[nova] Instance Even Scheduling

Sean Mooney smooney at redhat.com
Mon Jan 17 17:57:04 UTC 2022

On Mon, 2022-01-17 at 17:45 +0000, Tony Liu wrote:
> I recall weight didn't work as what I expected, that's why I used 
> shuffle_best_same_weighed_hosts.
> Here is what I experienced.
> With Ussuri and default Nova scheduling settings. All weighers are supposed
> to be enabled and all multipliers are positive.
yes by default all weighers are enabled and the shcduler spreads by default.
>  On 10x empty compute nodes
> with the same spec, say the first vm is created on compute-2. Because some
> memory and vCPU are consumed, the second vm should be created on some
> node other than compute-2, if weighers are working fine. But it's still created
> on compute-2, until I increased host_subset_size and enable shuffle_best_same_weighed_hosts.
i would guess that either the disk weigher or failed build wiehter is likely what results in teh behaivor different 
the default behavior is still to speread. before assuming there is a but you shoudl enable the schduler
in debug mode to look at the weighters that are assinged to each host and determin why you are seeing differnt behavior.

shuffle_best_same_weighed_hosts does as the name suggest. it shuffles the result if and only if there is a tie.
that means it will only have a effect if 2 hosts were judged by thge weigher as beeing equally good candiates.

host_subset_size instalead of looking at only the top host in the list enables you to consider the top n hosts.

host_subset_size does a random selection from the host_subset_size top element after the hosts are sorted by the weighers
intentionlaly adding randomness to the selection.
this should not be needed in general.

>  It seems that all compute nodes are equally
> weighted, although they don't have the same amount of resource.
> Am I missing anything there?
> Thanks!
> Tony
> ________________________________________
> From: Sean Mooney <smooney at redhat.com>
> Sent: January 17, 2022 09:06 AM
> To: openstack-discuss at lists.openstack.org
> Subject: Re: [nova] Instance Even Scheduling
> On Mon, 2022-01-17 at 16:35 +0000, Tony Liu wrote:
> > https://docs.openstack.org/nova/latest/admin/scheduling.html
> > 
> > Filter gives you a group of valid hosts, assuming they are equally weighted,
> > you may try with these two settings to pick up a host in a more even manner.
> > host_subset_size (increase the size)
> > shuffle_best_same_weighed_hosts (enable the shuffle)
> > 
> > https://docs.openstack.org/nova/latest/configuration/config.html
> yes the weighers are what will blance between the hosts and the filters determin which host are valid
> so if you want to spread based on ram then you need to adject the
> https://docs.openstack.org/nova/latest/configuration/config.html#filter_scheduler.ram_weight_multiplier
> for example set ram_weight_multiplier=10.0 to make it relitivly more important.
> the way the weigher work is all wheigher calulate the weight for a host,
> we then add them after multiplying them by the weights and then sort.
> > 
> > 
> > Tony
> > ________________________________________
> > From: Ammad Syed <syedammad83 at gmail.com>
> > Sent: January 16, 2022 11:53 PM
> > To: openstack-discuss
> > Subject: [nova] Instance Even Scheduling
> > 
> > Hi,
> > 
> > I have 5 compute nodes. When I deploy instances, the most of the instances automatically placed in node 1 or node 2. The other compute nodes remain empty or with one or two instances on it.
> > 
> > enabled_filters = ComputeFilter,ComputeCapabilitiesFilter,ImagePropertiesFilter,ServerGroupAntiAffinityFilter,ServerGroupAffinityFilter,AggregateInstanceExtraSpecsFilter
> > 
> > I have enabled the above filters. How to ensure that instances should be scheduled on compute nodes evenly on all compute hosts based on RAM only ? Like scheduler should schedule the instance on compute host which has a large amount of RAM available then other hosts.
> > 
> > - Ammad
> > 
> > 

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