On Mon, Oct 29, 2012 at 10:33 AM, Phil Mayers <p.may...@imperial.ac.uk> wrote: > On 10/28/2012 10:16 PM, Tristan Seligmann wrote: >> On Mon, Oct 29, 2012 at 12:05 AM, Phil Mayers <p.may...@imperial.ac.uk> >> wrote: > Sadly, this is not the case. As has already been pointed out, virtual > machine clocks can undergo stepping in "normal" oepration. A specfic > example: if a VMWare installation performs live migration of a host. We > often see: > > ntpd[1793]: time reset +0.263757 s > > ...when this happens. This can occur several times a day, as we're > running vCenter-controlled auto-migration - a very common setup. This is > forward stepping of course, so is relatively harmless (backward is a pain).
My original reply was perhaps a bit... exclusionary. But I think the point I was trying to make is still valid; if your VM is suspended for 250ms due to a migration (or CPU throttling, or ...) then it is impossible to maintain a consistent stream of time events because there isn't any code executing at all. Whether the clock stalls, or is jumped forward 250ms, you still have an unavoidable problem as far as time-keeping is concerned. >>> Who knows what newer implementations like chrony or openntpd do! >> >> If they're doing something silly, then maybe you shouldn't use them. I didn't intend it that way, but I can see how what I said could be read in a hostile / defensive tone. Perhaps this would be a better phrasing: If chrony or openntpd are doing something sensible, then there shouldn't be any serious issues with using them; if they're doing something that seriously destabilizes the system clock, then you probably shouldn't be using them, at least not if you want reliable timekeeping. I'm not familiar enough with either of them to know which case applies, so I didn't intend to come across as bashing chrony and/or openntpd. > Maybe "Twisted doesn't need a monotonic clock" is the right reply, but > it would be wrong to base that on the assumpton that "ntpd doesn't step" > - that's all I was trying to say. I don't think anyone in this thread is arguing *against* implementing this functionality; I think the point was just that this functionality is only of critical importance under a limited range of circumstances, as opposed to being something of urgent need for every Twisted-using program that needs to schedule timed events. -- mithrandi, i Ainil en-Balandor, a faer Ambar _______________________________________________ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
