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Thanks Melanie! It helps. VMs are spread very well.

Tony
> -----Original Message-----
> From: melanie witt <melwittt at gmail.com>
> Sent: Tuesday, October 27, 2020 2:25 PM
> To: Tony Liu <tonyliu0592 at hotmail.com>; openstack-
> discuss at lists.openstack.org
> Subject: Re: [nova-scheduler] always the same compute was selected?
> 
> On 10/27/20 14:13, Tony Liu wrote:
> > Hi,
> >
> > I have a Ussuri setup with 5 compute nodes (same resource spec).
> > When launch 5 Cirros VMs in a batch, they are all launched on the same
> > compute node. All 5 nodes have sufficient resource.
> >
> > AWIK, Nova scheduler picks a compute randomly after going through
> > filtering. But 5 VMs all on the same compute is quite coincidental.
> > Could it be something missing?
> > Is there any control to such random pick?
> > Any way to make a better balancing?
> 
> Nova scheduler will weigh all of the candidate hosts via the weighers
> configured and then by default, it will pick randomly from a set of only
> 1 [1]. This will result in a "packing" behavior if the weighers are not
> sufficiently configured to result in "spreading" instances (example:
> cpu/ram/disk weighers).
> 
> To make the scheduler pick randomly from a larger subset of candidate
> compute hosts, increase the host_subset_size [1].
> 
> To make the scheduler randomly shuffle candidate hosts when weighting is
> the same, set shuffle_best_same_weighed_hosts to True [2].
> 
> If you have > 1000 compute hosts (via multi-cell) and need to randomize
> the default 1000 results that come from placement [3], set
> randomize_allocation_candidates to True.
> 
> You likely will only be interested in the first two options I described.
> Tune host_subset_size until you get the desired scheduling behavior.
> 
> Hope this helps,
> -melanie
> 
> [1]
> https://docs.openstack.org/nova/ussuri/configuration/config.html#filter_
> scheduler.host_subset_size
> [2]
> https://docs.openstack.org/nova/ussuri/configuration/config.html#filter_
> scheduler.shuffle_best_same_weighed_hosts
> [3]
> https://docs.openstack.org/nova/ussuri/configuration/config.html#schedul
> er.max_placement_results
> [4]
> https://docs.openstack.org/placement/ussuri/configuration/config.html#pl
> acement.randomize_allocation_candidates