Re: [PATCH v3] migration: clear the memory region dirty bitmap when skipping free pages

[Peter Xu <peterx@redhat.com>]

On Fri, Jul 23, 2021 at 09:50:18AM +0200, David Hildenbrand wrote:
> On 22.07.21 19:41, Peter Xu wrote:
> > On Thu, Jul 22, 2021 at 04:51:48PM +0200, David Hildenbrand wrote:
> > > I'll give it a churn.
> > 
> > Thanks, David.
> > 
> 
> Migration of a 8 GiB VM
> * within the same host
> * after Linux is up and idle
> * free page hinting enabled
> * after dirtying most VM memory using memhog
> * keeping bandwidth set to QEMU defaults
> * On my 16 GiB notebook with other stuff running
> 
> 
> Current upstream with 63268c4970a, without this patch:
> 
> total time: 28606 ms
> downtime: 33 ms
> setup: 3 ms
> transferred ram: 3722913 kbytes
> throughput: 1066.37 mbps
> remaining ram: 0 kbytes
> total ram: 8389384 kbytes
> duplicate: 21674 pages
> skipped: 0 pages
> normal: 928866 pages
> normal bytes: 3715464 kbytes
> dirty sync count: 5
> pages-per-second: 32710
> 
> Current upstream without 63268c4970a, without this patch:
> 
> total time: 28530 ms
> downtime: 277 ms
> setup: 4 ms
> transferred ram: 3726266 kbytes
> throughput: 1070.21 mbps
> remaining ram: 0 kbytes
> total ram: 8389384 kbytes
> duplicate: 21890 pages
> skipped: 0 pages
> normal: 929702 pages
> normal bytes: 3718808 kbytes
> dirty sync count: 5
> pages-per-second: 32710
> 
> 
> Current upstream without 63268c4970a, with this patch:
> 
> total time: 5115 ms
> downtime: 37 ms
> setup: 5 ms
> transferred ram: 659532 kbytes
> throughput: 1057.94 mbps
> remaining ram: 0 kbytes
> total ram: 8389384 kbytes
> duplicate: 20748 pages
> skipped: 0 pages
> normal: 164516 pages
> normal bytes: 658064 kbytes
> dirty sync count: 4
> pages-per-second: 32710
> 
> 
> Current upstream with 63268c4970a, with this patch:
> 
> total time: 5205 ms
> downtime: 45 ms
> setup: 3 ms
> transferred ram: 659636 kbytes
> throughput: 1039.39 mbps
> remaining ram: 0 kbytes
> total ram: 8389384 kbytes
> duplicate: 20264 pages
> skipped: 0 pages
> normal: 164543 pages
> normal bytes: 658172 kbytes
> dirty sync count: 4
> pages-per-second: 32710
> 
> 
> 
> I repeated the last two measurements two times and took the "better"
> results.
> 
> Looks like this patch does it job and that 63268c4970a doesn't seem to
> degrade migration in this combination/setup significantly (if at all, we
> would have to do more measurements).

Thanks again for helping!

Just to double check: the loop in qemu_guest_free_page_hint() won't run for a
lot of iterations, right?  Looks like that only happens when over ramblock
boundaries.  Otherwise we may also want to move that mutex out of the loop at
some point because atomic looks indeed expensive on huge hosts.

-- 
Peter Xu