Re: [Qemu-devel] Re: [PATCH 10/10] Maintaing number of dirty pages

[Avi Kivity]

On 11/30/2010 04:46 PM, Juan Quintela wrote:
> Anthony Liguori<address@hidden>  wrote:
> >  On 11/23/2010 05:03 PM, Juan Quintela wrote:
> >>  From: Juan Quintela<address@hidden>
> >>
> >>  Calculate the number of dirty pages takes a lot on hosts with lots
> >>  of memory.  Just maintain how many pages are dirty.  Only sync bitmaps
> >>  if number is small enough.
> >>
> >
> >  There needs to be numbers that justify this as part of the commit message.
>
> They are on patch 0/6.
>
> Additionally, with 64GB of RAM, this bitmap is HUGE, having to walk over
> it in each ram_save_live() call is too onerous.

It's not so huge.  It's scaled down by a factor of 8 * 4096 = 32K.  So 
it's a 2MB bitmap.  If kept as a bitmap and accessed in longs, it can be 
read in less than a millisecond.

An optimization can be to look at the previous ram_save_live (which had 
to walk the bitmap).  If old_nr_dirty > 4*target_nr_dirty, assume we 
need one more pass and don't scan the bitmap.

Another optimization is to stop the count when we reach the target; 
instead of ram_save_remaining() have a ram_save_may_stop() which counts 
the number of dirty bits until it reaches target_nr_dirty or exhausts 
the bitmap.

--
error compiling committee.c: too many arguments to function