Open Stack

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 suitable hosts would be fine and allowing the scheduler to randomly pick one of these would add some spread for multiple requests that come in at the same time.  (I know we now have the retry mechanism if a particular host can't in fact handle a specific request - this is a complement to that rather an alternative).      Of course anyone who wants to schedule to host in strict weighted order would be able to configure N to be 1 (or we could keep the current host selection method as a separate default)

The first pass at this change is already in gerrit and I'd appreciate it if people could take a look: https://review.openstack.org/#/c/19494/


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.


Cheers,
Phil



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