[Xen-devel] RE: HPET/PIT timer accuracy

To:	"Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>, "xen-devel List" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject:	[Xen-devel] RE: HPET/PIT timer accuracy
From:	"Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date:	Fri, 29 Jul 2005 18:53:23 +0100
Delivery-date:	Fri, 29 Jul 2005 17:51:52 +0000
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Thread-topic:	HPET/PIT timer accuracy

> 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....

These systems haven't got a 'spread spectrum'[*] PCI bus clock per
chance? It might be worth seeing if there's a BIOS option for it.

Ian

[*] This is a horrible hack used by some system vendors to cheat EM
emmisions regulations.

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