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Re: [Qemu-ppc] [Qemu-devel] [libvirt] [PATCH 1/2] numa: deprecate 'mem'

From: Daniel P . Berrangé
Subject: Re: [Qemu-ppc] [Qemu-devel] [libvirt] [PATCH 1/2] numa: deprecate 'mem' parameter of '-numa node' option
Date: Mon, 4 Mar 2019 16:27:23 +0000
User-agent: Mutt/1.11.3 (2019-02-01)

On Mon, Mar 04, 2019 at 05:12:40PM +0100, Michal Privoznik wrote:
> On 3/4/19 4:19 PM, Igor Mammedov wrote:
> > Then I'd guess that most VMs end up with default '-numa node,mem'
> > which by design can produce only fake NUMA without ability to manage
> > guest RAM on host side. So such VMs aren't getting performance benefits
> > or worse run with performance regression (due to wrong sched/mm decisions
> > as guest kernel assumes NUMA topology is valid one).
> Specifying NUMA distances in libvirt XML makes it generate the modern cmd
> line.

AFAIK, specifying any guest NUMA -> Host NUMA affinity makes it use the
modern cmd line. eg I  just modified a plain 8 CPU / 2 GB RAM guest
with this:

    <memnode cellid='0' mode='strict' nodeset='0'/>
    <memnode cellid='1' mode='strict' nodeset='1'/>
  <cpu mode='host-model'>
      <cell id='0' cpus='0-3' memory='1024000' unit='KiB'/>
      <cell id='1' cpus='4-7' memory='1024000' unit='KiB'/>

and I can see libvirt decided to use memdev

  -numa node,nodeid=0,cpus=0-3,memdev=ram-node0
  -numa node,nodeid=1,cpus=4-7,memdev=ram-node1 

So unless I'm missing something, we aren't suffering from the problem
described by Igor above even today.

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