| Posted | Nick | Remark | |
|---|---|---|---|
| #openstack-nova - 2018-03-22 | |||
| 17:02:09 | johnthetubaguy | jaypipes: cool, gtk | |
| 17:02:16 | sean-k-mooney | cfriesen: that depends. if the resouces asks for 6 cpus but 5 for the vm the the vms sees and 1 for emulator thread that it does not i think its ok | |
| 17:02:19 | stephenfin | jaypipes: Heh, yeah. I'm not saying it's _not_ all crap as it is (quite the opposite, tbh) but I just don't see yet how we can abstract this kind of stuff better than we do | |
| 17:02:26 | jaypipes | sahid: and that is a resource accounting problem, no? | |
| 17:02:32 | sahid | we ask for 4 dedicated pCPUs but actually 5 are dedicated | |
| 17:02:34 | stephenfin | I'm just seeing "if we do this, stuff that does work at the moment breaks" | |
| 17:02:58 | johnthetubaguy | stephenfin: what about having those two options I just mentioned? | |
| 17:03:06 | sahid | jaypipes: it is yes, but saying I want 5 vCPUs and the user only have 4vCPU is also a mistake | |
| 17:03:16 | sean-k-mooney | cfriesen: we accounted for the extra cpu for the emulator for in placement and the guest still got what it expected | |
| 17:03:46 | stephenfin | johnthetubaguy: The problem is, what VZW wants is different from what E/// want | |
| 17:03:48 | openstackgerrit | Merged openstack/osc-placement master: Do not depend on jenkins user in devstack gate https://review.openstack.org/552476 | |
| 17:03:58 | sahid | we don't have any unit in our resource management to take that extra CPU into account | |
| 17:04:14 | cfriesen | sean-k-mooney: I want to distinguish between what's in the flavor and what nova asks from placement. If the flavor says "4 VCPU and I want isolated emulator threads" then I'm fine with nova asking for 5 VCPU. | |
| 17:04:30 | cfriesen | sean-k-mooney: but I don't want the flavor to have to ask for 5 VCPU | |
| 17:04:41 | stephenfin | We could also stop treating emulator threads as a VCPU and add another resource class to that | |
| 17:04:44 | stephenfin | *for that | |
| 17:04:49 | sean-k-mooney | sahid: again it depends if the flavour vcpu field is set to 4 the that means the guest will see 4 cpus. if the resouce:VCPUS is 5 we will claim 5 cpus in placemnt 4 of which would be used by the guest and on that could be used for the emulator | |
| 17:04:50 | jaypipes | sahid: so perhaps the way forward is indeed to have a completely separate CPU_SHARED and CPU_DEDICATED resource class, separte from VCPU. VCPU would represent guest vCPU threads that are using *shared* host CPUs. CPU_SHARED would represent emulator threads (and anything else not guest-related) that are on shared host CPUs, and CPU_DEDICATED would be the amount of dedicated CPUs. | |
| 17:04:59 | stephenfin | (for the emulator_threads_policy issue) | |
| 17:05:01 | cfriesen | stephenfin: just to simplify things. :) | |
| 17:05:01 | johnthetubaguy | stephenfin: I guess I am trying to suggest we document the two extreme cases really well, then document a few ways you might want to dial back in either direction from the extreme? | |
| 17:05:30 | stephenfin | So CPU_SHARED, CPU_DEDICATED, and CPU_EMULATOR_THREADS | |
| 17:05:55 | jaypipes | stephenfin: CPU_EMULATOR_THREADS is not a consumable resource, though... | |
| 17:06:05 | jaypipes | stephenfin: CPU_SHARED is the consumable resource that emulator threads consume. | |
| 17:06:08 | cfriesen | stephenfin: fundamentally right now, emulator threads run on the same host CPUs as the guest CPUs do by default. | |
| 17:06:15 | sahid | jaypipes: yes basically i thought that was you idea since the beginning :) | |
| 17:06:39 | stephenfin | jaypipes: It could be though, if we added the 'emulator_pin_set' list to nova.conf that sahid suggests | |
| 17:06:49 | dansmith | sahid: I left you minor comments in that spec | |
| 17:07:10 | sahid | thanks dansmith i will address them | |
| 17:07:13 | cfriesen | stephenfin: I don't like the idea of having a separate dedicated pool just for emulator threads when the "shared" pool is perfectly fine | |
| 17:07:29 | jaypipes | stephenfin: you are conflating two different things. one is allocation of resources. the other is assignment of emulator threads to a particular set of host CPUs. | |
| 17:08:20 | cfriesen | I think accounting for the emulator threads as a VCPU (specifically if all the cpus in a guest are "dedicated") is a reasonable approximation | |
| 17:08:38 | cfriesen | If any of the cpus in a guest are "shared", then we can run the emulator threads with them. | |
| 17:09:00 | jaypipes | omg, chris you're killing me. | |
| 17:09:08 | stephenfin | jaypipes: Am I? We're saying for the resources for CPU_DEDICATED will come from CONF.cpu_dedicated_set. I'm saying the resources for CPU_EMULATOR_THREADS could come from CONF.cpu_emulator_thread_set | |
| 17:09:32 | sean-k-mooney | jaypipes: can i sugges we create an etherpad and start listing exampels and then clearly state what each means in terms of host,guest and placement allocatiosn | |
| 17:09:40 | cfriesen | jaypipes: ah, forget it. just account for the emulator threads as a VCPU. | |
| 17:09:41 | stephenfin | Then, as a guest, I want N CPU_EMULATOR_THREADS | |
| 17:10:08 | stephenfin | sean-k-mooney: Not a bad idea | |
| 17:10:12 | jaypipes | stephenfin: but according to cfriesen, you can't request that. | |
| 17:10:23 | jaypipes | stephenfin: you can't say "I want 3 emulator threads". | |
| 17:10:48 | bauzas | jaypipes: stephenfin: I'm done with my meeting | |
| 17:10:54 | bauzas | can we jump into a hangout ? | |
| 17:10:58 | jaypipes | bauzas: uhm,.... | |
| 17:11:00 | cfriesen | stephenfin: yeah, there's just some random number of emulator threads that are all pinned the same. | |
| 17:11:05 | jaypipes | bauzas: you've missed quite a bit. | |
| 17:11:24 | bauzas | jaypipes: yeah, that's what I just see... | |
| 17:12:09 | stephenfin | Riight. OK, forget that | |
| 17:12:10 | sean-k-mooney | maybe this would help https://libvirt.org/formatdomain.html#elementsCPUTuning | |
| 17:12:24 | sean-k-mooney | and this https://libvirt.org/formatdomain.html#elementsIOThreadsAllocation | |
| 17:12:24 | bauzas | jaypipes: stephenfin: any possible tl;dr ? | |
| 17:12:25 | jaypipes | sean-k-mooney: that's the problem... | |
| 17:12:44 | stephenfin | You can tell libvirt what CPUs the emulator threads will be placed on. Currently we say to pin only to one dedicated CPU | |
| 17:12:52 | jaypipes | sean-k-mooney: I am not interested in the pinning questions. I am only concerned with how to do resource accounting. | |
| 17:13:02 | stephenfin | But in this new world, we'd be affining to the entire pool of shared CPUs | |
| 17:13:13 | stephenfin | That first bit was what I was mixing up | |
| 17:13:20 | sean-k-mooney | jaypipes: yes i know. | |
| 17:13:31 | cfriesen | jaypipes: if every guest CPU is "shared" then the emulator threads just float across the same set of host cpus. It's when you have "dedicated" guest CPUs that the question of how to account for the emulator thread work arises. | |
| 17:13:48 | jaypipes | cfriesen: ack, understood. | |
| 17:13:49 | openstackgerrit | Nguyen Hai proposed openstack/nova-specs master: Enhance nova-specs webpage and clean up repo https://review.openstack.org/551802 | |
| 17:13:53 | sean-k-mooney | so looking at the docs you can only have 1 emulator thread. you can have multiple io thread and multiple vcpu trheads | |
| 17:14:13 | cfriesen | sean-k-mooney: don't we spawn another emulator thread when live migrating? | |
| 17:14:17 | stephenfin | bauzas: We're enraging jaypipes to the point that I fear for his pugs' lives | |
| 17:14:23 | jaypipes | cfriesen: but it's not like that is something dynamically calculated, right? I mean, a guest either has dedicated CPUs or it doesn.t' | |
| 17:14:26 | jaypipes | https://tenor.com/view/angry-panda-mascot-mad-gif-3456638 | |
| 17:14:29 | sean-k-mooney | cfriesen: i dont know but in that case to we care | |
| 17:14:38 | cfriesen | jaypipes: correct. | |
| 17:14:41 | bauzas | stephenfin: hold my beer | |
| 17:14:45 | sean-k-mooney | cfriesen: its live migrating all sla are out the window at that point | |
| 17:14:53 | stephenfin | bauzas: Also, we're trying to figure out how we map things like emulator threads policy and the ISOLATE CPU threads policy features to placement | |
| 17:15:00 | bauzas | k | |
| 17:15:11 | jaypipes | cfriesen: so it would be a request for, say resources:CPU_DEDICATED=4 (the vCPU threads) and resources:CPU_SHARED=1 (the emulator thread) | |
| 17:15:50 | openstackgerrit | Nguyen Hai proposed openstack/nova-specs master: Enhance nova-specs webpage and clean up repo https://review.openstack.org/551802 | |
| 17:15:51 | cfriesen | jaypipes: if we assume that the emulator thread is doing a non-trivial amount of work then that would be accurate. | |
| 17:15:59 | jaypipes | cfriesen: and no request for resources:VCPU, since there's no "VCPU" resources being consumed by the guest. | |
| 17:16:16 | cfriesen | jaypipes: if we assumed the emulator thread is doing a trivial amount of work then we could avoid allocating resources for the emulator thread entirely | |
| 17:16:33 | jaypipes | cfriesen: it's not about whether the emulator thread is doing a lot of work or not. it's about whether we want to *account* for it -- which obviously would be up to the operator, yes? | |
| 17:17:01 | jaypipes | cfriesen: if the operator didn't care, they wouldn't put resources:CPU_SHARED=1 in the flavor extra spec | |
| 17:17:18 | cfriesen | jaypipes: okay, I get you. | |
| 17:17:46 | cfriesen | keeping resource allocation separate from the nitty-gritty of actually affining things, I think what you say is true | |
| 17:17:56 | jaypipes | ++ | |
| 17:17:57 | stephenfin | I wonder what operators _would_ care about this? It's not exactly a thing you'd use in a bog-standard cloud | |
| 17:18:12 | sean-k-mooney | jaypipes: i dodnt think we should assume resources:CPU_SHARED=1 is the emulator thread though | |
| 17:18:23 | stephenfin | Another idea: we could add sahid's emulator_thread_pin_set option and forget the whole thing accounting thing | |
| 17:18:52 | jaypipes | sean-k-mooney: we're not assuming that. | |
| 17:18:53 | stephenfin | Same way we don't account for things that currently run on the host outside of 'vcpu_pin_set' | |
| 17:19:19 | jaypipes | sean-k-mooney: it could be anything associated with the guest that is consuming some amount of shared CPU resources. | |
| 17:19:36 | stephenfin | That's a variant of the "use the shared pool and ignore the impact idea", but this would ensure our emulator threads wouldn't impact the guests running on said shared pool | |
| 17:19:44 | sean-k-mooney | ok can i sugges we document this here https://etherpad.openstack.org/p/cpu-resource-accounting | |
| 17:19:48 | stephenfin | e.g. during a live migration | |
| 17:19:52 | sean-k-mooney | then we can pull it into the spec | |
| 17:20:02 | jaypipes | sure | |
| 17:20:12 | cfriesen | jaypipes: the thing here though is that "emulator_thread_policy=isolate" is what we currently ask for. I'm not enthusiastic about saying that they *also* need to set CPU_SHARED=1 or similar | |
| 17:20:16 | jaypipes | I need to take a quick walk to screw my head back on, though. | |
| 17:20:34 | cfriesen | I need coffee | |
| 17:20:54 | jaypipes | cfriesen: they wouldn't *need* to. if they didn't, the resource accounting would just continue to be messed up and inaccurate. | |
| 17:20:57 | bauzas | I need to catch up the convo... | |
| 17:21:08 | jaypipes | anyway, I will now take a walk. | |
| 17:21:40 | openstackgerrit | sahid proposed openstack/nova-specs master: libvirt: add support for virtio-net rx/tx queue sizes https://review.openstack.org/539605 | |
| 17:21:42 | sean-k-mooney | bauzas: the reason i suggested the etherpad is i think we all need to catch up and get it donw on "paper" | |