On Fri, Jun 23, 2017 at 10:34:26AM -0600, Chris Friesen wrote: > On 06/23/2017 09:35 AM, Henning Schild wrote: > > Am Fri, 23 Jun 2017 11:11:10 +0200 > > schrieb Sahid Orentino Ferdjaoui <sferd...@redhat.com>: > > > > In Linux RT context, and as you mentioned, the non-RT vCPU can acquire > > > some guest kernel lock, then be pre-empted by emulator thread while > > > holding this lock. This situation blocks RT vCPUs from doing its > > > work. So that is why we have implemented [2]. For DPDK I don't think > > > we have such problems because it's running in userland. > > > > > > So for DPDK context I think we could have a mask like we have for RT > > > and basically considering vCPU0 to handle best effort works (emulator > > > threads, SSH...). I think it's the current pattern used by DPDK users. > > > > DPDK is just a library and one can imagine an application that has > > cross-core communication/synchronisation needs where the emulator > > slowing down vpu0 will also slow down vcpu1. You DPDK application would > > have to know which of its cores did not get a full pcpu. > > > > I am not sure what the DPDK-example is doing in this discussion, would > > that not just be cpu_policy=dedicated? I guess normal behaviour of > > dedicated is that emulators and io happily share pCPUs with vCPUs and > > you are looking for a way to restrict emulators/io to a subset of pCPUs > > because you can live with some of them beeing not 100%. > > Yes. A typical DPDK-using VM might look something like this: > > vCPU0: non-realtime, housekeeping and I/O, handles all virtual interrupts > and "normal" linux stuff, emulator runs on same pCPU > vCPU1: realtime, runs in tight loop in userspace processing packets > vCPU2: realtime, runs in tight loop in userspace processing packets > vCPU3: realtime, runs in tight loop in userspace processing packets > > In this context, vCPUs 1-3 don't really ever enter the kernel, and we've > offloaded as much kernel work as possible from them onto vCPU0. This works > pretty well with the current system. > > > > For RT we have to isolate the emulator threads to an additional pCPU > > > per guests or as your are suggesting to a set of pCPUs for all the > > > guests running. > > > > > > I think we should introduce a new option: > > > > > > - hw:cpu_emulator_threads_mask=^1 > > > > > > If on 'nova.conf' - that mask will be applied to the set of all host > > > CPUs (vcpu_pin_set) to basically pack the emulator threads of all VMs > > > running here (useful for RT context). > > > > That would allow modelling exactly what we need. > > In nova.conf we are talking absolute known values, no need for a mask > > and a set is much easier to read. Also using the same name does not > > sound like a good idea. > > And the name vcpu_pin_set clearly suggest what kind of load runs here, > > if using a mask it should be called pin_set. > > I agree with Henning. > > In nova.conf we should just use a set, something like > "rt_emulator_vcpu_pin_set" which would be used for running the emulator/io > threads of *only* realtime instances.
I'm not agree with you, we have a set of pCPUs and we want to substract some of them for the emulator threads. We need a mask. The only set we need is to selection which pCPUs Nova can use (vcpus_pin_set). > We may also want to have "rt_emulator_overcommit_ratio" to control how many > threads/instances we allow per pCPU. Not really sure to have understand this point? If it is to indicate that for a pCPU isolated we want X guest emulator threads, the same behavior is achieved by the mask. A host for realtime is dedicated for realtime, no overcommitment and the operators know the number of host CPUs, they can easily deduct a ratio and so the corresponding mask. > > > If on flavor extra-specs It will be applied to the vCPUs dedicated for > > > the guest (useful for DPDK context). > > > > And if both are present the flavor wins and nova.conf is ignored? > > In the flavor I'd like to see it be a full bitmask, not an exclusion mask > with an implicit full set. Thus the end-user could specify > "hw:cpu_emulator_threads_mask=0" and get the emulator threads to run > alongside vCPU0. Same here, I'm not agree, the only set is the vCPUs of the guest. Then we want a mask to substract some of them. > Henning, there is no conflict, the nova.conf setting and the flavor setting > are used for two different things. > > Chris > > __________________________________________________________________________ > OpenStack Development Mailing List (not for usage questions) > Unsubscribe: openstack-dev-requ...@lists.openstack.org?subject:unsubscribe > http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev __________________________________________________________________________ OpenStack Development Mailing List (not for usage questions) Unsubscribe: openstack-dev-requ...@lists.openstack.org?subject:unsubscribe http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
