[Numpy-discussion] Byte aligned arrays

Henry Gomersall heng@cantab....
Wed Dec 19 12:25:34 CST 2012


On Wed, 2012-12-19 at 19:03 +0100, Francesc Alted wrote:
<snip>
> > Finally, I think there is significant value in auto-aligning the
> array
> > based on an appropriate inspection of the cpu capabilities (or
> > alternatively, a function that reports back the appropriate SIMD
> > alignment). Again, this makes it easier to wrap libraries that may
> > function with any alignment, but benefit from optimum alignment.
> 
> Hmm, NumPy seems to return data blocks that are aligned to 16 bytes
> on 
> systems (Linux and Mac OSX):
<snip>

That is not true at least under Windows 32-bit. I think also it's not
true for Linux 32-bit from my vague recollections of testing in a
virtual machine. (disclaimer: both those statements _may_ be out of
date).

But yes, under Linux 64-bit I always get my arrays aligned to 16 bytes.
> 
> The only scenario that I see that this would create unaligned arrays
> is 
> for machines having AVX.  But provided that the Intel architecture is 
> making great strides in fetching unaligned data, I'd be surprised
> that 
> the difference in performance would be even noticeable.
> 
> Can you tell us which difference in performance are you seeing for an 
> AVX-aligned array and other that is not AVX-aligned?  Just curious.

I don't know; I don't own a machine with AVX ;)

It might be that the difference is negligible, though I do think it
would be _nice_ to have the arrays properly aligned if it's not too
difficult.

Cheers,

Henry



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