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

David Cournapeau cournape@gmail....
Fri Sep 10 19:41:55 CDT 2010

On Sat, Sep 11, 2010 at 2:57 AM, Charles R Harris
<charlesr.harris@gmail.com> wrote:
> 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
> multiplication.

That's actually most likely a denormal issue. The a and b matrix (from
mm.py) have many very small numbers, which could cause numbers to be
denormal. Maybe a has more denormals than b. Denormal cause
significant performance issues on Intel hardware at least.

Unfortunately, we don't have a way in numpy to check for denormal that
I know of.



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