Re: [Xen-devel] [PATCH] Disable Xen PowerNow! support on Opteron 2nd gen and earlier processors

["Stephen C. Tweedie" <sct@xxxxxxxxxx>]

Hi,

On Wed, 2008-01-23 at 15:26 +0000, Keir Fraser wrote:

> > I spent a month trying to debug this and I made no significant
> > progress.  If someone who understands Xen timekeeping wants to
> > help me dive into this, I'll be glad to take another stab at
> > it.  As it is, even with informing Xen of frequency changes,
> > Xen does not ensure monotinicity.
> 
> Seeing them at that frequency (less than one a minute) is not too serious.
> Perhaps we should get rid of the message, or reduce its priority.

They are most definitely serious --- gettimeofday() going backwards
breaks Unix semantics.  It breaks applications.  It _will_ corrupt
application data --- things like database rollback logs that rely on
timestamp monotonicity to order things correctly will break.

I think this is something that Xen upstream ought to be pretty concerned
about.  If we can't fix it properly, then the workaround of disabling
powernow seems to be a must-have.

--Stephen



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