On Mon, Oct 28, 2013 at 05:20:45PM +0100, Ingo Molnar wrote: > > * Neil Horman <nhor...@tuxdriver.com> wrote: > > > Base: > > 0.093269042 seconds time elapsed > > ( +- 2.24% ) > > Prefetch (5x64): > > 0.079440009 seconds time elapsed > > ( +- 2.29% ) > > Parallel ALU: > > 0.087666677 seconds time elapsed > > ( +- 4.01% ) > > Prefetch + Parallel ALU: > > 0.080758702 seconds time elapsed > > ( +- 2.34% ) > > > > So we can see here that we get about a 1% speedup between the base > > and the both (Prefetch + Parallel ALU) case, with prefetch > > accounting for most of that speedup. > > Hm, there's still something strange about these results. So the > range of the results is 790-930 nsecs. The noise of the measurements > is 2%-4%, i.e. 20-40 nsecs. > > The prefetch-only result itself is the fastest of all - > statistically equivalent to the prefetch+parallel-ALU result, within > the noise range. > > So if prefetch is enabled, turning on parallel-ALU has no measurable > effect - which is counter-intuitive. Do you have an > theory/explanation for that? > > Thanks, I mentioned it farther down, loosely theorizing that running with parallel alu's in conjunction with a prefetch, puts more pressure on the load/store unit causing stalls while both alu's wait for the L1 cache to fill. Not sure if that makes sense, but I did note that in the both (prefetch+alu case) our data cache hit rate was somewhat degraded, so I was going to play with the prefetch stride to see if that fixed the situation. Regardless I agree, the lack of improvement in the both case is definately counter-intuitive.
Neil > > Ingo > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
