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Le_Forgeron <lef### [at] free fr> wrote:
> Le 14/02/2014 07:45, jhu a écrit :
> > Apparently it does by about 4%. Not quite sure how or why though.
> >
> > Intel Core i5 3317U (Ivy Bridge) @ 1.7 GHz (turbo 2.4 GHz), Ubuntu 12.04
> >
> > gcc 4.8, -march=core-avix-i -mavx
> > Photon Time: 0 hours 0 minutes 2 seconds (2.558 seconds)
> > using 7 thread(s) with 2.981 CPU-seconds total
> > Radiosity Time: No radiosity
> > Trace Time: 0 hours 7 minutes 58 seconds (478.870 seconds)
> > using 4 thread(s) with 1909.554 CPU-seconds total
> >
> > gcc 4.8, -march=core-avx-i -msse2
> > Photon Time: 0 hours 0 minutes 2 seconds (2.607 seconds)
> > using 7 thread(s) with 3.055 CPU-seconds total
> > Radiosity Time: No radiosity
> > Trace Time: 0 hours 8 minutes 18 seconds (498.217 seconds)
> > using 4 thread(s) with 1986.597 CPU-seconds total
>
> There is a few operations (functions in povray) applied to vector (3 or
> more double/float) which might enjoy such optimisation.
>
> SSE2 is using only 128 bits of data (2 doubles), whereas AVX is able to
> use 256 bits (4 doubles).
>
> The operations are simple, but repeated a lot of time so grabbing a few
> cycles per operation gives that change.
The original comparison is a little flawed. Both binaries were compiled with
different versions of gcc. Here's a more apt comparison running on Ubuntu 13.10,
both binaries compiled with gcc 4.8
-march=core-avx-i -mavx
Photon Time: 0 hours 0 minutes 2 seconds (2.590 seconds)
using 7 thread(s) with 3.034 CPU-seconds total
Radiosity Time: No radiosity
Trace Time: 0 hours 8 minutes 9 seconds (489.257 seconds)
using 4 thread(s) with 1954.087 CPU-seconds total
-march=core-avx-i -msse2
Photon Time: 0 hours 0 minutes 2 seconds (2.583 seconds)
using 7 thread(s) with 3.015 CPU-seconds total
Radiosity Time: No radiosity
Trace Time: 0 hours 8 minutes 8 seconds (488.213 seconds)
Result: no significant performance difference.
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