Re: [Xen-devel] RFC: automatic NUMA placement

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>]

Hi Andre,

thanks for your thoughts.

On 09/27/10 23:46, Andre Przywara wrote:
> Juergen Gross wrote:
> > Hi,
> >
> > I just stumbled upon the automatic pinning of vcpus on domain creation in
> > case of NUMA.
> > This behaviour is questionable IMO, as it breaks correct handling of
> > scheduling weights on NUMA machines.
> > I would suggest to switch this feature off per default and make it a
> > configuration option of xend. It would make sense, however, to change
> > cpu pool
> > processor allocation to be NUMA-aware.
> > Switching NUMA off via boot option would remove NUMA-optimized memory
> > allocation, which would be sub-optimal :-)
> Hi Jürgen,
>
> stumbled over your mail just now, so sorry for the delay.
> First: Don't turn off automatic NUMA placement ;-)
> In my tests it helped a lot to preserve performance on NUMA machines.
>
> I was just browsing through the ML archive to find your original CPU
> pools description from April, and it seems to fit the requirements in
> NUMA machines quite well.
> I haven't done any experiments with Cpupools nor haven't looked at the
> code yet, but just a quick idea:
> What about if we marry static NUMA placement and Cpupools?
>
> I'd suggest to introduce static NUMA pools, one for each node. The CPUs
> assigned to each pool are fixed and cannot be removed nor added (because
> the NUMA topology is fixed).
> Is that possible? Can we assign one physical CPUs to multiple pools (to
> Pool-0 and to NUMA-0?) Or are they exclusive or hierarchical like the
> Linux' cpusets?
A cpu is always member of only one pool.

> We could introduce magic names for each NUMA pool, so that people just
> say cpupool="NUMA-2" and get their domain pinned to that pool. Without
> any explicit assignment the system would pick a NUMA node (like it does
> today) and would just use the respective Cpupool. I think that is very
> similar to what it does today, only that the pinning nature is more
> evident to the user (as it uses the Cpupool name space).
> Also it would allow for users to override the pinning by specifying a
> different Cpupool explicitly (like Pool-0).
>
> Just tell me what you think about this and whether I am wrong with my
> thinking ;-)
With your proposal it isn't possible to start a domU with more vcpus than
cpus in a node without changing cpu pools.

I would suggest to do it the following way:
- use automatic NUMA placement only in Pool-0 (this won't change anything
  for users not using cpu pools), perhaps with an option to switch it off
- change the cpu allocation for pools to be NUMA aware
- optionally add a xl and/or xm command to create one cpu pool per NUMA node


Juergen

--
Juergen Gross                 Principal Developer Operating Systems
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