On 10/12/2014 17:53, Marcelo.Tosatti@xxxxxxxx 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. > > Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx> What is the latency value that you find in practice, for both apic_timer_fn to vmentry? Or for apic_timer_fn to just before vmrun? Let's start with a kvm-unit-tests patch to measure this value. We can then decide whether to hardcode a small default value (e.g. 1000-3000) and make it a module parameter? Or perhaps start with a higher value (twice what you find in practice?) and adjust it towards a target every time wait_lapic_expire is called. But in order to judge the correct approach, I need to see the numbers. Paolo -- 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
