[libvirt] [PATCH v4 2/5] numa: describe siblings distances within cells

[Jim Fehlig]

On 09/08/2017 08:47 AM, Wim Ten Have wrote:
> From: Wim ten Have <wim.ten.have at oracle.com>
> 
> Add libvirtd NUMA cell domain administration functionality to
> describe underlying cell id sibling distances in full fashion
> when configuring HVM guests.

May I suggest wording this paragraph as:

Add support for describing sibling vCPU distances within a domain's vNUMA cell 
configuration.

> Schema updates are made to docs/schemas/cputypes.rng enforcing domain
> administration to follow the syntax below the numa cell id and
> docs/schemas/basictypes.rng to add "numaDistanceValue".

I'm not sure this paragraph is needed in the commit message.

> A minimum value of 10 representing the LOCAL_DISTANCE as 0-9 are
> reserved values and can not be used as System Locality Distance Information.
> A value of 20 represents the default setting of REMOTE_DISTANCE
> where a maximum value of 255 represents UNREACHABLE.
> 
> Effectively any cell sibling can be assigned a distance value where
> practically 'LOCAL_DISTANCE <= value <= UNREACHABLE'.
> 
> [below is an example of a 4 node setup]
> 
>    <cpu>
>      <numa>
>        <cell id='0' cpus='0' memory='2097152' unit='KiB'>
>          <distances>
>            <sibling id='0' value='10'/>
>            <sibling id='1' value='21'/>
>            <sibling id='2' value='31'/>
>            <sibling id='3' value='41'/>
>          </distances>
>        </cell>
>        <cell id='1' cpus='1' memory='2097152' unit='KiB'>
>          <distances>
>            <sibling id='0' value='21'/>
>            <sibling id='1' value='10'/>
>            <sibling id='2' value='31'/>
>            <sibling id='3' value='41'/>
>          </distances>
>        </cell>
>        <cell id='2' cpus='2' memory='2097152' unit='KiB'>
>          <distances>
>            <sibling id='0' value='31'/>
>            <sibling id='1' value='21'/>
>            <sibling id='2' value='10'/>
>            <sibling id='3' value='21'/>
>          </distances>
>        <cell id='3' cpus='3' memory='2097152' unit='KiB'>
>          <distances>
>            <sibling id='0' value='41'/>
>            <sibling id='1' value='31'/>
>            <sibling id='2' value='21'/>
>            <sibling id='3' value='10'/>
>          </distances>
>        </cell>
>      </numa>
>    </cpu>

How would this look when having more than one cpu in a cell? I suppose something 
like

  <cpu>
     <numa>
       <cell id='0' cpus='0-3' memory='2097152' unit='KiB'>
         <distances>
           <sibling id='0' value='10'/>
           <sibling id='1' value='10'/>
           <sibling id='2' value='10'/>
           <sibling id='3' value='10'/>
           <sibling id='4' value='21'/>
           <sibling id='5' value='21'/>
           <sibling id='6' value='21'/>
           <sibling id='7' value='21'/>
         </distances>
       </cell>
       <cell id='1' cpus='4-7' memory='2097152' unit='KiB'>
         <distances>
           <sibling id='0' value='21'/>
           <sibling id='1' value='21'/>
           <sibling id='2' value='21'/>
           <sibling id='3' value='21'/>
           <sibling id='4' value='10'/>
           <sibling id='5' value='10'/>
           <sibling id='6' value='10'/>
           <sibling id='7' value='10'/>
         </distances>
      </cell>
    </numa>
  </cpu>

In the V3 thread you mentioned "And to reduce even more we could also
remove LOCAL_DISTANCES as they make a constant factor where; (cell_id == 
sibling_id)". In the above example cell_id 1 == sibling_id 1, but it is not 
LOCAL_DISTANCE.

> Whenever a sibling id the cell LOCAL_DISTANCE does apply and for any
> sibling id not being covered a default of REMOTE_DISTANCE is used
> for internal computations.

I'm having a hard time understanding this sentence...

I didn't look closely at the patch since I'd like to understand how multi-cpu 
cells are handled before doing so.

Regards,
Jim