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 ? Do you want to that as a special tsc clocksource driver ? > Even better: have the VM offer to invalidate the physical page > containing the kernel's clock data on migration and interrupt one CPU. > If another CPU races, it'll fault and wait for the guest kernel to > update its timing. Perhaps that is a good idea. > Does the current kvmclock stuff track CLOCK_MONOTONIC and > CLOCK_REALTIME separately? No. kvmclock counting is interrupted on vm pause (the "hw" clock does not count during vm pause). > > Can you explain why you consider it so bad ? How you think it could be > > improved ? > > The second rdtsc_barrier looks unnecessary. Even better, if rdtscp is > available, then rdtscp can replace rdtsc_barrier, rdtsc, and the > getcpu call. > > It would also be nice to avoid having two sets of rescalings of the timing data. Yep, probably good improvements, patches are welcome :-) -- 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
