libvirt users - Jan 2016 - Re: starting a domain only when you have enough resources

Hi all,

I am running KVM on a 3.18 kernel. The system runs and Atom processor with
2Gb RAM.

Using KVM you obviously can over allocate your resources: say you have 4
guests each configured with 1GB ram. Running all four at the same time,
depending on the workload, can crash the system - I get a kernel trace when
this happens.

But let's consider a simpler case: one guest with 1.5 Gb RAM, ubuntu 14.03.
During the installation the system will again crash.
The memory statistics (top or proc/meminfo) will show that the FreeMemory
goes down to 12Mb when this happens - which kind of makes sense considering
the host will require some RAM to run.

But the question is: does libvirt offer any way to prevent this from
happening?

Some way of not allowing the user to start a guest unless you have enough
free memory. I know how much ram each guest has configured but that is not
enough. I need to know how much the system has available, and just reading
the free memory statistic does not help much since that is only a snapshot
- when running a guest you can have 1gb free now, and 10 mb free 2 min
later.

Any ideas?

Thanks,
Andrei

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On 26.01.2016 12:30, Andrei Perietanu wrote:
> Hi all,
> 
> I am running KVM on a 3.18 kernel. The system runs and Atom processor with
> 2Gb RAM.
> 
> Using KVM you obviously can over allocate your resources: say you have 4
> guests each configured with 1GB ram. Running all four at the same time,
> depending on the workload, can crash the system - I get a kernel trace when
> this happens.
> 
> But let's consider a simpler case: one guest with 1.5 Gb RAM, ubuntu
14.03.
> During the installation the system will again crash.
> The memory statistics (top or proc/meminfo) will show that the FreeMemory
> goes down to 12Mb when this happens - which kind of makes sense considering
> the host will require some RAM to run.
> 
> But the question is: does libvirt offer any way to prevent this from
> happening?
> 
> Some way of not allowing the user to start a guest unless you have enough
> free memory. I know how much ram each guest has configured but that is not
> enough. I need to know how much the system has available, and just reading
> the free memory statistic does not help much since that is only a snapshot
> - when running a guest you can have 1gb free now, and 10 mb free 2 min
> later.
> 
> Any ideas?
There is one option I see, use -mem-prealloc. Either you can passthrough
it onto qemu commandline [1] or use locked memoryBacking [2]. I advocate
for the latter though. Not only it will allocate all the memory at qemu
startup it will also lock it so it won't get swapped off.

Michal

1: http://libvirt.org/drvqemu.html#qemucommand
2: http://libvirt.org/formatdomain.html#elementsMemoryBacking

On Tue, Jan 26, 2016 at 12:39 PM, Michal Privoznik <mprivozn@redhat.com>
wrote:
> On 26.01.2016 12:30, Andrei Perietanu wrote:
> > Hi all,
> >
> > I am running KVM on a 3.18 kernel. The system runs and Atom processor
> with
> > 2Gb RAM.
> >
> > Using KVM you obviously can over allocate your resources: say you have
4
> > guests each configured with 1GB ram. Running all four at the same
time,
> > depending on the workload, can crash the system - I get a kernel trace
> when
> > this happens.
> >
> > But let's consider a simpler case: one guest with 1.5 Gb RAM,
ubuntu
> 14.03.
> > During the installation the system will again crash.
> > The memory statistics (top or proc/meminfo) will show that the
FreeMemory
> > goes down to 12Mb when this happens - which kind of makes sense
> considering
> > the host will require some RAM to run.
> >
> > But the question is: does libvirt offer any way to prevent this from
> > happening?
> >
> > Some way of not allowing the user to start a guest unless you have
enough
> > free memory. I know how much ram each guest has configured but that is
> not
> > enough. I need to know how much the system has available, and just
> reading
> > the free memory statistic does not help much since that is only a
> snapshot
> > - when running a guest you can have 1gb free now, and 10 mb free 2 min
> > later.
> >
> > Any ideas?
>
> There is one option I see, use -mem-prealloc. Either you can passthrough
> it onto qemu commandline [1] or use locked memoryBacking [2]. I advocate
> for the latter though. Not only it will allocate all the memory at qemu
> startup it will also lock it so it won't get swapped off.
>
> Michal
>
> 1: http://libvirt.org/drvqemu.html#qemucommand
> 2: http://libvirt.org/formatdomain.html#elementsMemoryBacking
>
I tried memoryBacking: I added this to the domain xml:
<memoryBacking>
  <locked/>
</memoryBacking>

And got an error when attempting to start the vm: memory locking no
supported by QEMU binary.

Aside from that I don't really understand how this helps solve my issue -
if you don't mind going a bit into details I'd appreciate it.

Thanks!
Andrei

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The information transmitted is intended only for the person or entity to 
which it is addressed and may contain confidential and/or privileged 
material. Any review, retransmission, dissemination or other use of or 
taking of any action in reliance upon this information by persons or 
entities other than the intended recipient is prohibited. If you receive 
this in error please contact the sender and delete the material from any 
computer immediately. It is the policy of Klas  Limited to disavow the 
sending of offensive material and should you consider that the material 
contained in the message is offensive you should contact the sender 
immediately and also your I.T. Manager.

Klas Telecom Inc., a Virginia Corporation with offices at 1101 30th St. NW, 
Washington, DC 20007.

Klas Limited (Company Number 163303) trading as Klas Telecom, an Irish 
Limited Liability Company, with its registered office at Fourth Floor, One 
Kilmainham Square, Inchicore Road, Kilmainham, Dublin 8, Ireland.