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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Jidong Xiao (jidong.xiao@gmail.com) 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
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK