libvirt users - May 2019 - Re: Running all my virtual machines with a low priority

On Thu, May 02, 2019 at 11:33:06AM +0200, Martin Kletzander
wrote:
> On Wed, May 01, 2019 at 11:58:23AM +0200, R. Diez wrote:
> > 
> > > You would have to set the priority of all vCPUs in each guest, as
well
> > > as emulator threads in each guest (the latter needs very new
libvirt).
> > 
> > This does not seem a viable option then with Ubuntu 18.04. And
changing XML on all guest VMs is extra work.
> > 
> > Is there no other way? For example, are all such virtual machines not
some daemon or systemd unit or set of units? Would it be feasible to lower the
> > priority on all of those? Or is that not easy to do either?
> > 
> 
> I'm not sure if we're resetting all the cgroup values, so it should
be possible
> by setting this for the machine.slice cgroup (or somewhere around there)
which
> would make it effective for all the VMs, right?  I have not tested it,
though,
> sorry.
You could perhaps set  'cpu.shares' cgroup setting on /machine.slice.
This isn't really a nice level in the traditional sense, rather it is
a relative weighting evaluated against other cgroups at the same level.
So if you change it from 1024 to 512, then processes under /machine.slice
will get 1/2 the CPU time vs stuff under /system.slice or /user.slice,
when there is contention for CPU time. 

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|

> You could perhaps set  'cpu.shares' cgroup setting on
/machine.slice.
> This isn't really a nice level in the traditional sense, rather it is
> a relative weighting evaluated against other cgroups at the same level.
> So if you change it from 1024 to 512, then processes under /machine.slice
> will get 1/2 the CPU time vs stuff under /system.slice or /user.slice,
> when there is contention for CPU time.
Thanks for your help.

There is no "machine.slice" on my system. This is an excerpt from
"systemd-cgls".


??? ??????system.slice
???   ??????irqbalance.service
???   ??? ??????865 /usr/sbin/irqbalance --foreground
[... blah blah ...]
???   ??????wpa_supplicant.service
???   ??? ??????878 /sbin/wpa_supplicant -u -s -O /run/wpa_supplicant
???   ??????libvirtd.service
???   ??? ??????  987 /usr/sbin/libvirtd
???   ??? ?????? 1395 /usr/sbin/dnsmasq
--conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro
--dhcp-script=/usr/lib/libvirt/libvirt_leaseshelper
???   ??? ?????? 1396 /usr/sbin/dnsmasq
--conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro
--dhcp-script=/usr/lib/libvirt/libvirt_leaseshelper
???   ??? ??????12285 /usr/bin/qemu-system-x86_64 -name
guest=UbuntuMATE1804,debug-threads=on -S -object
secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-1-UbuntuMATE1804/master-key.aes
-machine...


But I guess there are differences depending on the libvirt version.

I thought of 'nice' because it is an easy way to lower the priority for
everything (CPU and disk) at once.

This is actually getting too complicated. I am a lone developer and sysadmin in
a small company. I would need to learn too many things about
control groups etc. One of the reasons I am trying libvirt is because of the
"Virtual Machine Manager" GUI.

Is there not an easier way? I realised there is a "libvirtd.service"
on my system. Is there a simple way in systemd to just lower the
priority of everything under that service? I am assuming that this service is
starting all the virtual machines.

Best regards,
   rdiez

On Thu, May 02, 2019 at 12:10:54PM +0200, R. Diez wrote:
>
>> You could perhaps set  'cpu.shares' cgroup setting on
/machine.slice.
>> This isn't really a nice level in the traditional sense, rather it
is
>> a relative weighting evaluated against other cgroups at the same level.
>> So if you change it from 1024 to 512, then processes under
/machine.slice
>> will get 1/2 the CPU time vs stuff under /system.slice or /user.slice,
>> when there is contention for CPU time.
>
>Thanks for your help.
>
>There is no "machine.slice" on my system. This is an excerpt from
"systemd-cgls".
>
>
>??? ??????system.slice
>???   ??????irqbalance.service
>???   ??? ??????865 /usr/sbin/irqbalance --foreground
>[... blah blah ...]
>???   ??????wpa_supplicant.service
>???   ??? ??????878 /sbin/wpa_supplicant -u -s -O /run/wpa_supplicant
>???   ??????libvirtd.service
>???   ??? ??????  987 /usr/sbin/libvirtd
>???   ??? ?????? 1395 /usr/sbin/dnsmasq
--conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro
>--dhcp-script=/usr/lib/libvirt/libvirt_leaseshelper
>???   ??? ?????? 1396 /usr/sbin/dnsmasq
--conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro
>--dhcp-script=/usr/lib/libvirt/libvirt_leaseshelper
>???   ??? ??????12285 /usr/bin/qemu-system-x86_64 -name
guest=UbuntuMATE1804,debug-threads=on -S -object
>secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-1-UbuntuMATE1804/master-key.aes
-machine...
>
Runing:

  $ systemctl set-property machine.slice CPUShares=512

works for me on some random machine I tried.  As far as I understand, Nice= was
supported (for backward compatibility) but it is not now, or at least not for
slices.

Feel free to set other things with similar commands or just use systemctl edit
machine.slice and configure that however you want.  Looking for the proper
documentation is left as an exercise for the reader.
>
>But I guess there are differences depending on the libvirt version.
>
>I thought of 'nice' because it is an easy way to lower the priority
for everything (CPU and disk) at once.
>
>This is actually getting too complicated. I am a lone developer and sysadmin
in a small company. I would need to learn too many things about
>control groups etc. One of the reasons I am trying libvirt is because of the
"Virtual Machine Manager" GUI.
>
I believe such basic settings should *not* be complicated, and I hope the above
got you closer to the goal.
>Is there not an easier way? I realised there is a
"libvirtd.service" on my system. Is there a simple way in systemd to
just lower the
>priority of everything under that service? I am assuming that this service
is starting all the virtual machines.
>
We are also counting with the use case where you want the machine to take full
advantage of the system and its resources, but you want to restrict the
management of such virtual machines (libvirt in this case), so libvirt has to
explicitly override any possibly inherited configuration for virtual machines it
is starting.  That's why setting anything on the libvirtd.service will not
work.
>Best regards,
>   rdiez