[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Is there a way to get consistant time across different VMs?
On 09/07/2010 02:12 PM, walmart wrote: > Hi, all: > > I want to test some program, which requires a high precise time. (some > thing as rdtsc). > > I tested rdtsc on different VMs, they are not consistant. (I am using > Xen 4.0, Fedora 13, 64 bit). > > Is there a way to get consistant high precise time across different VMs? Hm, you're walking into a bit of a minefield. What are your precise requirements for: * accuracy * resolution * monotonicity * cross-cpu synchronization * cross-process synchronization ? In general rdtsc isn't very useful as a timesource, since there are many ways in which it can fail/do strange things from processor to processor and system to system, so "s[a]me thing as rdtsc" doesn't tell us much. However, in modern versions of Xen, you can turn on rdtsc emulation which makes rdtsc generate a guaranteed global monotonic time value at a nominal 1GHz rate (I think, or did that change to the starting CPU speed?). But the downside is that it results in a trap'n'emulate of the instruction which is a bit more expensive than a raw rdtsc. Or if you have a new Intel system with a really, truly nonstop synchronized tsc, you can use rdtsc directly. But if neither of those are acceptable/possible, you need to use the normal Xen system time, which has a 1ns resolution, is fairly precise, but not generally completely monotonic between cpus. It also isn't usable from usermode without some extra kernel patches. J _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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