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

[Avi Kivity]

On 04/19/2010 01:56 PM, Peter Zijlstra wrote:
>    
> > > Right, do bear in mind that the x86 implementation of atomic64_read() is
> > > terrifyingly expensive, it is better to not do that read and simply use
> > > the result of the cmpxchg.
> > >
> > >
> > >        
> > atomic64_read() _is_ cmpxchg64b.  Are you thinking of some clever
> > implementation for smp i386?
> >      
>
> No, what I was suggesting was to rewrite that loop no to need the
> initial read but use the cmpxchg result of the previous iteration.
>
> Something like:
>
>    u64 last = 0;
>
>    /* more stuff */
>
>    do {
>      if (ret<  last)
>        return last;
>      last = cmpxchg64(&last_value, last, ret);
>    } while (last != ret);
>
> That only has a single cmpxchg8 in there per loop instead of two
> (avoiding the atomic64_read() one).
>    

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.

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

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