Re: [Xen-devel] [PATCH] Fix clock_gettime to increment monotonically onPV-domain/x86

[John Levon <levon@xxxxxxxxxxxxxxxxx>]

On Fri, Jun 15, 2007 at 01:59:36PM +0100, Keir Fraser wrote:

> >> An alternative to a spin lock would be to use a 64-bit variable storing the
> >> raw
> >> nanosecond value, and use cmpxchg to check/update it. I did it this way for
> >> the clocksource monotonicity:
> > 
> > This is essentially what we've got now in Solaris. It seems like a
> > terrible shame not to just fix it in Xen, especially given all that
> > traffic from all CPUs to 'last_ret'.
> 
> How would we fix it in Xen in a way that is faster and more scalable?

A good question :)

One thing we've considered is losing some precision based upon how much
of a delta there is between the real CPUs (i.e. drop lower bits and
round up). But we're still (slowly) looking into the problem.

regards
john

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