On Thu, 2020-03-12 at 14:40 +0000, George Dunlap wrote: > > On Mar 12, 2020, at 1:55 PM, Andrew Cooper < > > [email protected]> wrote: > > > > Certainly as far as XenServer is concerned, we haven't seen > > symptoms > > like this in a decade of running credit1. > > One reason could be because the symptoms are different. On credit1, > credits and “priority” are separated; it’s not possible in credit1 > for a vcpu to end up with a lower priority than the idle domain, and > no matter how low the credits become, a vcpu will always end up with > some “peers” at the same priority level, meaning it always has a > chance at some cpu. > Indeed. Under many respects, Credit1 is basically a slightly more powerful variant of Round-Robin.
The actual time vCPUs spend executing influences the scheduling decisions a lot less than in Credit2 (well, it influences the scheduling decisions a lot less than in any scheduler of any OS or hypervisor conceived and implemented in the last decade, or maybe even more). As a matter of fact, this property happens to have shield it from the nastiest effects of the issue at hand. > What Dario is saying (if I understand him correctly) is that the > *proximate* cause (allowing a vcpu to have an effective priority of > less than idle) is certainly credit2-only; but the *deeper* cause > (vcpus racking up massive amounts of negative credit) is not. > Yes, that's exactly what I meant. :-) Regards -- Dario Faggioli, Ph.D http://about.me/dario.faggioli Virtualization Software Engineer SUSE Labs, SUSE https://www.suse.com/ ------------------------------------------------------------------- <<This happens because _I_ choose it to happen!>> (Raistlin Majere)
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