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

[Peter Lieven]

Am 19.09.2017 um 16:41 schrieb Dr. David Alan Gilbert:
> * Peter Lieven (address@hidden) wrote:
>> Am 19.09.2017 um 16:38 schrieb Dr. David Alan Gilbert:
>>> * Peter Lieven (address@hidden) 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.
>>> Hmm that's unfortunate; do you have a bandwidth set lower than your
>>> actual network connection? I'm just wondering if it's actually going
>>> between the block and RAM iterative sections or getting stuck in ne.
>> It happens also if source and dest are on the same machine and speed is set 
>> to 100G.
> But does it happen if they're not and the speed is set low?

Yes, it does. I noticed it in our test environment between different nodes with 
a 10G
link in between. But its totally clear why it happens. During block migration 
we transfer
all dirty memory pages in each round (if there is moderate memory load), but 
all dirty
pages are obviously more than 50% of the transferred ram in that round.
It is more exactly 100%. But the current logic triggers on this condition.

I think I will go forward and send a patch which disables auto converge during
block migration bulk stage.

Thanks for your feedback,
Peter