Re: [Qemu-devel] Block Migration and CPU throttling

[Peter Lieven]

Am 19.09.2017 um 16:41 schrieb Paolo Bonzini:
> On 19/09/2017 15:36, Peter Lieven wrote:
>> Hi,
>>
>> I just noticed that CPU throttling and Block Migration don't work
>> together very well.
>> During block migration the throttling heuristic detects that we
>> obviously make no progress
>> in ram transfer. But the reason is the running block migration and not a
>> too high dirty pages rate.
>>
>> The result is that any VM is throttled by 99% during block migration.
>>
>> I wonder what the best way would be fix this. I came up with the
>> following ideas so far:
>>
>> - disable throttling while block migration is in bulk stage
>> - check if absolute number of num_dirty_pages_period crosses a threshold
>> and not if its just
>>   greater than 50% of transferred bytes
>> - check if migration_dirty_pages > 0. This slows down throttling, but
>> does not avoid it completely.
> If you can use nbd-server and drive-mirror for block migration (libvirt
> would do it), then you will use multiple sockets and be able to migrate
> block and RAM at the same time.
>
> Otherwise, disabling throttling during the bulk stage is the one that
> seems nicest and most promising.

Okay, but this can be done independently of the nbd approach.
If someone uses classic block migration and auto converge his
vserver will freeze.

I will send a patch to fix that.

Thanks,
Peter