[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: [xen-devel] System time monotonicity
> If we wanted to be more certain we could maintain a > last_system_time fields per VCPU and If you mean per VCPU *and* per guest this seems like a good idea. > backwards, by turning small backwards deltas into very short > periods of stalled time. The stalled time may be a problem, but only if the tsc skew between processors is "bad". Your estimate of 100us seems like it could be unacceptable for some applications. Any idea how expensive arch/x86/time.c:local_time_calibration() is? If it's not too bad, one option might be to add a xen boot parameter "calibratehz" to calibrate more frequently. Then systems running time-sensitive guests can be instructed to increase the parameter accordingly to ensure tsc skew is small enough. > > If you are not confident that this will be OK on existing and > > (within-reason) future Xen platforms, perhaps the hvm virtual > > platform timers should (at least optionally) be built on physical > > platform timers (Dave Winchell cc'ed), which would ensure time > > never goes backwards. > > If we wanted to be more certain we could maintain a > last_system_time fields > per VCPU and, whenever using system time to compute current > value for a > virtual timer for an HVM VCPU, we could actually use max(system time, > last_system_time). This would mean we were 100% sure that > time didn't go > backwards, by turning small backwards deltas into very short > periods of > stalled time. > > As it is: no, since system time 'free runs' on each CPU over > one-second > periods, there can be drift between CPUs if they are driven > by different > oscillators. Also there are tolerances in our software > calibration code to > consider. Which is why Linux guests implement the max(curr > time, last time) > in their gettimeofday() code. It would be quite reasonable to > the same, > inside Xen, for HVM guests. We can at least be pretty certain that any > drifts across CPUs/VCPUs will be on the order of less than 100us. > > -- Keir > > > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |