Re: Synchronized time with kvm_clock
On Monday 26 April 2010, I wrote: host: cat /sys/devices/system/clocksource/clocksource0/current_clocksource tsc guest: cat /sys/devices/system/clocksource/clocksource0/current_clocksource kvm-clock Forgotten some info which might be essential: Kernel (host and guest): 2.6.32-trunk-amd64 qemu-kvm: 0.12.3+dfsg-4 Please keep me on the cc, I'm not on the list. -- Greetings, Alex Hermann -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Synchronized time with kvm_clock
Alex, You don't need to run ntp on each guest. You can enable rtc support in the guest kernel and on the hypervisor. Run ntp client on the hypervisor via cron, and use hwclock -w on the hypervisor after you run ntp, to sync the hardware clock to the system clock (which is now updated by ntpdate). On the guests, periodically run hwclock -s to set the system clock from the hw clock. This seems to work extremely well, the clocksource on the guests as kvm_clock, and as long as you have the clocksource as hpet or acpi_pm on the hypervisor, there doesn't seem to be any problems with keeping time. The only thing I've noticed is that when you reboot, the very first guest will have the wrong time on boot, so the uptime is messed up. Regards Alex Hermann wrote: On Monday 26 April 2010, I wrote: host: cat /sys/devices/system/clocksource/clocksource0/current_clocksource tsc guest: cat /sys/devices/system/clocksource/clocksource0/current_clocksource kvm-clock Forgotten some info which might be essential: Kernel (host and guest): 2.6.32-trunk-amd64 qemu-kvm: 0.12.3+dfsg-4 Please keep me on the cc, I'm not on the list. -- John Buswell CEO, Carbon Mountain LLC http://www.carbonmountain.com -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: Synchronized time with kvm_clock
On Mon, Apr 26, 2010 at 10:32:51AM -0400, John Buswell wrote: You don't need to run ntp on each guest. You can enable rtc support in the guest kernel and on the hypervisor. Run ntp client on the hypervisor via cron, and use hwclock -w on the hypervisor after you run ntp, to sync the hardware clock to the system clock (which is now updated by ntpdate). On the guests, periodically run hwclock -s to set the system clock from the hw clock. What a *horribly* hacky way to do it, meaning you'll get time warps all over the place, admittedly of short intervals if you run those cron jobs often enough. It seems much simpler to me to simply run ntpd in all the guests. It's not like the extra CPU or bandwidth is going to be a problem. At the very least you want to run ntpd, not ntpdate out of cron, in the hypervisor, and only use cron for those hwclock -w's. This seems to work extremely well, the clocksource on the guests as kvm_clock, and as long as you have the clocksource as hpet or acpi_pm on the hypervisor, there doesn't seem to be any problems with keeping time. The only thing I've noticed is that when you reboot, the very first guest will have the wrong time on boot, so the uptime is messed up. And I think many people would find this unacceptable. Really, I appreciate that keep the time sync'd via ntpd on the hypervisor and have it passed accurately to the guests has a certain elegant simplicity about it. But if you achieve the latter by periodically resyncing against what the guest sees as its hardware clock you've lost that elegance again. It really needs to 'just work' via KVM code in the guest kernel using the exact same time as the hypervisor kernel is supplying. -- - Athanasius = Athanasius(at)miggy.org / http://www.miggy.org/ Finger athan(at)fysh.org for PGP key And it's me who is my enemy. Me who beats me up. Me who makes the monsters. Me who strips my confidence. Paula Cole - ME signature.asc Description: Digital signature
Re: Synchronized time with kvm_clock
Athanasius wrote: On Mon, Apr 26, 2010 at 10:32:51AM -0400, John Buswell wrote: You don't need to run ntp on each guest. You can enable rtc support in the guest kernel and on the hypervisor. Run ntp client on the hypervisor via cron, and use hwclock -w on the hypervisor after you run ntp, to sync the hardware clock to the system clock (which is now updated by ntpdate). On the guests, periodically run hwclock -s to set the system clock from the hw clock. What a *horribly* hacky way to do it, meaning you'll get time warps all over the place, admittedly of short intervals if you run those cron jobs often enough. It seems much simpler to me to simply run ntpd in all the guests. It's not like the extra CPU or bandwidth is going to be a problem. At the very least you want to run ntpd, not ntpdate out of cron, in the hypervisor, and only use cron for those hwclock -w's. Not really. You don't get time warps at all, the only place you get a time warp is on the initial guest, and thats not a problem with the workaround I suggested. It seems to be an issue with the clock on the initial guest. There is no point wasting resources by running ntpd on each guest when you don't have to. This seems to work extremely well, the clocksource on the guests as kvm_clock, and as long as you have the clocksource as hpet or acpi_pm on the hypervisor, there doesn't seem to be any problems with keeping time. The only thing I've noticed is that when you reboot, the very first guest will have the wrong time on boot, so the uptime is messed up. And I think many people would find this unacceptable. This particular problem has nothing to do with what I suggested above. This is some kind of issue with kvm_clock on the first guest starting up. Really, I appreciate that keep the time sync'd via ntpd on the hypervisor and have it passed accurately to the guests has a certain elegant simplicity about it. But if you achieve the latter by periodically resyncing against what the guest sees as its hardware clock you've lost that elegance again. It really needs to 'just work' via KVM code in the guest kernel using the exact same time as the hypervisor kernel is supplying. I agree. Unfortunately, kvm_clock doesn't seem to be quite there yet. So using rtc0 as a comparison, and keeping the hypervisor clock in sync with reality, is a good way to avoid having to run N+1 copies of ntpd on the guests :) -- John Buswell CEO, Carbon Mountain LLC http://www.carbonmountain.com -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
