On Tue, Apr 1, 2014 at 5:12 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > On Tue, Apr 01, 2014 at 12:17:16PM -0700, Andy Lutomirski wrote: >> On Tue, Apr 1, 2014 at 11:01 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: >> > On Mon, Mar 31, 2014 at 10:33:41PM -0700, Andy Lutomirski wrote: >> >> On Mar 31, 2014 8:45 PM, "Marcelo Tosatti" <mtosatti@xxxxxxxxxx> wrote: >> >> > >> >> > On Mon, Mar 31, 2014 at 10:52:25AM -0700, Andy Lutomirski wrote: >> >> > > On 03/29/2014 01:47 AM, Zhanghailiang wrote: >> >> > > > Hi, >> >> > > > I found when Guest is idle, VDSO pvclock may increase host consumption. >> >> > > > We can calcutate as follow, Correct me if I am wrong. >> >> > > > (Host)250 * update_pvclock_gtod = 1500 * gettimeofday(Guest) >> >> > > > In Host, VDSO pvclock introduce a notifier chain, pvclock_gtod_chain in timekeeping.c. It consume nearly 900 cycles per call. So in consideration of 250 Hz, it may consume 225,000 cycles per second, even no VM is created. >> >> > > > In Guest, gettimeofday consumes 220 cycles per call with VDSO pvclock. If the no-kvmclock-vsyscall is configured, gettimeofday consumes 370 cycles per call. The feature decrease 150 cycles consumption per call. >> >> > > > When call gettimeofday 1500 times,it decrease 225,000 cycles,equal to the host consumption. >> >> > > > Both Host and Guest is linux-3.13.6. >> >> > > > So, whether the host cpu consumption is a problem? >> >> > > >> >> > > Does pvclock serve any real purpose on systems with fully-functional >> >> > > TSCs? The x86 guest implementation is awful, so it's about 2x slower >> >> > > than TSC. It could be improved a lot, but I'm not sure I understand why >> >> > > it exists in the first place. >> >> > >> >> > VM migration. >> >> >> >> Why does that need percpu stuff? Wouldn't it be sufficient to >> >> interrupt all CPUs (or at least all cpus running in userspace) on >> >> migration and update the normal timing data structures? >> > >> > Are you suggesting to allow interruption of the timekeeping code >> > at any time to update frequency information ? >> >> I'm not sure what you mean by "interruption of the timekeeping code". >> I'm suggesting sending an interrupt to the guest (via a virtio device, >> presumably) to tell it that it has been paused and resumed. > > code: > > 1) disable interrupts > 2) A = RDTSC > 3) B = SCALE(A, TSC.FREQ) > > If migration happens between 2 and 3, you've got an incorrect value. > Fair enough. I guess 1) disable interrupts 2) A = RDTSC 3) B = SCALE(A, TSC.FREQ) is also bad if (3) blocks due to magic invalidation of the physical page. --Andy -- 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
