Re: [PATCH 07 of 10 v2] libxl: optimize the calculation of how many VCPUs can run on a candidate

[George Dunlap <george.dunlap@eu.citrix.com>]

On 19/12/12 19:07, Dario Faggioli wrote:
> For choosing the best NUMA placement candidate, we need to figure out
> how many VCPUs are runnable on each of them. That requires going through
> all the VCPUs of all the domains and check their affinities.
>
> With this change, instead of doing the above for each candidate, we
> do it once for all, populating an array while counting. This way, when
> we later are evaluating candidates, all we need is summing up the right
> elements of the array itself.
>
> This reduces the complexity of the overall algorithm, as it moves a
> potentially expensive operation (for_each_vcpu_of_each_domain {})
> outside from the core placement loop, so that it is performed only
> once instead of (potentially) tens or hundreds of times.
>
> Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>

You know this code best. :-)  I've looked it over and just have one 
minor suggestion:

>           for (j = 0; j < nr_dom_vcpus; j++) {
> +            /* For each vcpu of each domain, increment the elements of
> +             * the array corresponding to the nodes where the vcpu runs */
> +            libxl_bitmap_set_none(&vcpu_nodemap);
> +            libxl_for_each_set_bit(k, vinfo[j].cpumap) {
> +                int node = tinfo[k].node;

I think I might rename "vcpu_nodemap" to something that suggests better 
how it fits with the algorithm -- for instance, "counted_nodemap" or 
"nodes_counted" -- something to suggest that this is how we avoid 
counting the same vcpu on the same node multiple times.

  -George