Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

[Avi Kivity <avi@xxxxxxxxxx>]

On 11/02/2009 05:46 PM, Dan Magenheimer wrote:
> > > I don't have any public data available for this DB usage,
> > >        
> > Sorry, that doesn't explain anything.
> >      
> Well for now just consider the DB usage as another use
> of profiling.  But one can easily draw scenarios where
> a monotonic timestamp is also used to guarantee transaction
> ordering.
>    
In this case we should provide a facility for this.  Providing a global 
monotonic counter may be easier than providing a monotonic clock.  Hence 
my question.
> > > Search for "flight recorder".  This feature is intended to
> > > be enabled all the time, but with non-vsyscall gettimeofday
> > > the performance impact is unacceptably high, so they are using
> > >        
> > For profiling work fast timestamping is of course great, but surely
> > there is no monotonicity requirement?
> >      
> Yes and no.  Monotonicity is a poor substitute for a more
> generic mechanism that might provide an indication that a
> discontinuity has occurred (forward or backward); if an app
> could get both the timestamp AND some kind of "continuity
> generation counter" (basically a much more sophisticated
> form of TSC_AUX that changes whenever the timestamp is
> coming from a different source), perhaps all problems could be solved.
>    
I doubt it.  A discontinuity has occured, but what do we know about it?  
nothing.
> > I don't think we'll be able to provide monotonicity with vsyscall on
> > tsc-broken hosts, so we'll be limited to correcting the tsc frequency
> > after migration for good-tsc hosts.
> >      
> True, though clock_gettime(CLOCK_MONOTONIC) can provide
> the monotonicity where it is required.
>    
We have that already.  The question is how to implement it in a vsyscall.

--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.


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