Hi Robert, On Thu, Feb 10, 2011 at 8:16 PM, Robert Kern <robert.k...@gmail.com> wrote:
> On Thu, Feb 10, 2011 at 11:53, eat <e.antero.ta...@gmail.com> wrote: > > Thanks Chuck, > > > > for replying. But don't you still feel very odd that dot outperforms sum > in > > your machine? Just to get it simply; why sum can't outperform dot? > Whatever > > architecture (computer, cache) you have, it don't make any sense at all > that > > when performing significantly less instructions, you'll reach to spend > more > > time ;-). > > These days, the determining factor is less often instruction count > than memory latency, and the optimized BLAS implementations of dot() > heavily optimize the memory access patterns. Can't we have this as well with simple sum? > Additionally, the number > of instructions in your dot() probably isn't that many more than the > sum(). The sum() is pretty dumb But does it need to be? > and just does a linear accumulation > using the ufunc reduce mechanism, so (m*n-1) ADDs plus quite a few > instructions for traversing the array in a generic manner. With fused > multiply-adds, being able to assume contiguous data and ignore the > numpy iterator overhead, and applying divide-and-conquer kernels to > arrange sums, the optimized dot() implementations could have a > comparable instruction count. Couldn't sum benefit with similar logic? > If you were willing to spend that amount of developer time and code > complexity to make platform-specific backends to sum() Actually I would, but I'm not competent at all in that detailed level (:, But I'm willing to spend more on my own time for example for testing, debugging, analysing various improvements and suggestions if such emerge. > , you could make > it go really fast, too. Typically, it's not all that important to make > it worthwhile, though. One thing that might be worthwhile is to make > implementations of sum() and cumsum() that avoid the ufunc machinery > and do their iterations more quickly, at least for some common > combinations of dtype and contiguity. > Well I'm allready perplexd before reaching that 'ufunc machinery', it's actually anyway trivial (for us more mortal ;-) to figure out what's happening with sum on fromnumeric.py! Regards, eat > > -- > Robert Kern > > "I have come to believe that the whole world is an enigma, a harmless > enigma that is made terrible by our own mad attempt to interpret it as > though it had an underlying truth." > -- Umberto Eco > _______________________________________________ > NumPy-Discussion mailing list > NumPy-Discussion@scipy.org > http://mail.scipy.org/mailman/listinfo/numpy-discussion >
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