Re: Strange Performance Hit on 2D-Loop

[Richard Guenther <richard.guenther@gmail.com>]

On Thu, Jul 9, 2009 at 4:19 PM, Andreas Schäfer<gentryx@gmx.de> wrote:
> Hey guys,
>
> I noticed a strange performance hit in one of our stencil codes,
> causing it to run twice as long.
>
> To nail down the error, I reduced our code to the two attached demo
> programs. Basically they take two matrices and average each matrix
> element with its four direct neighbors. Depending on how these
> matrices are allocated, the performance hit occurs -- or does not.
>
> Here is the diff of the two files:
> @@ -17,8 +17,7 @@
>
>  void test(double (*grid)[GRID_WIDTH])
>  {
> -    double (*gridOld)[GRID_WIDTH] =
> -        malloc(GRID_WIDTH * GRID_HEIGHT * sizeof(double));
> +    double (*gridOld)[GRID_WIDTH] = gridOldArray;
>     double (*gridNew)[GRID_WIDTH] = gridNewArray;
>     printAddress(&gridNew[0][0]);
>     printAddress(&gridOld[0][0]);
>
> where gridOldArray is a statically allocated array. Depending on the
> machines processor the performance hit varies from negligible to
> dramatic:
>
>
> Processor          GCC Version Time(slow) Time(fast) Performance Hit
> ------------------ ----------- ---------- ---------- ---------------
> Core 2 Quad Q9550  4.3.3       12.19s      5.11s     138%
> Athlon 64 X2 3800+ 4.3.3        7.34s      6.61s      11%
> Opteron 2378       4.3.2        6.13s      5.60s       9%
> Opteron 2352       4.3.3        8.16s      7.96s       2%
> Xeon 3.00GHz       4.3.3       18.98s     14.67s      29%
>
> Apparently Intel systems are more susceptible to this effect.
>
> Can anyone reproduce these results?
> And could anyone explain, why this happens?

Depends on the GCC version used.  First of all

    printAddress(&gridNew[0][0]);
    printAddress(&gridOld[0][0]);

makes the addresses escape and GCC versions other than the
current development trunk think that the malloced address can
alias the global variables.

Richard.

> Thanks in advance
> -Andreas
>
>
> --
> ============================================
> Andreas Schäfer
> Cluster and Metacomputing Working Group
> Friedrich-Schiller-Universität Jena, Germany
> 0049/3641-9-46376
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