On 12/12/2017 04:28 PM, Cornelia Huck wrote: > On Tue, 12 Dec 2017 16:17:17 +0100 > David Hildenbrand <da...@redhat.com> wrote: > >> On 12.12.2017 15:29, Cornelia Huck wrote: >>> On Tue, 12 Dec 2017 15:13:46 +0100 >>> Christian Borntraeger <borntrae...@de.ibm.com> wrote: >>> >>>> On 12/12/2017 02:49 PM, Cornelia Huck wrote: >>> >>>>> One thing I noticed: You removed the caching of the flic (in the old >>>>> kvm inject routine), and you generally do more qom invocations (first, >>>>> to find the common flic; then, to translate to the qemu or kvm flic). >>>>> Not sure if this might be a problem (probably not). >>>> >>>> Is any of these calls on a potential fast path (e.g. guest without adapter >>>> interrupts)? If yes, then QOM is a no-go since it is really slow. >>> >>> At least the new airq interface was using QOM without caching before. >>> >>> It's basically about any interrupt; but otoh we are (for kvm) in >>> userspace already. Caching the flic and just keeping the casting to the >>> specialized flic might be ok (I'd guess that the lookup is the slowest >>> path.) >>> >> >> Please note that the lookup is already cached in s390_get_flic(); That >> should be sufficient, as it does the expensive lookup. One cache should >> be enough, no? > > Ah, missed that. So the old code actually did double caching... > >> >> The other conversions should be cheap (and already were in place in a >> couple of places before). > > Yes, object_resolve_path() is probably the most expensive one. > > Did anyone ever check if the (existing) conversions are actually > measurable?
Did some quick measurement. the initial object resolve takes about 20000ns, the cached ones then is 2-5ns. (measuring s390_get_flic function). The other conversions (S390FLICStateClass *fsc = S390_FLIC_COMMON_GET_CLASS(fs);) takes about 15-25ns (up to 100 cycles). For irqfd users this does not matter, but things like plan9fs might benefit if we speedup this lookup as well? PS: We can improve the initial object_resolve_path by doing the resolve not for TYPE_KVM_S390_FLIC but "/machine/" TYPE_KVM_S390_FLIC (2500ns instead of 20000) but its still way too slow.
