Re: [gentoo-user] openmp flag

[Florian Philipp]

Am 25.09.2012 17:01, schrieb Michael Mol:
> On Tue, Sep 25, 2012 at 10:42 AM, James <wireless@×××××××××××.com> wrote:
>> Hello,
>>
>> background:
>> It seems there is a major push now to put openmp:
>> [1,2] into  embedded systems [3].
>>
>> So I looked at these [4] packages to find something
>> interesting to look deeper into related to openMP.
>>
>> Blender immediately jumped out at me as a good example,
>> cause an old friend Ken Hughes is, imho, one of the
>> world's most amazing C programmers, and a stalwart at
>> the blender project.
>>
>>
>> OK, here's the question, I went to emerge blender
>> and found that the openmp flag is already set. {?}
>> Yet I looked everywhere and did not see the openmp flag
>> set (/etc/make.conf, /etc/portage/package.use)
>> so where is it getting set on my AMD workstation?
>>
>> [ebuild  N     ] media-gfx/blender-2.49b-r2  USE="ffmpeg
>>  nls ogg openmp -blender-game -openal -verse"
>>
>> I feel like I should know (profiles etc) but, I'm a little
>> bit brain_dead this am, so any help is appreciated.
>
> Packages can choose to have USE flags enabled or disabled for them by
> default. So blender likely has openmp enabled by default, without that
> affecting any other packages.
>
>>
>> OH, anyone is encouraged to "chime in" about openmp
>> and your thoughts as to it's viability and usefulness.
>> Do you believe it will become a core technology,
>> embedded into GCC? Used widely?
>
> If you can use it, use it. OpenMP is little more than a set of
> extensions to C (and C++) which allows the normally-scalar language to
> do some things in a parallel fashion without resorting to the costs of
> multithreading. This is good, because vector instructions have been
> available in x86 since MMX came out, and improvements to the vector
> instructions available to x86 still goes on.
>

I guess this is just poorly phrased but to clarify: OpenMP *does* use
multithreading and nothing else. It does not, in any way, make more use
of vector instructions than GCC without -fopenmp. I guess what you mean
is avoiding the costs of *manual* multithreading using POSIX threads and
the like.

If you want to use vector instructions for your own code, you should
look into compiler intrinsics (i.e. vector instructions as built-in C
functions).
http://ds9a.nl/gcc-simd/

And, just to nit-pick: OpenMP also works for Fortran.

> Related are CUDA and OpenCL, which are two other systems for
> parallelizing code. CUDA assumes you have access to an nVidia GPU (and
> have a CUDA-enabled driver installed). OpenCL is a big more generic,
> and supports dispatching to CUDA, CPU vector instructions or even
> thread pools.
>
> Personally, my recommendation is to enable everything you can get
> working (be it, OpenMP, CUDA or OpenCL); vector processing is going to
> be generally more efficient than scalar processing. You don't need to
> worry about which is better unless you're a software developer. (And
> if you're a software developer, go study up on their differences;
> tradeoffs happen.)
>

+1

By the way: Did anyone get good results out of dev-util/intel-ocl-sdk
for OpenCL? Some time ago I tested it with a package that supported both
OpenMP and OpenCL (not sure which) and OpenCL didn't really make an
impact on my Core i5.

Regards,
Florian Philipp