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RE: [Xen-devel] [PATCH] Add a timer mode that disables pending missed ti

To: "dan.magenheimer@xxxxxxxxxx" <dan.magenheimer@xxxxxxxxxx>, "Dave Winchell" <dwinchell@xxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH] Add a timer mode that disables pending missed ticks
From: "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>
Date: Wed, 20 Feb 2008 16:40:47 -0700
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Deepak Patel <deepak.patel@xxxxxxxxxx>
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> > >percentage.  However, our 32-bit clock skew seems to
> > >show a measureable problem now.
> > >
> > For the 32 bit guest, which timesource did it pick?
> The dmesg output is hard to interpret on a 32-bit guest,
> but based on what we've seen, I think it was selecting
> hpet as timesource (because we specified clocksource=pit
> which would have been ignored on RHEL4-32).  We are
> running another test with "clock=pit" to see if the
> skew goes away.

Yep, that was the problem.  No clock skew with clock=pit
on RHEL4-32 anymore.

> > >For Xen RHEL5 HVM guests:
> > >- I *think* clock=pit is sufficient for RHEL5-32
> > >
> > But still poor accuracy, right?
> Unproven yet but I hope not.  The nohpet and nopmtimer
> parameters are ignored on RHEL5-32 so the clock=pit
> (or clocksource=pit) is the only way to choose the
> clock source, and thus the only way to get good
> accuracy on RHEL5-32.

We had an interesting firedrill with RHEL5-32.
It seems that the RHEL-specific "divider=10" doesn't
get along very well with "clock=pit" and causes the
system clock to go whacko, gaining literally HOURS
every second.  Until Deepak discovered this, it was
impossible to boot.  So we haven't tested this
hypothesis yet, but the dmesg output looks promising.


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