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On 3/29/2018 12:05 PM, Chris Dent wrote:
>>>> Other suggestions? I'm looking at things like turning off 
>>>> scheduler_tracks_instance_changes, since affinity scheduling is not 
>>>> needed (at least so-far), but not sure that that will help with 
>>>> placement load (seems like it might, though?)
>>>
>>> This won't impact the placement service itself.
>>
>> It seemed like it might be causing the compute nodes to make calls to 
>> update allocations, so I was thinking it might reduce the load a bit, 
>> but I didn't confirm that. This was "clutching at straws" - hopefully 
>> I won't need to now.
> 
> There's duplication of instance state going to both placement and
> the nova-scheduler. The number of calls from nova-compute to
> placement reduces a bit as you updgrade to newer releases. It's
> still more than we'd prefer.

As Chris said, scheduler_tracks_instance_changes doesn't have anything 
to do with Placement, and it will add more RPC load to your system 
because all computes are RPC casting to the scheduler for every instance 
create/delete/move operation along with a periodic that runs, by 
default, every minute on each compute service to sync things up.

The primary need for scheduler_tracks_instance_changes is the 
(anti-)affinity filters in the scheduler (and maybe if you're using the 
CachingScheduler). If you don't enable the (anti-)affinity filters (they 
are enabled by default), then you can disable 
scheduler_tracks_instance_changes.

Note that you can still disable scheduler_tracks_instance_changes and 
run the affinity filters, but the scheduler will likely make poor 
decisions in a busy cloud which can result in reschedules, which are 
also expensive.

Long-term, we hope to remove the need for 
scheduler_tracks_instance_changes at all because we should have all of 
the information we need about the instances in the Placement service, 
which is generally considered global to the deployment. However, we 
don't yet have a way to model affinity/distance in Placement, and that's 
what's holding us back from removing scheduler_tracks_instance_changes 
and the existing affinity filters.

-- 

Thanks,

Matt