Re: [Qemu-devel] [PATCH] numa: warn if numa 'mem' option or default RAM splitting between nodes is used.

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 7 Mar 2019 10:04:56 +0000
Daniel P. Berrangé <berrange@redhat.com> wrote:

> On Wed, Mar 06, 2019 at 07:48:22PM +0100, Igor Mammedov wrote:
> > On Wed, 6 Mar 2019 17:10:37 +0000
> > Daniel P. Berrangé <berrange@redhat.com> wrote:
> >   
> > > On Wed, Mar 06, 2019 at 05:58:35PM +0100, Igor Mammedov wrote:  
> > > > On Wed, 6 Mar 2019 16:39:38 +0000
> > > > Daniel P. Berrangé <berrange@redhat.com> wrote:
> > > >   
> > > > > On Wed, Mar 06, 2019 at 05:30:25PM +0100, Igor Mammedov wrote:  
> > > > > > Ammend -numa option docs and print warnings if 'mem' option or default RAM
> > > > > > splitting between nodes is used. It's intended to discourage users from using
> > > > > > configuration that allows only to fake NUMA on guest side while leading
> > > > > > to reduced performance of the guest due to inability to properly configure
> > > > > > VM's RAM on the host.
> > > > > > 
> > > > > > In NUMA case, it's recommended to always explicitly configure guest RAM
> > > > > > using -numa node,memdev={backend-id} option.
> > > > > > 
> > > > > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > > > > ---
> > > > > >  numa.c          |  5 +++++
> > > > > >  qemu-options.hx | 12 ++++++++----
> > > > > >  2 files changed, 13 insertions(+), 4 deletions(-)
> > > > > > 
> > > > > > diff --git a/numa.c b/numa.c
> > > > > > index 3875e1e..c6c2a6f 100644
> > > > > > --- a/numa.c
> > > > > > +++ b/numa.c
> > > > > > @@ -121,6 +121,8 @@ static void parse_numa_node(MachineState *ms, NumaNodeOptions *node,
> > > > > >  
> > > > > >      if (node->has_mem) {
> > > > > >          numa_info[nodenr].node_mem = node->mem;
> > > > > > +        warn_report("Parameter -numa node,mem is obsolete,"
> > > > > > +                    " use -numa node,memdev instead");  
> > > > > 
> > > > > I don't think we should do this. Libvirt isn't going to stop using this
> > > > > option in the near term. When users see warnings like this in logs  
> > > > well when it was the only option available libvirt had no other choice,
> > > > but since memdev became available libvirt should try to use it whenever
> > > > possible.  
> > > 
> > > As we previously discussed, it is not possible for libvirt to use it
> > > in all cases.
> > >   
> > > >   
> > > > > they'll often file bugs reports thinking something is broken which is
> > > > > not the case here.   
> > > > It's the exact purpose of the warning, to force user asking questions
> > > > and fix configuration, since he/she obviously not getting NUMA benefits
> > > > and/or performance-wise  
> > > 
> > > That's only useful if it is possible to do something about the problem.
> > > Libvirt wants to use the new option but it can't due to the live migration
> > > problems. So this simply leads to bug reports that will end up marked
> > > as CANTFIX.  
> > The problem could be solved by user though, by reconfiguring and restarting
> > domain since it's impossible to (at least as it stands now wrt migration).
> >   
> > > I don't believe libvirt actually  suffers from the performance problem
> > > you describe wrt lack of pinning.   When we attempt to pin guest NUMA
> > > nodes to host NUMA nodes, libvirt *will* use "memdev". IIUC, we
> > > use "mem" in the case where there /no/ requested pinning of guest
> > > NUMA nodes, and so we're not suffering from the limitations of "mem"
> > > in that case.  
> > What would be the use-case for not pinning numa nodes?
> > If user isn't asking for pinning, VM would run with degraded performance and
> > it would be better of being non-numa.  
> 
> The guest could have been originally booted on a host which has 2 NUMA
> nodes and have been migrated to a host with 1 NUMA node, in which case
> pinnning is not relevant.
> 
> For CI purposes too it is reasonable to create guests with NUMA configurations
> that bear no resemblance to the host NUMA configuration. This allows for testing
> the operation of guest applications. This is something that is relevant to
> OpenStack for testnig Nova's handling of NUMA placement logic, since almost
> all their testing is in VMs not bare metal.
>
> Not pinning isn't common, but it is reasonable to do it.

 
 
> > Even if user doesn't ask for pinning, non-pinned memdev (for new VMs where
> > available) could be used as well. Users of 'mem' with new QEMU will see the warning
> > and probably think about if they are configured VM correctly.  
> 
> We can't change to memdev due to migration problems. Since there is no
> functional problem with using 'mem' in this scenario there's no pressing
> reason to stop using 'mem'.
The problem with it is that it blocks fixing bug in QEMU for the multi node case
 '-numa node,mem + -mem-path + -mem-prealloc + -object "memdev",policy=bind'
and it also gets in the way of unifying RAM handling using device-memory
for initial RAM due to the same migration issue since RAM layout in migration
stream is different.

> > Desire to deprecate 'mem' at least for new machines is to be able to prevent user
> > from creating broken configs and manage all RAM in the same manner (frontend/backend).
> > (I can't do it for old machine types but for new it is possible).  
> 
> As before libvirt can't make its CLI config dependant on machine type
> version as that breaks live migration to old libvirt versions.
> 
> Regards,
> Daniel