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. Can you explain why you consider it so bad ? How you think it could be improved ? > I certainly understand the goal of keeping the guest CLOCK_REALTIME is > sync with the host, but pvclock seems like overkill for that. VM migration. -- 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
