[libvirt] [PATCH] numa: warn if numa 'mem' option or default RAM splitting between nodes is used.

[Daniel P. Berrangé]

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 at 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 at 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 +        warn_report("Parameter -numa node,mem is obsolete,"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.

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.

> I have to disagree here, I don't like ducking our head and ignoring broken
> configuration when there is working alternative. As end user I'd hate that
> problem was hidden from me.



Regards,
Daniel
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