Re: Xen system skew MUCH worse than tsc skew (was RE: RE: [PATCH] record max stime skew (was RE: [PATCH] strictly increasing hvm guest time))

[Keir Fraser <keir.fraser@eu.citrix.com>]

On 10/7/08 23:42, "Dan Magenheimer" <dan.magenheimer@oracle.com> wrote:

> SO XEN SYSTEM TIME MAX SKEW IS >30X WORSE THAN TSC MAX SKEW!
> 
> Looks to me like there's still something algorithmically wrong
> and its not just natural skew and jitter.  Maybe some corner
> case in the scale-delta code?  Also, should interrupts be turned
> off during the calibration part of init_pit_and_calibrate_tsc()
> (which might cause different scaling factors for each CPU)?

I didn't measure skew across CPUs. I measured jitter between one local TSC
and the chosen platform timer for calibration (in my case I think this was
the HPET). I did this because getting a consistent tick rate from the
platform timer, and from each local TSC, is the basis for the calibration
algorithm. The more jitter there is between them, the less well it will
work.

I implemented a user-space program to collect the required stats. It used
CLI/STI to prevent getting interrupted when reading the timer pair.

 -- Keir