Re: [PATCH 1/5] Add a global synchronization point for pvclock

[Avi Kivity]

On 04/19/2010 02:19 PM, Peter Zijlstra wrote:
> > Still have two cmpxchgs in the common case.  The first iteration will
> > fail, fetching last_value, the second will work.
> >
> > It will be better when we have contention, though, so it's worthwhile.
> >      
> Right, another option is to put the initial read outside of the loop,
> that way you'll have the best of all cases, a single LOCK'ed op in the
> loop, and only a single LOCK'ed op for the fast path on sensible
> architectures ;-)
>
>      last = atomic64_read(&last_value);
>      do {
>        if (ret<  last)
>          return last;
>        last = atomic64_cmpxchg(&last_value, last, ret);
>      } while (unlikely(last != ret));
>
> Or so.
>
>    

Yes, looks optimal when !NONSTOP_TSC.

--
error compiling committee.c: too many arguments to function

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