[Xen-devel] HPET/PIT timer accuracy

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

Does anyone on this list have an idea of expected accuracy of chipset 
time sources (e.g., PIT/HPET)? I've been looking at relative frequency 
stability of the CPU-local TSC and platform HPET on a few different 
systems, and see weird instabilities that I cannot explain.
On a couple of systems based on Intel E7520 (Lindenhurst) chipset I see 
the relative frequency is stable to within a couple of parts per 
million (which is within measurement error). Either the two are clocked 
from the same crystal, or two crystals with low frequency drift/wander 
(but not unexpectedly low -- I don't think crystals should drift over 
short time periods).
In contrast, two different systems based on AMD 8111 chipset perform 
very badly. The relative frequency of TSC vs HPET is stable only to 
within +/- 30ppm! This means that, during one second, timers based on 
these two sources can diverge by around 30us, which is a *lot*. Clearly 
the TSC and HPET are driven off different time sources, but I almost 
cannot believe that both are driven off crystals -- one of the two must 
have a 1Hz frequency drift/wander of 30ppm which is totally outside the 
specs I'd expect of a crystal source.
Any ideas what could be going on?! This is something of a pain for the 
new time code, which syncs the local TSCs to the chipset timer....
 -- Keir

(I am talking frequency stability here, by the way, not frequency 
tolerance. I know a crystal can be 100s of ppm out of whack from what 
is stamped on the can, but I think over short time periods it should 
not drift noticeably from its natural frequency).

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