Open Stack

----- Original Message -----
> From: "Jean-Philippe Methot" <jp.methot at planethoster.info>
> To: "Edmund Rhudy" <erhudy at bloomberg.net>
> Cc: openstack-operators at lists.openstack.org
> Sent: Thursday, March 23, 2017 3:49:26 PM
> Subject: Re: [Openstack-operators] Memory usage of guest vms, ballooning and nova
> 
> 
> On 2017-03-23 15:15, Edmund Rhudy (BLOOMBERG/ 120 PARK) wrote:
> > What sort of memory overcommit value are you running Nova with? The
> > scheduler looks at an instance's reservation rather than how much
> > memory is actually being used by QEMU when making a decision, as far
> > as I'm aware (but please correct me if I am wrong on this point). If
> > the HV has 128GB of memory, the instance has a reservation of 96GB,
> > you have 16GB reserved via reserved_host_memory_mb,
> > ram_allocation_ratio is set to 1.0, and you try to launch an instance
> > from a flavor with 32GB of memory, it will fail to pass RamFilter in
> > the scheduler and the scheduler will not consider it a valid host for
> > placement. (I am assuming you are using FilterScheduler still, as I
> > know nothing about the new placement API or what parts of it do and
> > don't work in Newton.)
> The overcommit value is set to 1.5 in the scheduler. It's not the
> scheduler that was preventing the instance from being provisionned, it
> was qemu returning that there was not enough ram when libvirt was trying
> to provision the instance (that error was not handled well by openstack,
> btw, but that's something else). So the instance does pass every filter.
> It just ends up in error when getting provisioned in the compute node
> because of a lack of ram, with the actual full error message only
> visible in the QEMU logs.
> > As far as why the memory didn't automatically get reclaimed, maybe KVM
> > will only reclaim empty pages and memory fragmentation in the guest
> > prevented it from doing so? It might also not actively try to reclaim
> > memory unless it comes under pressure to do so, because finding empty
> > pages and returning them to the host may be a somewhat time-consuming
> > operation.
> 
> That's entirely possible, but according to the doc, libvirt is supposed
> to have a memory balloon function that does the operation of reclaiming
> empty pages from guest processes, or so I understand. Now, how this
> function works is not exactly clear to me, or even if nova uses it or
> not. Another user suggested it might not be automatic, which is in
> accordance to what you're conjecturing.

As a general rule Libvirt provides an interface to facilitate various actions on the guest, but does not perform them without intervention - that is generally it needs to be triggered to do something either by a management layer (OpenStack, oVirt, virt-manager, Boxes, etc.) or explicit call from the operator (e.g. via virsh).

In this case as Chris noted while the memory stats are exposed by default, and while Libvirt exposes an API for interacting with the balloon, there is no process in Nova currently - or commonly deployed with it - that will actually exercise the ballooning mechanism to expand/contract the memory balloon. In oVirt/RHEV the traditional way to do it was using Memory Overcommitt Manager (MOM) to define and apply policies for managing it - the guest also needs to have a driver for the virtio balloon device IIRC. 

Such things have been proposed in the past [2] in OpenStack but never made it to implementation to my knowledge though as you've discovered it still seems like something that is generally desirable.

Thanks,

Steve

[1] http://www.ovirt.org/develop/projects/mom/
[2] https://blueprints.launchpad.net/nova/+spec/libvirt-memory-ballooning


> > From: jp.methot at planethoster.info
> > Subject: Re: [Openstack-operators] Memory usage of guest vms,
> > ballooning and nova
> >
> >     Hi, This is indeed linux, CentOS 7 to be more precise, using
> >     qemu-kvm as hypervisor. The used ram was in the used column. While
> >     we have made adjustments by moving and resizing the specific guest
> >     that was using 96 GB (verified in top), the ram usage is still
> >     fairly high for the amount of allocated ram. Currently the ram
> >     usage looks like this : total used free shared buff/cache
> >     available Mem: 251G 190G 60G 42M 670M 60G Swap: 952M 707M 245M I
> >     have 188.5GB of ram allocated to 22 instances on this node. I
> >     believe it's unrealistic to think that all these 22 instances have
> >     cached/are using up all their ram at this time. On 2017-03-23
> >     13:07, Kris G. Lindgren wrote: > Sorry for the super stupid
> >     question. > > But if this is linux are you sure that the memory is
> >     not actually being consumed via buffers/cache? > > free -m > total
> >     used free shared buff/cache available > Mem: 128751 27708 2796
> >     4099 98246 96156 > Swap: 8191 0 8191 > > Shows that of 128GB 27GB
> >     is used, but buffers/cache consumes 98GB of ram. > >
> >     ___________________________________________________________________
> >     > Kris Lindgren > Senior Linux Systems Engineer > GoDaddy > > On
> >     3/23/17, 11:01 AM, "Jean-Philippe Methot"
> >     <jp.methot at planethoster.info <mailto:jp.methot at planethoster.info>>
> >     wrote: > > Hi, > > Lately, on my production openstack Newton
> >     setup, I've ran into a > situation that defies my assumptions
> >     regarding memory management on > Openstack compute nodes and I've
> >     been looking for explanations. > Basically, we had a VM with a
> >     flavor that limited it to 96 GB of ram, > which, to be quite
> >     honest, we never thought we could ever reach. This is > a very
> >     important VM where we wanted to avoid running out of memory at >
> >     all cost. The VM itself generally uses about 12 GB of ram. > > We
> >     were surprised when we noticed yesterday that this VM, which has
> >     been > running for several months, was using all its 96 GB on the
> >     compute host. > Despite that, in the guest, the OS was indicating
> >     a memory usage of > about 12 GB. The only explanation I see to
> >     this is that at some point in > time, the host had to allocate all
> >     the 96GB of ram to the VM process and > it never took back the
> >     allocated ram. This prevented the creation of > more guests on the
> >     node as it was showing it didn't have enough memory left. > > Now,
> >     I was under the assumption that memory ballooning was integrated >
> >     into nova and that the amount of allocated memory to a specific
> >     guest > would deflate once that guest did not need the memory.
> >     After > verification, I've found blueprints for it, but I see no
> >     trace of any > implementation anywhere. > > I also notice that on
> >     most of our compute nodes, the amount of ram used > is much lower
> >     than the amount of ram allocated to VMs, which I do > believe is
> >     normal. > > So basically, my question is, how does openstack
> >     actually manage ram > allocation? Will it ever take back the
> >     unused ram of a guest process? > Can I force it to take back that
> >     ram? > > -- > Jean-Philippe Méthot > Openstack system
> >     administrator > PlanetHoster inc. > www.planethoster.net
> >     <http://www.planethoster.net> > > >
> >     _______________________________________________ >
> >     OpenStack-operators mailing list >
> >     OpenStack-operators at lists.openstack.org
> >     <mailto:OpenStack-operators at lists.openstack.org> >
> >     http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-operators
> >     > > -- Jean-Philippe Méthot Openstack system administrator
> >     PlanetHoster inc. www.planethoster.net
> >     <http://www.planethoster.net>
> >     _______________________________________________
> >     OpenStack-operators mailing list
> >     OpenStack-operators at lists.openstack.org
> >     <mailto:OpenStack-operators at lists.openstack.org>
> >     http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-operators