[ovirt-devel] [ovirt-users] [SCALE][RFC] ksmd bound to one core

[Daniel Helgenberger]

On 05.11.2014 21:07, Adam Litke wrote:
> On 04/11/14 13:11 +0100, Sven Kieske wrote:
>> Hi,
>>
>> currently ksmd is a single process
>> and is thus bound to one core.
>>
>> This leads to some scaling problems such as:
>>
>> If you got a lot of vms on one host with huge amounts
>> of ram you can observe that the cpu usage by ksmd
>> goes easily to 100%.
I wonder what would be the benefit here... I think spending CPU cycles 
on something like memory compression is not what (most) users would do. 
Already I think this is an annoyance; maybe thats why the process is 
niced to +5. A multi process daemon would require careful confining 
(with cgroups).
To my understanding the for KSM to really work well is to have many 
(idle / high mem) guests which are quite similar?

>>
>> I wonder if ksmd could not be split up
>> in child/worker threads, thus enabling higher density
>> of vms on one host.
>
> It's likely going to be trickier than you imagine with the added
> locking that would be required to synchronize the ksmd threads.
Actually I was refraining to answer Svens question since I lack some 
knowledge here. But I guessed synchronization was the reason for the 
single threadted design.
Some serous work needs to be done on KSM do achieve this goal. I was 
thinking about some map/reduce aglo to do that... OTOH mangeling with 
mem pages of guests very requires special care.
>
>>
>> or can this just be tweaked by altering values in
>> /etc/ksmtuned.conf ?
>
> I don't think it can.
>
>> What do you think?
>
> Interesting idea.
>

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Daniel Helgenberger
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