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

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 15/6/07 14:08, "John Levon" <levon@xxxxxxxxxxxxxxxxx> wrote:

>>> 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.

IIUC this would make it less likely to see time going backwards, but when
you do it'll be by a lot more (the size of your precision granularity), and
it'll occur when your time values are unlucky enough to be just on the wrong
sides of a boundary between two time intervals which map to different
lower-precision time values.

I believe it's a pretty fundamental property of a monotonically-increasing
counter in a distributed system that communication is required to implement
it. Time is a funny thing.

 -- Keir


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