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

From: Peter Lieven
Subject: Re: [Qemu-block] Block Migration and CPU throttling
Date: Wed, 13 Dec 2017 17:26:50 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

Am 12.12.2017 um 18:05 schrieb Dr. David Alan Gilbert:
> * Peter Lieven (address@hidden) wrote:
>> Am 21.09.2017 um 14:36 schrieb Dr. David Alan Gilbert:
>>> * Peter Lieven (address@hidden) wrote:
>>>> 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.
>>> Yes, that's fair;  it probably would also make sense to throttle the RAM
>>> migration during the block migration bulk stage, since the chances are
>>> it's not going to get far.  (I think in the nbd setup, the main
>>> migration process isn't started until the end of bulk).
>> Catching up with the idea of delaying ram migration until block bulk has 
>> completed.
>> What do you think is the easiest way to achieve this?
> <excavates inbox, and notices I never replied>
> I think the answer depends whether we think this is a 'special' or we
> need a new general purpose mechanism.
> If it was really general then we'd probably want to split the iterative
> stage in two somehow, and only do RAM in the second half.
> But I'm not sure it's worth it; I suspect the easiest way is:
>    a) Add a counter in migration/ram.c or in the RAM state somewhere
>    b) Make ram_save_inhibit increment the counter
>    c) Check the counter at the head of ram_save_iterate and just exit
>      if it's none 0
>    d) Call ram_save_inhibit from block_save_setup
>    e) Then release it when you've finished the bulk stage
> Make sure you still count the RAM in the pending totals, otherwise
> migration might think it's finished a bit early.

I will look into this for 2.12. Thanks for the cookbook.


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