[openstack-dev] Potential enhancement to filter scheduler

Chris Behrens cbehrens at codestud.com
Wed Jan 16 23:32:57 UTC 2013

On Jan 16, 2013, at 11:22 AM, "Day, Phil" <philip.day at hp.com> wrote:

> Hi Folks,
> In working with the Filter Scheduler we’ve come with a couple of ideas on enhancement to make the final host selection stage configurable.  Whilst its sometimes fine to just pick the first host from the list of weighted hosts, the more general case is that I’d like to be able to have the scheduler pick one of the first N hosts on the weighted list.    The specific use cases that I have in mind are:
> i) On a large system there is very rarely a single ideal / optimal host for a particular instance to be placed on.  In practice any of the N most 

Hey cool, I actually have a patch for picking at random from the top N hosts as well, but I haven't gotten around to proposing it. :)   Consider that as support for the idea. ;)   Not sure I like the implementation, but I can comment on that in the review.

> ii) The second idea is that when creating M instances in one request we could randomly select M hosts out of the first N hosts in the list (since these have all been filtered as being suitable) which would save having to iterate through the filter / weighting functions for each successive instance.   Haven’t done any coding for this yet, but would appreciate thoughts.

You don't need to re-filter the whole list of hosts.  However, you do need to re-filter the host that has been chosen on every pass.  It may have only been suitable for 1 instance.

- Chris

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