[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: (progress on hpet accuracy) and Re: [Xen-devel] [PATCH] Add a timer mode that disables pending missed ticks
Keir, Dan: This is an update on the hpet work. With limited testing, the accuracy of this method appears to be at least 10 times better than the pit/tsc method for usex loads. Also, as mentioned before, it does not have the time going backwards problem. Another interesting property is that the same policy is used for a 32 bit Linux guest and 64 bit Linux. Two recent 14-16 hour tests were run, one with usex e48 and the other with usex b48 as loads. The same load was run on three 8 vcpu guests on an 8 cpu platform. No ntpd running. ntpdate -b used for initial setting of clock and ntpdate -q used for monitoring drift. The guests were 4u464, 4u564, 4u432 Linux. The results are: test duration drift (secs) 4u464,4u564,4u432 drift % usex b48 16 hrs -.68, -.60, -.68 -.0012 usex e48 15 hrs -.58, -.55, -.58 -.0011 The drift with pit/tsc as checked in reported a few months ago: The error tests confirm the equivalence. With overnight cpu loads, the checked in version was accurate to +.048% for sles and +.038% for red hat. Recall that .05% is the goal for accuracy so that ntpd can synchronize. With pit/tsc we are rather close to that limit. So far, with hpet, we are well under that goal. As for the code structure, I tried layering on vpt.c and had trouble doing so. Therefore, I focused on a direct approach. I'll describe the approach shortly, after I run tests with other loads and guests. Regards, Dave Keir Fraser wrote: On 26/2/08 14:45, "Dave Winchell" <dwinchell@xxxxxxxxxxxxxxx> wrote:If this is an integration of hpet into the vpt.c infrastructure then that would be very welcome.So far it is not, however I may head in that direction. Last night's test had an error of .065% on one guest so I still have some work to do.Then I hope that the accuracy improvements have come from a set of changes whose effects are both explicable and generally for the good (rather than artefacts of how a particular sub-point release of Linux drives the HPET). -- Keir _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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