[fedora-virt] F12 and ksm/ksmtuned: info required...

[Dan Kenigsberg]

On Tue, Nov 24, 2009 at 11:53:46AM +0100, Gianluca Cecchi wrote:
> Hello,
> F12 x86_64 with Qemu/KVM booted yesterday evening.
> It has two guests with CentOS 5.3 x86_64 configured to auto-start and
> now they are running...
> I see this morning, some minutes ago:
> [root at virtfed ~]# service ksm status
> ksm is not running
> 
> My config:
> [root at virtfed ~]# chkconfig --list|grep ksm
> ksm            	0:off	1:off	2:on	3:on	4:on	5:on	6:off
> ksmtuned       	0:off	1:off	2:on	3:on	4:on	5:on	6:off
> 
> [root at virtfed ~]# cat /sys/kernel/mm/ksm/pages_shared
> 3786
> 
> Then I start two other guests, both with CentOS 5.4 x86_64.
> After a while:
>  [root at virtfed ~]# cat /sys/kernel/mm/ksm/pages_shared
> 4752
> [root at virtfed ~]# cat /sys/kernel/mm/ksm/pages_shared
> 5219
> ...
> [root at virtfed ~]# cat /sys/kernel/mm/ksm/pages_shared
> 90895
> 
> and now, without any action at my side:
> [root at virtfed ~]# service ksm status
> ksm is running
> 
> Is this normal/expected?
> Any deeper doc/link about ksm/ksmtuned logic?
> Is it ksmtuned that spins ksm as it is needed? Any log file to check/configure?
> I read https://fedoraproject.org/wiki/Features/KSM but probably I'm
> missing something...

yes, the purpose of ksmtuned is to tune ksm parameters, or stop it
altogether if it is not needed. In case your two virtual machines
required less than available memeory, ksm is not used. If ksmtune senses
that memory stress has risen, it fires up ksm again.

I don't think this is written anywhere but in the code.. We probably
should add something to the "User experience" section to that feature page.
How about:

Fedora's kvm comes with 2 services controlling the behavior of ksm. One,
simply called ksm, is just a nice means to start and stop ksm's kernel
thread. The other, called ksmtuned, controls the first service and tunes
its parameters according to the memory stress that is generated by KVM
virtual machines.

> 
> Thanks,
> Gianluca
> 
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