On Thu, Dec 11, 2014 at 12:48:36PM -0800, Andy Lutomirski wrote: > On 12/10/2014 07:07 PM, Marcelo Tosatti wrote: > > On Thu, Dec 11, 2014 at 12:37:57AM +0100, Paolo Bonzini wrote: > >> > >> > >> On 10/12/2014 21:57, Marcelo Tosatti wrote: > >>> For the hrtimer which emulates the tscdeadline timer in the guest, > >>> add an option to advance expiration, and busy spin on VM-entry waiting > >>> for the actual expiration time to elapse. > >>> > >>> This allows achieving low latencies in cyclictest (or any scenario > >>> which requires strict timing regarding timer expiration). > >>> > >>> Reduces cyclictest avg latency by 50%. > >>> > >>> Note: this option requires tuning to find the appropriate value > >>> for a particular hardware/guest combination. One method is to measure the > >>> average delay between apic_timer_fn and VM-entry. > >>> Another method is to start with 1000ns, and increase the value > >>> in say 500ns increments until avg cyclictest numbers stop decreasing. > >> > >> What values are you using in practice for the parameter? > > > > 7us. > > > It takes 7us to get from TSC deadline expiration to the *start* of > vmresume? That seems rather extreme. > > Is it possible that almost all of that latency is from deadline > expiration to C-state exit? If so, can we teach the timer code to wake > up early to account for that? We're supposed to know our idle exit > latency these days. 7us includes: idle thread wakeup idle schedout ksoftirqd schedin ksoftirqd schedout qemu schedin vm-entry C-states are disabled of course. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
