On Thu, Feb 25, 2016 at 4:20 AM, Radim Krčmář <rkrcmar@xxxxxxxxxx> wrote: > 2016-02-24 19:50-0800, Owen Hofmann: >> On Wed, Feb 24, 2016 at 5:19 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >>> Of course the guest can run its own NTP daemon or similar adjtimex >>> caller and cause the guest to stop tracking the host. But if the host >>> passed CLOCK_MONOTONIC through, then the guest would, by default, >>> treat kvm-clock as an exactly 1GHz source and would then expose a >>> disciplined NTP-tracking CLOCK_MONOTONIC through to its user apps even >>> without an NTP client on the guest. >>> >>> If integration with the POSIX clock core were provided, the guest >>> would learn to consume the host's CLOCK_REALTIME as well, as long as >>> the host uses the tsc as its clocksource. >> >> Your proposal, which I'd describe as a direct passthrough (to the >> extent possible) of the host gettimeofday vdso to a kvm guest, sounds >> like a much better way to get clock frequency adjustments from the >> host to the guest. But I don't know if I can think of a reason to do >> this besides "hey you don't have to run ntp". Is there a situation you >> have in mind that this helps out? > > Running NTP only on the host is a good reason. > (And probably the only reason I'd call good, because any software that > passes TSC or CLOCK_MONOTONIC timestamps between hosts needs to handle > their differences.) There are handful of distributed algorithms that benefit from clocks with a bounded worst-case synchronization error. I think that Google uses some. If some cloud provider were to provide, say, 10ms max CLOCK_REALTIME error and pass CLOCK_REALTIME through using kvm-clock, it could be quite useful. --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
