| Posted | Nick | Remark | |
|---|---|---|---|
| #openstack-nova - 2020-02-11 | |||
| 19:01:07 | efried | which simply splits your CPU and mem "evenly" across $n nodes. | |
| 19:01:15 | dansmith | that just seems to naive to me | |
| 19:01:19 | efried | yes | |
| 19:01:20 | efried | it is | |
| 19:01:32 | efried | but so is "I don't care about NUMA"? | |
| 19:01:41 | efried | And it ought to work 99% of the time | |
| 19:01:52 | efried | and if it doesn't, switch on your [workaround] for a couple of hosts. | |
| 19:02:07 | dansmith | I want the workaround to go away, remember | |
| 19:02:23 | efried | Yes, the workaround goes away completely once we beef up placement to understand can_split | |
| 19:02:53 | dansmith | okay your even splitting is just the first pass at sanity? then that's fine | |
| 19:04:27 | efried | oh, yeah, eventually we want the utopia where all of this happens in one call with can_split, whose ratios are tunable (via placement side conf? via nova conf fed into placement qparams?) | |
| 19:04:43 | dansmith | has to be nova-side | |
| 19:04:52 | dansmith | communicated to placement via the query | |
| 19:05:15 | efried | then we won't need the workaround anymore, because everything will be able to land (and if it can't, it's because it *shouldn't*). | |
| 19:05:43 | efried | In practice, we may even find that nobody needs the workaround. But we'll see. | |
| 19:09:39 | efried | Did we decide how we're going to deal with "control plane is updated but some computes are not"? Does the control plane wait to start using the new query style until all the computes are updated? We have that capability, right? | |
| 19:11:05 | efried | okay, I see that discussed in the spec. | |
| 19:18:58 | sean-k-mooney | efried: i think if we go that route in ussuri we wont be able to land it in time | |
| 19:19:36 | efried | why not? | |
| 19:19:47 | efried | we're not talking about trying to implement can_split in any form in ussuri | |
| 19:20:04 | efried | are you concerned that the progressive-splitting algorithm is too complicated? | |
| 19:20:28 | sean-k-mooney | just multiple queries wehre we try to progressively split | |
| 19:20:36 | sean-k-mooney | efried: yes | |
| 19:21:01 | efried | meh, I don't see how it's any worse than the proposed fallback. | |
| 19:21:28 | sean-k-mooney | efried: it will have to take into accoung numa, native request groups in teh flavor, external requests form cyborng and netorn ports and not suck form a performnce point of view | |
| 19:21:58 | sean-k-mooney | the proposed fallback is two queries, the native nuam one followed by the query we do today | |
| 19:22:08 | sean-k-mooney | and it only impacts the perfomce of numa instances | |
| 19:22:20 | sean-k-mooney | the other way imacpts the perfomce of all non numa instnaces | |
| 19:22:37 | efried | sean-k-mooney: I don't think it has to take all that stuff into account at all. | |
| 19:23:07 | efried | sean-k-mooney: It should behave *exactly* as if you said hw:numa_nodes=$n with no other hw:numa*-isms. | |
| 19:23:40 | efried | (except I think we said we would bounce if we couldn't split evenly; that restriction would have to be lifted for this case.) | |
| 19:24:12 | sean-k-mooney | so we would create a fake flaovr where we overrid that and pass it to the current get numa constratis funct | |
| 19:24:19 | sean-k-mooney | *function | |
| 19:24:25 | efried | if you like | |
| 19:24:33 | efried | that would be the spirit, anyway. | |
| 19:24:36 | sean-k-mooney | i gues that makes it simpler | |
| 19:25:14 | sean-k-mooney | so the get_numa_constraits function will reject any invalid toplogy with regard to even spliting with an excetion | |
| 19:25:32 | sean-k-mooney | so we would just loop and contiue if an excption is raised up to the limit | |
| 19:26:09 | efried | or we relax the constraint to split as close to evenly as possible. Or do that split first. | |
| 19:26:33 | efried | Implementation detail. Point is, it shouldn't be super hard to figure out. | |
| 19:26:49 | sean-k-mooney | we could use the asemetric numa modeling support that is there yes | |
| 19:27:19 | sean-k-mooney | e.g. if you had a 9 core vm and we are on numa=2 do 4cpus+5cpus | |
| 19:28:01 | sean-k-mooney | instead of going to 3 numa nodes with 3 cpus | |
| 19:28:14 | efried | right | |
| 19:28:36 | sean-k-mooney | im not sure which is more likely to cause fragmentation of the top of my head | |
| 19:28:50 | sean-k-mooney | we should tell people to just use powers of 2 | |
| 19:28:51 | efried | example I gave above was with 10 VCPUs, we would try 10, then 5/5, then 3/3/4, then 2/3/2/3. | |
| 19:28:55 | efried | no | |
| 19:29:01 | sean-k-mooney | i was jokeing | |
| 19:29:10 | efried | we should tell people to use real numa specs if they care. | |
| 19:29:19 | sean-k-mooney | it does make life easier when they do but ya i can see that working | |
| 19:30:51 | sean-k-mooney | ok if we caluate the split in the tempory flavor we pass to the numa constratis function then it would populate the instance numa toplogy object as if the user had set it manuyaly in teh falvor | |
| 19:31:07 | sean-k-mooney | then if we save that in the instnace we could ensure we dont break live migration by changing it | |
| 19:31:08 | efried | just so. | |
| 19:31:32 | efried | well, I would expect we shouldn't save it in the flavor, because we want the instance to be able to morph to fit somewhere else if it needs to. | |
| 19:31:46 | sean-k-mooney | right | |
| 19:31:57 | efried | but I don't know how that works, do you break an instance if you "change" its topo from under it? | |
| 19:31:58 | sean-k-mooney | i ment save the instance_numa_toplogy object | |
| 19:32:01 | sean-k-mooney | not the falavor | |
| 19:32:21 | sean-k-mooney | efried: during live migration you would | |
| 19:32:38 | sean-k-mooney | cold migration it might mess up some manula config but it should not break it in general | |
| 19:32:58 | sean-k-mooney | i was suggesting once we select a toplogy we store it in the request_spec and instnace | |
| 19:32:58 | efried | hm, well that's a bummer. So how do we migrate numa-agnostic instances today? | |
| 19:33:11 | sean-k-mooney | so that it stays the same for the liftime of the instance unless you resize | |
| 19:33:14 | sean-k-mooney | or rebuild | |
| 19:33:34 | sean-k-mooney | am today | |
| 19:33:44 | sean-k-mooney | non numa isntace are alwasy exposed as 1 numa node | |
| 19:33:54 | sean-k-mooney | so it never chagnes form the gest point of view | |
| 19:34:02 | sean-k-mooney | so we just lie to the guest | |
| 19:34:10 | efried | couldn't we continue lying to the guest? | |
| 19:34:21 | sean-k-mooney | we can yes | |
| 19:34:39 | efried | does the guest do things differently if it knows CPU x is affined to memory y? | |
| 19:34:51 | sean-k-mooney | so we would do the progessive spliting to select the host resouces and present it as 1 numa node to the guest | |
| 19:35:04 | sean-k-mooney | but that will have worse performace then telling it its actull toplogy | |
| 19:35:10 | sean-k-mooney | yes | |
| 19:35:22 | sean-k-mooney | the kernel will take that into account wehn allocation memroy for a proces | |
| 19:35:36 | sean-k-mooney | trying to use numa local memory ahead of remote numa memory | |
| 19:35:44 | spatel | sean-k-mooney: hey | |
| 19:35:50 | efried | well, I guess this is a problem we would have eventually anyway, right dansmith? | |
| 19:35:52 | sean-k-mooney | if we dotn expose the toplogy to the vm the vm kernel wont know how to optimise | |
| 19:36:01 | spatel | did you see my last mesg? | |
| 19:36:43 | dansmith | efried: what? needing everything to support guests with numa? | |
| 19:37:01 | spatel | running vm on single numa0 give very high performance compare to running on both numa node (with cpu_socket=2 and cpu_threads=2 option) | |
| 19:37:32 | efried | Which will then limit where you can fit it on migration. | |
| 19:37:32 | efried | If you migrate that instance, you have to preserve that topo on the target; you can't just munge it to a new shape. | |
| 19:37:32 | efried | dansmith: TL;DR: In the world where you say you don't care about NUMA, we give you an instance that's NUMA-ified, but with whatever split we were able to fit. | |
| 19:37:36 | sean-k-mooney | spatel: i was away but just saw it now. fundemtally i think erlang is jut not optimising correctly. im not really sure how to help other then suggestin run more small vms if you can scale out the application horrizontally instead of vertically | |
| 19:37:44 | dansmith | sean-k-mooney: presumably if the flavor on the instance is not numa-aware then we can just find a new host during cold migration and give it a new topology when it moves | |
| 19:37:59 | sean-k-mooney | dansmith: yes we could | |
| 19:38:00 | dansmith | efried: only on cold migration | |
| 19:38:02 | dansmith | sorry | |
| 19:38:05 | dansmith | efried: only on live migration | |
| 19:38:28 | dansmith | efried: on cold migration it could change because you're rebooting and there shouldn't be anything in the guest that persistently cares what the topology is.. | |
| 19:38:29 | efried | Okay, that would be a new limitation. | |
| 19:38:32 | spatel | sean-k-mooney: totally understand but i am going to loose 16 cpu in that case. but anyway i can live with that | |
| 19:38:45 | dansmith | efried: we have that limitation today and it's handled by the numa live migration stuff, AFAIK | |
| 19:39:34 | efried | dansmith: oh, my understanding was that, by lying to the guest and saying it only has one NUMA node, regardless of how many are under the covers, we can change the under-the-covers on live migration without "affecting" the guest. It would still have crappy performance on both sides, but it wouldn't notice that anything had changed. | |
| 19:39:38 | sean-k-mooney | efried: well the limitation would be dont change the view of hardwar the vm sees in live migration | |
| 19:39:46 | dansmith | efried: and selecting a host in that situation should be the same as selecting a host for a new instance boot where the flavor cares deeply about the topology | |
| 19:39:47 | sean-k-mooney | when framed that way its what we do today | |