GOn Fri, May 20, 2016 at 10:18:47PM +0000, Jiang, Yunhong wrote: > > > > -----Original Message----- > > From: kvm-owner@xxxxxxxxxxxxxxx [mailto:kvm-owner@xxxxxxxxxxxxxxx] On > > Behalf Of Marcelo Tosatti > > Sent: Friday, May 20, 2016 11:19 AM > > To: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx> > > Cc: kvm@xxxxxxxxxxxxxxx; rkrcmar@xxxxxxxxxx; pbonzini@xxxxxxxxxx > > Subject: Re: [RFC PATCH 0/5] Utilizing VMX preemption for timer > > virtualization > > > > On Thu, May 19, 2016 at 06:44:58PM -0700, Yunhong Jiang wrote: > > > The VMX-preemption timer is a feature on VMX, it counts down, from the > > > value loaded by VM entry, in VMX nonroot operation. When the timer > > > counts down to zero, it stops counting down and a VM exit occurs. > > > > > > The VMX preemption timer for tsc deadline timer virtualization. The > > > VMX preemption timer is armed when the vCPU is running, and a VMExit > > > will happen if the virtual TSC deadline timer expires. > > > > > > When the vCPU thread is scheduled out, the tsc deadline timer > > > virtualization will be switched to use the current solution, i.e. use > > > the timer for it. It's switched back to VMX preemption timer when the > > > vCPU thread is scheduled int. > > > > > > This solution replace the complex OS's hrtimer system, and also the > > > host timer interrupt handling cost, with a preemption_timer VMexit. It > > > fits well for some NFV usage scenario, when the vCPU is bound to a > > > pCPU and the pCPU is isolated, or some similar scenarioes. > > > > > > However, it possibly has impact if the vCPU thread is scheduled in/out > > > very frequently, because it switches from/to the hrtimer emulation a > > > lot. A module parameter is provided to turn it on or off. > > > > > > Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxx> > > > > Hi Yunhong Jiang, > > > > This adds cost to the VM-exit and VM-entry paths (additional > > instructions and i-cache pressure). Also it adds cost to > > kvm_sched_out. > > Hi, Marcelo, > Thanks for reply. As reply to Paolo's previous mail, I will change it so > that there will be no extra cost to VM-exit/entry paths. > > Yes, it add costs to kvm_sched_out/in as stated in the commit > message, so it's not good if the thread is scheduled in/out a lot. > > > > > What is the benefit the switch from external interrupt to VMX preemption > > timer brings? > > The cost is the interrupt exit cost + HRtimer handler cost. I will get data on my next patch set. > > Thanks > --jyh The data will be useful. Paolo's argument about hrtimer_start/stop makes sense. -- 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
