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