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

[David Hildenbrand <david@redhat.com>]

On 23.07.21 14:51, Peter Xu wrote:
> 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.

I'd expect it never ever happens.

-- 
Thanks,

David / dhildenb