Messages in this thread | | | Date | Sat, 01 Mar 2014 14:56:01 -0800 | From | "H. Peter Anvin" <> | Subject | Re: Final: Add 32 bit VDSO time function support |
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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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