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.
From: [email protected]
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"
<[email protected] <mailto:[email protected]>>
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 >
[email protected]
<mailto:[email protected]> >
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
[email protected]
<mailto:[email protected]>
http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-operators
--
Jean-Philippe Méthot
Openstack system administrator
PlanetHoster inc.
www.planethoster.net
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