Re: [PATCH 10 of 10 v2] Some automatic NUMA placement documentation

[Dario Faggioli <raistlin.df@gmail.com>]

On Fri, 2012-06-15 at 19:04 +0200, Dario Faggioli wrote:
> +### Automatic Guest Placement with xl ###
> +
> +In case no "cpus=" option is specified in the config file, xl tries
> +to figure out on its own on which node(s) the domain could fit best.
> +
> +First of all, it needs to find a node (or a set of nodes) that have
> +enough free memory for accommodating the domain. After that, the actual
> +decision on where to put the new guest happens by generating all the
> +possible combinations of nodes that satisfies the above and chose among
> +them according to the following heuristics:
> +
> +  *  candidates involving fewer nodes come first. In case two (or more)
> +     candidates span the same number of nodes,
> +  *  candidates with greater amount of free memory come first. In case
> +     two (or more) candidates differ in their amount of free memory by
> +     less than 10%,
> +  *  candidates with fewer domains already placed on them come first.
> +
> +Giving preference to small candidates ensures better performance for
> +the guest, as it avoid spreading its memory among different nodes.
> +Using the nodes that have the biggest amounts of free memory helps
> +keeping the memory fragmentation small, from a system wide perspective.
> +Finally, in case more candidates fulfil these criteria by the same
> +extent, choosing the candidate that is hosting fewer domain helps
> +balancing the load on the various nodes.
> +

This part below is basically a leftover from previous version. Things
does not work like that any longer. Basically, what we do is looking for
all the nodes, or a sets of nodes, that have enough free memory and at
least as much pCPUs as the domain has vCPUs and then apply the
heuristics outlined above.
> +The last step is figuring out whether the selected candidate contains
> +at least as much CPUs as the number of VCPUs of the VM. The current
> +solution for the case when this is not verified is just to add some
> +more nodes, until the condition turns into being true. When doing
> +this, the nodes with the least possible distance from the ones
> +already in the nodemap are considered.
> +

Sorry, (already fixed in my local patchqueue)
Dario

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