep(1000) // sleep for 1 ms
> > >> >         }
> > >> >
> > >> >         tighten_free_page_bmap =
> > >> tighten_guest_free_pages(free_page_bitmap);
> > >> >         filter_out_guest_free_pages(tighten_free_page_bmap);
> > >> >     }
> > >>
> > >> Given the typical speed of networks; it wouldn't do too much harm
> > >> to start sending assuming all pages are dirty and then when the
> > >> guest finally gets around to finishing the bitmap then update, so
> > >> it's asynchronous - and then if the guest never responds we don't really
> care.
> > >
> > >Indeed, thanks!
> > >
> >
> > This is interesting. By doing so, the threshold I mentioned in another
> > mail is not necessary, since we can do it in parallel.
> 
> Actually I just realised it's a little more complex; we can't sync the dirty
> bitmap again from the guest until after we've received the guests 'free'
> bitmap; that's because we wouldn't know if a 'dirty' page reflected that a
> page declared as 'free' had now been reused - so there is still an ordering
> there.
> 
> Dave

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?

Liang
> 
> >
> > >Liang