Re: [Xen-devel] PV-vNUMA issue: topology is misinterpreted by the guest

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Wed, 2015-07-22 at 16:09 +0200, Juergen Gross wrote:
> On 07/22/2015 03:58 PM, Boris Ostrovsky wrote:

> > What if I configure a guest to follow HW topology? I.e. I pin VCPUs to
> > appropriate cores/threads? With elfnote I am stuck with disabled topology.
> 
> Add an option to do exactly that: follow HW topology (pin vcpus,
> configure vnuma)?
> 
I thought about configuring things in such a way that they match the
host topology, as Boris is suggesting, too. And in that case, I think
arranging for doing so in toolstack, if PV vNUMA is identified (as I
think Juergen is suggesting) seems a good approach.

However, when I try to do that on my box, manually, but I don't seem to
be able to.

Here's what I tried. Since I have this host topology:
cpu_topology           :
cpu:    core    socket     node
  0:       0        1        0
  1:       0        1        0
  2:       1        1        0
  3:       1        1        0
  4:       9        1        0
  5:       9        1        0
  6:      10        1        0
  7:      10        1        0
  8:       0        0        1
  9:       0        0        1
 10:       1        0        1
 11:       1        0        1
 12:       9        0        1
 13:       9        0        1
 14:      10        0        1
 15:      10        0        1

I configured the guest like this:
vcpus       = '4'
memory      = '1024'
vnuma = [ [ "pnode=0","size=512","vcpus=0-1","vdistances=10,20"  ],
          [ "pnode=1","size=512","vcpus=2-3","vdistances=20,10"  ] ]
cpus=["0","1","8","9"]

This means vcpus 0 and 1, which are assigned to vnode 0, are pinned to
pcpu 0 and 1, which are siblings, per the host topology.
Similarly, vcpus 2 and 3, assigned to vnode 1, are assigned to two
siblings pcpus on pnode 1.

This seems to be honoured:
# xl vcpu-list 4
Name                                ID  VCPU   CPU State   Time(s) Affinity 
(Hard / Soft)
test                                 4     0    0   -b-      10.9  0 / 0-7
test                                 4     1    1   -b-       7.6  1 / 0-7
test                                 4     2    8   -b-       0.1  8 / 8-15
test                                 4     3    9   -b-       0.1  9 / 8-15

And yet, no joy:
# ssh root@xxxxxxxxxxxxx "yes > /dev/null 2>&1 &"
# ssh root@xxxxxxxxxxxxx "yes > /dev/null 2>&1 &"
# ssh root@xxxxxxxxxxxxx "yes > /dev/null 2>&1 &"
# ssh root@xxxxxxxxxxxxx "yes > /dev/null 2>&1 &"
# xl vcpu-list 4
Name                                ID  VCPU   CPU State   Time(s) Affinity 
(Hard / Soft)
test                                 4     0    0   r--      16.4  0 / 0-7
test                                 4     1    1   r--      12.5  1 / 0-7
test                                 4     2    8   -b-       0.2  8 / 8-15
test                                 4     3    9   -b-       0.1  9 / 8-15

So, what am I doing wrong at "following the hw topology"?

> > Besides, this is not necessarily a NUMA-only issue, it's a scheduling
> > one (inside the guest) as well.
> 
> Sure. That's what Jan said regarding SUSE's xen-kernel. No toplogy info
> (or a trivial one) might be better than the wrong one...
> 
Yep. Exacty. As Boris says, this is a generic scheduling issue, although
it's tru that it's only (as far as I can tell) with vNUMA that it bite
us so hard... I mean, performance are always going to be inconsistent,
but it's only in that case that you basically _loose_ some of the
vcpus! :-O

Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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