RE: [Xen-devel] Issues with multiple differing guest HZ rates?

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

Hi Chris --

We've noticed that a 100Hz RHEL5-64bit kernel (compiled 100 HZ, not
divider=10) has less clock skew with timer_mode=0 than with
timer_mode=2.  This would be the only such we've found for
a 64-bit kernel and this is worrisome because it complicates
the selection algorithm for setting timer_mode.  In our clock skew
tests, we run six 2vcpu 2GB guests on a 8pcpu 16GB system; this
was the only guest with 100Hz tick, all the rest are 1000Hz.
timer_mode=2 works fine if that guest is 1000Hz.

We'll continue to investigate but Alan's comment in that patch
was intriguing enough to ask for more info.

Thanks,
Dan

> -----Original Message-----
> From: Chris Lalancette [mailto:clalance@xxxxxxxxxx]
> Sent: Sunday, March 02, 2008 8:14 PM
> To: dan.magenheimer@xxxxxxxxxx
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Issues with multiple differing guest 
> HZ rates?
> 
> 
> Dan Magenheimer wrote:
> > In a timer patch in RHEL5u1 (by Alan Cox), is the comment:
> >
> > "Xen itself has issues with multiple differing guest GZ rates"
> > (and I assume that GZ is a typo, should be HZ)
> >
> > Can anyone explain this further or are there experimental
> > results to back it up?  We are seeing some strangeness with
> > running multiple pvm guests with different HZ rates and wonder
> > if this is related.
> 
> I highly doubt it; in that comment, Alan was describing why 
> he *didn't*
> implement the tick divider for Xen kernels.  So that patch doesn't
> actually touch any Xen code in particular, and should in 
> general not be
> touching much code that the -xen kernels share with the bare-metal
> kernel either.  What's the problem you are seeing with different HZ
> rates in guests?
> 
> Chris Lalancette
>


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