Re: [Qemu-devel] Question regarding two variables in qemu migration code

[Dr. David Alan Gilbert]

* Jidong Xiao (address@hidden) wrote:
> On Tue, Jan 13, 2015 at 1:38 AM, Dr. David Alan Gilbert
> <address@hidden> wrote:
> > * Jidong Xiao (address@hidden) wrote:
> >> Hi,
> >
> > Hi,
> >
> >> I am looking at the qemu source code, and trying to understand the
> >> migration part. In arch_init.c, there are two variables which seems
> >> quite confusing to me,
> >>
> >> They are:
> >>
> >> static uint64_t migration_dirty_pages;
> >
> > 'migration_dirty_pages' is the number of pages that are currently known
> > that need to be sent to the destination; it goes down whenever we send
> > a page, but goes up when we sync the dirty bitmap that tells us that
> > something changed the data in the page (see migration_bitmap_sync_range )
> >
> >> static int64_t num_dirty_pages_period; // defined in function
> >> migration_bitmap_sync()
> >
> > This is looking how many pages we've noticed are now dirty within
> > a particular time - to try and get an estimate of how fast memory is 
> > changing
> > If you see migration_bitmap_sync  has an:
> >        if (end_time > start_time + 1000) {
> >
> > and inside there it uses num_dirty_pages_period to update dirty_pages_rate.
> >
> >>
> >> Can anyone kindly explain that what does these two variables mean? Thanks.
> >>
> >> -Jidong
> >
> > Dave
> > --
> Thanks Dave, your explanation is really really helpful.
> 
> But in function migration_bitmap_sync(), I see this:
> 
>     num_dirty_pages_period += migration_dirty_pages - num_dirty_pages_init;
> 
> If as you said, num_dirty_pages_period refers to the pages get dirty
> within a particular time, then why it is "+=???, instead of "="? i.e.,
> something like this:
> 
>     num_dirty_pages_period = migration_dirty_pages - num_dirty_pages_init;
> 
> I just don't see why num_dirty_pages_period has to be accumulated with
> its previous value.

It's because there are two periods:
   migration_dirty_pages - num_dirty_pages_init
             is the number of pages found dirty in this call to 
migration_bitmap_sync

   num_dirty_pages_period
             is the number of pages found dirty in a ~1 second periods, as 
tested by
             the code in migration_bitmap_sync after the:
                 if (end_time > start_time + 1000) {

migration_bitmap_sync is called multiple times within that ~1 second period, and
num_dirty_pages_period accumulates across the calls.

Dave

> 
> -Jidong
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK