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This patch was backported:
https://review.opendev.org/c/openstack/nova/+/805649. Once this is in
place, new VMs always get assigned correctly to the numa node with
more free memory. But when existing VMs (created with vm flavor with
hw:numa_node=1 set) already running on numa node #0 got live migrated,
it would always be stuck on numa node #0 after live migration.

So it seems we would also need to set hw:mem_page_size=small on the vm
flavor, so that new VMs created from that flavor would be able to land
on different numa node other than node#0 after its live migration?

On Mon, May 15, 2023 at 2:33 PM Sean Mooney <smooney at redhat.com> wrote:
>
> On Mon, 2023-05-15 at 13:03 -0500, hai wu wrote:
> > > > Another question: Let's say a VM runs on one host's numa node #0. If
> > > > we live-migrate this VM to another host, and that host's numa node #1
> > > > has more free memory, is it possible for this VM to land on the other
> > > > host's numa node #1?
> > > yes it is
> > > on newer relsese we will prefer to balance the load across numa nodes
> > > on older release nova woudl fill the first numa node then move to the second.
> >
> > About the above point, it seems even with the numa patch back ported
> > and in place, the VM would be stuck in its existing numa node. Per my
> > tests, after its live migration, the VM will end up on the other
> > host's numa node #0, even if numa node#1 has more free memory. This is
> > not the case for newly built VMs.
> >
> > Is this a design issue?
> if you are using a release that supprot numa live migration (train +)
> https://specs.openstack.org/openstack/nova-specs/specs/train/implemented/numa-aware-live-migration.html
> then the numa affintiy is recalulated on live migration however numa node 0 is prefered.
>
> as of xena [compute]/packing_host_numa_cells_allocation_strategy has been added to contol how vms are balanced acros numa nodes
> in zed the default was changed form packing vms per host numa node to balancing vms between host numa nodes
> https://docs.openstack.org/releasenotes/nova/zed.html#relnotes-26-0-0-stable-zed-upgrade-notes
>
> even without the enhanchemt in xena and zed it was possible for the scheduler to select a numa node
>
> if you dont enable memory or cpu aware numa placment with
> hw:mem_page_size or hw:cpu_policy=dedicated then it will always select numa 0
>
> if you do not request cpu pinnign or a specifc page size the sechudler cant properly select the host nuam node
> and will alwasy use numa node 0. That is one of the reason i said that if hw:numa_nodes is set then hw:mem_page_size shoudl be set.
>
> from a nova point of view using  numa_nodes without mem_page_size is logically incorrect as you asked for
> a vm to be affinites to n host numa nodes but did not enable numa aware memory scheduling.
>
> we unfortnally cant provent this in the nova api without breaking upgrades for everyone who has made this mistake.
> we woudl need to force them to resize all affected instances which means guest downtime.
> the other issue si multiple numa nodes are supproted by Hyper-V but they do not supprot mem_page_size
>
> we have tried to document this in the past but never agreed on how becasuse it subtel and requries alot of context.
> the tl;dr is if the instace has a numa toplogy it should have mem_page_size set in the image or flavor but
> we never foudn a good place to capture that.
>
> >
> > On Thu, May 11, 2023 at 2:42 PM Sean Mooney <smooney at redhat.com> wrote:
> > >
> > > On Thu, 2023-05-11 at 08:40 -0500, hai wu wrote:
> > > > Ok. Then I don't understand why 'hw:mem_page_size' is not made the
> > > > default in case if hw:numa_node is set. There is a huge disadvantage
> > > > if not having this one set (all existing VMs with hw:numa_node set
> > > > will have to be taken down for resizing in order to get this one
> > > > right).
> > > there is an upgrade impact to changign the default.
> > > its not impossibel to do but its complicated if we dont want to break exisitng deployments
> > > we woudl need to recored a value for eveny current instance that was spawned before
> > > this default was changed that had hw:numa_node without hw:mem_page_size so they kept the old behavior
> > > and make sure that is cleared when the vm is next moved so it can have the new default
> > > after a live migratoin.
> > > >
> > > > I could not find this point mentioned in any existing Openstack
> > > > documentation: that we would have to set hw:mem_page_size explicitly
> > > > if hw:numa_node is set. Also this slide at
> > > > https://www.linux-kvm.org/images/0/0b/03x03-Openstackpdf.pdf kind of
> > > > indicates that hw:mem_page_size `Default to small pages`.
> > > it defaults to unset.
> > > that results in small pages by default but its not the same as hw:mem_page_size=small
> > > or hw:mem_page_size=any.
> > >
> > >
> > > >
> > > > Another question: Let's say a VM runs on one host's numa node #0. If
> > > > we live-migrate this VM to another host, and that host's numa node #1
> > > > has more free memory, is it possible for this VM to land on the other
> > > > host's numa node #1?
> > > yes it is
> > > on newer relsese we will prefer to balance the load across numa nodes
> > > on older release nova woudl fill the first numa node then move to the second.
> > > >
> > > > On Thu, May 11, 2023 at 4:25 AM Sean Mooney <smooney at redhat.com> wrote:
> > > > >
> > > > > On Wed, 2023-05-10 at 15:06 -0500, hai wu wrote:
> > > > > > Is it possible to update something in the Openstack database for the
> > > > > > relevant VMs in order to do the same, and then hard reboot the VM so
> > > > > > that the VM would have this attribute?
> > > > > not really adding the missing hw:mem_page_size requirement to the flavor chagnes the
> > > > > requirements for node placement and numa affinity
> > > > > so you really can only change this via resizing the vm to a new flavor
> > > > > >
> > > > > > On Wed, May 10, 2023 at 2:47 PM Sean Mooney <smooney at redhat.com> wrote:
> > > > > > >
> > > > > > > On Wed, 2023-05-10 at 14:22 -0500, hai wu wrote:
> > > > > > > > So there's no default value assumed/set for hw:mem_page_size for each
> > > > > > > > flavor?
> > > > > > > >
> > > > > > > correct this is a known edgecase in the currnt design
> > > > > > > hw:mem_page_size=any would be a resonable default but
> > > > > > > techinially if just set hw:numa_nodes=1 nova allow memory over subscription
> > > > > > >
> > > > > > > in pratch if you try to do that you will almost always end up with vms
> > > > > > > being killed due to OOM events.
> > > > > > >
> > > > > > > so from a api point of view it woudl be a change of behvior for use to default
> > > > > > > to hw:mem_page_size=any but i think it would be the correct thign to do for operators
> > > > > > > in the long run.
> > > > > > >
> > > > > > > i could bring this up with the core team again but in the past we
> > > > > > > decided to be conservitive and just warn peopel to alwasy set
> > > > > > > hw:mem_page_size if using numa affinity.
> > > > > > >
> > > > > > > >  Yes https://bugs.launchpad.net/nova/+bug/1893121 is critical
> > > > > > > > when using hw:numa_nodes=1.
> > > > > > > >
> > > > > > > > I did not hit an issue with 'hw:mem_page_size' not set, maybe I am
> > > > > > > > missing some known test cases? It would be very helpful to have a test
> > > > > > > > case where I could reproduce this issue with 'hw:numa_nodes=1' being
> > > > > > > > set, but without 'hw:mem_page_size' being set.
> > > > > > > >
> > > > > > > > How to ensure this one for existing vms already running with
> > > > > > > > 'hw:numa_nodes=1', but without 'hw:mem_page_size' being set?
> > > > > > > you unfortuletly need to resize the instance.
> > > > > > > tehre are some image porpeties you can set on an instance via nova-manage
> > > > > > > but you cannot use nova-mange to update the enbedd flavor and set this.
> > > > > > >
> > > > > > > so you need to define a new flavour and resize.
> > > > > > >
> > > > > > > this is the main reason we have not changed the default as it may requrie you to
> > > > > > > move instnace around if there placement is now invalid now that per numa node memory
> > > > > > > allocatons are correctly being accounted for.
> > > > > > >
> > > > > > > if it was simple to change the default without any enduser or operator impact we would.
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > >
> > > > > > > > On Wed, May 10, 2023 at 1:47 PM Sean Mooney <smooney at redhat.com> wrote:
> > > > > > > > >
> > > > > > > > > if you set hw:numa_nodes there are two things you should keep in mind
> > > > > > > > >
> > > > > > > > > first if hw:numa_nodes si set to any value incluing hw:numa_nodes=1
> > > > > > > > > then hw:mem_page_size shoudl also be defiend on the falvor.
> > > > > > > > >
> > > > > > > > > if you dont set hw:mem_page_size then the vam will be pinned to a host numa node
> > > > > > > > > but the avaible memory on the host numa node will not be taken into account
> > > > > > > > >
> > > > > > > > > only the total free memory on the host so this almost always results in VMs being killed by the OOM reaper
> > > > > > > > > in the kernel.
> > > > > > > > >
> > > > > > > > > i recomend setting hw:mem_page_size=small hw:mem_page_size=large or hw:mem_page_size=any
> > > > > > > > > small will use your kernels default page size for guest memory, typically this is 4k pages
> > > > > > > > > large will use any pages size other then the smallest that is avaiable (i.e. this will use hugepages)
> > > > > > > > > and any will use small pages but allow the guest to request hugepages via the hw_page_size image property.
> > > > > > > > >
> > > > > > > > > hw:mem_page_size=any is the most flexable as a result but generally i recommend using  hw:mem_page_size=small
> > > > > > > > > and having a seperate flavor for hugepages. its really up to you.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > the second thing to keep in mind is using expict numa toplolig8ies including hw:numa_nodes=1
> > > > > > > > > disables memory oversubsctipion.
> > > > > > > > >
> > > > > > > > > so you will not be able ot oversubscibe the memory on the host.
> > > > > > > > >
> > > > > > > > > in general its better to avoid memory oversubscribtion anyway but jsut keep that in mind.
> > > > > > > > > you cant jsut allocate a buch of swap space and run vms at a 2:1 or higher memory over subscription ratio
> > > > > > > > > if you are using numa affinity.
> > > > > > > > >
> > > > > > > > > https://that.guru/blog/the-numa-scheduling-story-in-nova/
> > > > > > > > > and
> > > > > > > > > https://that.guru/blog/cpu-resources-redux/
> > > > > > > > >
> > > > > > > > > are also good to read
> > > > > > > > >
> > > > > > > > > i do not think stephen has a dedicated block on the memory aspect
> > > > > > > > > but https://bugs.launchpad.net/nova/+bug/1893121 covers some of the probelem that only setting
> > > > > > > > > hw:numa_nodes=1 will casue.
> > > > > > > > >
> > > > > > > > > if you have vms with hw:numa_nodes=1 set and you do not have hw:mem_page_size set in the falvor or
> > > > > > > > > hw_mem_page_size set in the image then that vm is not configure properly.
> > > > > > > > >
> > > > > > > > > On Wed, 2023-05-10 at 11:52 -0600, Alvaro Soto wrote:
> > > > > > > > > > Another good resource =)
> > > > > > > > > >
> > > > > > > > > > https://that.guru/blog/cpu-resources/
> > > > > > > > > >
> > > > > > > > > > On Wed, May 10, 2023 at 11:50 AM Alvaro Soto <alsotoes at gmail.com> wrote:
> > > > > > > > > >
> > > > > > > > > > > I don't think so.
> > > > > > > > > > >
> > > > > > > > > > > ~~~
> > > > > > > > > > > The most common case will be that the admin only sets hw:numa_nodes and
> > > > > > > > > > > then the flavor vCPUs and memory will be divided equally across the NUMA
> > > > > > > > > > > nodes. When a NUMA policy is in effect, it is mandatory for the instance's
> > > > > > > > > > > memory allocations to come from the NUMA nodes to which it is bound except
> > > > > > > > > > > where overriden by hw:numa_mem.NN.
> > > > > > > > > > > ~~~
> > > > > > > > > > >
> > > > > > > > > > > Here are the implementation documents since Juno release:
> > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > https://opendev.org/openstack/nova-specs/src/branch/master/specs/juno/implemented/virt-driver-numa-placement.rst
> > > > > > > > > > >
> > > > > > > > > > > https://opendev.org/openstack/nova-specs/commit/45252df4c54674d2ac71cd88154af476c4d510e1
> > > > > > > > > > > ?
> > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > On Wed, May 10, 2023 at 11:31 AM hai wu <haiwu.us at gmail.com> wrote:
> > > > > > > > > > >
> > > > > > > > > > > > Is there any concern to enable 'hw:numa_nodes=1' on all flavors, as
> > > > > > > > > > > > long as that flavor can fit into one numa node?
> > > > > > > > > > > >
> > > > > > > > > > > >
> > > > > > > > > > >
> > > > > > > > > > > --
> > > > > > > > > > >
> > > > > > > > > > > Alvaro Soto
> > > > > > > > > > >
> > > > > > > > > > > *Note: My work hours may not be your work hours. Please do not feel the
> > > > > > > > > > > need to respond during a time that is not convenient for you.*
> > > > > > > > > > > ----------------------------------------------------------
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