On 02/28/2014 06:00 PM, Andy Lutomirski wrote:
>
> This leads to a potentially interesting question: is rdtsc_barrier()
> actually necessary on UP? IIRC the point is that, if an
> rdtsc_barrier(); rdtsc in one thread is "before" (in the sense of
> being synchronized by some memory operation) an rdtsc_barrier(); rdtsc
> in another thread, then the first rdtsc needs to return an earlier or
> equal time to the second one.
>
> I assume that no UP CPU is silly enough to execute two rdtsc
> instructions out of order relative to each other in the absence of
> barriers. So this is a nonissue on UP.
>
> On the other hand, suppose that some code does:
>
> volatile long x = *(something that's not in cache)
> clock_gettime
>
> I can imagine a modern CPU speculating far enough ahead that the rdtsc
> happens *before* the cache miss. This won't cause visible
> non-monotonicity as far as I can see, but it might annoy people who
> try to benchmark their code.
>
> Note: actually making this change might be a bit tricky. I don't know
> if the alternatives code is smart enough.
>
Let's put it this way... this is at best a third-order optimization...
let's not worry about it right now.
-hpa
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