Re: [Qemu-devel] [RFC PATCH] Exporting Guest RAM information for NUMA binding

[Dipankar Sarma]

On Thu, Dec 01, 2011 at 06:36:23PM +0100, Andrea Arcangeli wrote:
> On Thu, Dec 01, 2011 at 10:55:20PM +0530, Dipankar Sarma wrote:
> > On Wed, Nov 30, 2011 at 06:41:13PM +0100, Andrea Arcangeli wrote:
> > > On Wed, Nov 30, 2011 at 09:52:37PM +0530, Dipankar Sarma wrote:
> > > > create the guest topology correctly and optimize for NUMA. This
> > > > would work for us.
> > > 
> > > Even on the case of 1 guest that fits in one node, you're not going to
> > > max out the full bandwidth of all memory channels with this.
> > > 
> > > qemu all can do with ms_mbind/tbind is to create a vtopology that
> > > matches the hardware topology. It has these limits:
> > > 
> > > 1) requires all userland applications to be modified to scan either
> > >    the physical topology if run on host, or the vtopology if run on
> > >    guest to get the full benefit.
> > 
> > Not sure why you would need that. qemu can reflect the
> > topology based on -numa specifications and the corresponding
> > ms_tbind/mbind in FDT (in the case of Power, I guess ACPI
> > tables for x86) and guest kernel would detect this virtualized
> > topology. So there is no need for two types of topologies afaics.
> > It will all be reflected in /sys/devices/system/node in the guest.
> 
> The point is: what a vtopology gives you? If you don't modify all apps
> running in the guest to use it? vtopology on guest, helps exactly like
> the topology on host -> very little unless you modify qemu on host to
> use ms_tbind/mbind.

Sure, ms_tbind/mbind will be needed in qemu. For the rest, NUMA aware apps 
already use topology while running on physical systems and they wouldn't need 
modification for this kind of virtualized topology.

Thanks
Dipankar