[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] time-xen : Reset monotonic time when sync up time from dom0 to domU
At 15:09 +0100 on 15 Oct (1287155382), Dan Magenheimer wrote: > > At 17:48 +0100 on 13 Oct (1286992083), Jeremy Fitzhardinge wrote: > > > > There was a paper about this at OSDI last week: > > > > http://www.usenix.org/events/osdi10/tech/full_papers/Broomhead.pdf > > > > > > Ooh, look, RADclock, just what I was thinking about. > > > > Yes, it looks pretty good. Also they can use Xen stime as the local > > oscillator and distribute drift numbers from xenstore, so no hypervisor > > patches (and no hypervisor-interface changes) required. :) > > Maybe I'm misunderstanding the paper, but isn't it required > that Xen stime be directly readable for every attempt to > sample time (e.g. requiring at least some small interface > change such as adding a hypercall to obtain Xen stime)? You can obtain xen stime directly from the shared-into page and RDTSC. There might need to be _kernel_ changes to make that available to userspace, though my impression is that they push packet-timestamping into the kernel. > Also, for those certain enterprise applications that want > to sample time 10000+ samples/second/processor, and need > to know "immediately" when a sample might be bad (due > to, for example, live migration), I think each sample would > need to check xenstore. Is xenstore up to that kind of > pounding (and is it fast enough)? No it's certainly not (either of those things), but: - the problem of turning a distributed wallclock into something suitable for timestamping that aggressively is orthogonal to the problem of distributing that wallclock in the first place; and - all the user really needs is a generation counter to know that the clock correction values are stale, and the kernel can provide that alongside the stime. Cheers, Tim. -- Tim Deegan <Tim.Deegan@xxxxxxxxxx> Principal Software Engineer, XenServer Engineering Citrix Systems UK Ltd. (Company #02937203, SL9 0BG) _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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