[Numpy-discussion] dot() performance depends on data?

Charles R Harris charlesr.harris@gmail....
Fri Sep 10 12:57:05 CDT 2010

On Fri, Sep 10, 2010 at 11:36 AM, Hagen Fürstenau <hagen@zhuliguan.net>wrote:

> Hi,
> I'm multiplying two 1000x1000 arrays with numpy.dot() and seeing
> significant performance differences depending on the data. It seems to
> take much longer on matrices with many zeros than on random ones. I
> don't know much about optimized MM implementations, but is this normal
> behavior for some reason?
Multiplication by zero used to be faster than multiplication by random
numbers. However, modern hardware and compilers may have changed that to
pretty much a wash. More likely you are seeing cache issues due to data
localization or even variations in the time given the thread running the

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