std::vector vs Vc::Memory

[Sandro Wenzel]

Dear Tijskens,

I was intrigued by your observations  and tried to reproduce them but I
failed. Actually, I feel like Matthias that measuring such short
minimalistic code section is really tough.

Would you be able  to share your benchmark code and the way you compile it
such that I can have a more thorough look?

Best

Sandro



2014-03-04 18:57 GMT+01:00 Tijskens Engelbert <
[please enable javascript to see the address]>:

>  Dear all,
>
>  I am trying to figure out how to use std::vector<float> efficiently in
> combination with Vc. (to have dynamic arrays and performance)
>
>      std::vector<float> x(1024);
>     for( int i=0; i<ne; ++i ) {//initialize
>         x[i]=1.0;
>     }
> // scalar loop using std::vector
>     for( int i=0; i<ne; ++i ) {
>         x[i] -= 1.0;
>     }
> // vector loop using std::vector
>     for( int i=0; i<ne; i+=Vc::float_v::Size )
>     {
>         Vc::float_v vx( &x[i] );
>         vx -= 1.0;
>         vx.store( &x[i] );
>     }
> // vector loop using Vc::Memory instead of std::vector
>     Vc::Memory<Vc::float_v,ne> Vx;
>     for( int i=0; i<ne; ++i ) {//initialize
>         Vx[i] = 1.0;
>     }
>     Vc::float_v one(1.);
>     ET_TIME_THIS
>      ( "Vc::Memory<Vc::float_v,ne>  vector",
>         for( int i=0; i<nv; ++i ) {
>             Vx.vector(i) -= one;
>         }
> When i time these loops i get the following results
>  scalar loop using std::vector : 2162 cycles/repetition, 9.4e-07
> seconds/repetition, 1   x speedup, 2.3  GHz, 100 repetitions.
> vector loop using std::vector :  357 cycles/repetition, 1.6e-07
> seconds/repetition, 6.04x speedup, 2.24 GHz, 100 repetitions.
> vector loop using Vc::Memory  :  288 cycles/repetition, 1.2e-07
> seconds/repetition, 7.49x speedup, 2.4  GHz, 100 repetitions.
>
>
>  is there a way to improve the vector loop using std::vector? By the way
> if i write the second loop as
>  // vector loop using std::vector
>     for( int i=0; i<ne; i+=Vc::float_v::Size )
>     {
>         Vc::float_v vx( &x[i] );
>         Vx.vector(i) -= one;
>         vx.store( &x[i] );
>     }
>  things get even worse, the speedup being only 4.2x roughly.
>
>
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>


-- 
Dr. Sandro Wenzel
PH / SFT
CERN
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