On Wed, Oct 11, 2017 at 04:19:38PM -0400, Waiman Long wrote: > On 10/10/2017 03:41 PM, Eduardo Habkost wrote: > > On Tue, Oct 10, 2017 at 02:07:25PM -0400, Waiman Long wrote: > >> On 10/10/2017 11:50 AM, Eduardo Habkost wrote: > >>>> Yes. Another possibility is to enable it when there is >1 NUMA node in > >>>> the guest. We generally don't do this kind of magic but higher layers > >>>> (oVirt/OpenStack) do. > >>> Can't the guest make this decision, instead of the host? > >> By guest, do you mean the guest OS itself or the admin of the guest VM? > > It could be either. But even if action is required from the > > guest admin to get better performance in some cases, I'd argue > > that the default behavior of a Linux guest shouldn't cause a > > performance regression if the host stops hiding a feature in > > CPUID. > > > >> I am thinking about maybe adding kernel boot command line option like > >> "unfair_pvspinlock_cpu_threshold=4" which will instruct the OS to use > >> unfair spinlock if the number of CPUs is 4 or less, for example. The > >> default value of 0 will have the same behavior as it is today. Please > >> let me know what you guys think about that. > > If that's implemented, can't Linux choose a reasonable default > > for unfair_pvspinlock_cpu_threshold that won't require the admin > > to manually configure it on most cases? > > It is hard to have a fixed value as it depends on the CPUs being used as > well as the kind of workloads that are being run. Besides, using unfair > locks have the undesirable side effect of being subject to lock > starvation under certain circumstances. So we may not work it to be > turned on by default. Customers have to take their own risk if they want > that.
Probably I am not seeing all variables involved, so pardon my confusion. Would unfair_pvspinlock_cpu_threshold > num_cpus just disable usage of kvm_pv_unhalt, or make the guest choose a completely different spinlock implementation? Is the current default behavior of Linux guests when kvm_pv_unhalt is unavailable a good default? If using kvm_pv_unhalt is not always a good idea, why do Linux guests default to eagerly trying to use it only because the host says it's available? -- Eduardo -- libvir-list mailing list libvir-list@redhat.com https://www.redhat.com/mailman/listinfo/libvir-list
