Re: [Qemu-devel] [libvirt] [PATCH 1/2] numa: deprecate 'mem' parameter of '-numa node' option

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 1 Mar 2019 15:49:47 +0000
Daniel P. Berrangé <berrange@redhat.com> wrote:

> On Fri, Mar 01, 2019 at 04:42:15PM +0100, Igor Mammedov wrote:
> > The parameter allows to configure fake NUMA topology where guest
> > VM simulates NUMA topology but not actually getting a performance
> > benefits from it. The same or better results could be achieved
> > using 'memdev' parameter. In light of that any VM that uses NUMA
> > to get its benefits should use 'memdev' and to allow transition
> > initial RAM to device based model, deprecate 'mem' parameter as
> > its ad-hoc partitioning of initial RAM MemoryRegion can't be
> > translated to memdev based backend transparently to users and in
> > compatible manner (migration wise).
> > 
> > That will also allow to clean up a bit our numa code, leaving only
> > 'memdev' impl. in place and several boards that use node_mem
> > to generate FDT/ACPI description from it.  
> 
> Can you confirm that the  'mem' and 'memdev' parameters to -numa
> are 100% live migration compatible in both directions ?  Libvirt
> would need this to be the case in order to use the 'memdev' syntax
> instead.
Unfortunately they are not migration compatible in any direction,
if it where possible to translate them to each other I'd alias 'mem'
to 'memdev' without deprecation. The former sends over only one
MemoryRegion to target, while the later sends over several (one per
memdev).

Mixed memory issue[1] first came from libvirt side RHBZ1624223,
back then it was resolved on libvirt side in favor of migration
compatibility vs correctness (i.e. bind policy doesn't work as expected).
What worse that it was made default and affects all new machines,
as I understood it.

In case of -mem-path + -mem-prealloc (with 1 numa node or numa less)
it's possible on QEMU side to make conversion to memdev in migration
compatible way (that's what stopped Michal from memdev approach).
But it's hard to do so in multi-nodes case as amount of MemoryRegions
is different.

Point is to consider 'mem' as mis-configuration error, as the user
in the first place using broken numa configuration
(i.e. fake numa configuration doesn't actually improve performance).

CCed David, maybe he could offer a way to do 1:n migration and other
way around.


> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > ---
> >  numa.c               |  2 ++
> >  qemu-deprecated.texi | 14 ++++++++++++++
> >  2 files changed, 16 insertions(+)
> > 
> > diff --git a/numa.c b/numa.c
> > index 3875e1e..2205773 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 deprecated,"
> > +                    " use -numa node,memdev instead");
> >      }
> >      if (node->has_memdev) {
> >          Object *o;
> > diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
> > index 45c5795..73f99d4 100644
> > --- a/qemu-deprecated.texi
> > +++ b/qemu-deprecated.texi
> > @@ -60,6 +60,20 @@ Support for invalid topologies will be removed, the user must ensure
> >  topologies described with -smp include all possible cpus, i.e.
> >    @math{@var{sockets} * @var{cores} * @var{threads} = @var{maxcpus}}.
> >  
> > +@subsection -numa node,mem=@var{size} (since 4.0)
> > +
> > +The parameter @option{mem} of @option{-numa node} is used to assign a part of
> > +guest RAM to a NUMA node. But when using it, it's impossible to manage specified
> > +size on the host side (like bind it to a host node, setting bind policy, ...),
> > +so guest end-ups with the fake NUMA configuration with suboptiomal performance.
> > +However since 2014 there is an alternative way to assign RAM to a NUMA node
> > +using parameter @option{memdev}, which does the same as @option{mem} and has
> > +an ability to actualy manage node RAM on the host side. Use parameter
> > +@option{memdev} with @var{memory-backend-ram} backend as an replacement for
> > +parameter @option{mem} to achieve the same fake NUMA effect or a properly
> > +configured @var{memory-backend-file} backend to actually benefit from NUMA
> > +configuration.
> > +
> >  @section QEMU Machine Protocol (QMP) commands
> >  
> >  @subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0)
> > -- 
> > 2.7.4
> > 
> > --
> > libvir-list mailing list
> > libvir-list@redhat.com
> > https://www.redhat.com/mailman/listinfo/libvir-list  
> 
> Regards,
> Daniel