[Gdal-dev] performance problem with several DEM reading/updating at the same time

[LALLAURET Fabrice - <SSII>]

Hello,
I'm playing with gdal and I don't understand some performance problem I 
have:
I'm reading several geotiff dems pixel by pixel at the same time (I 
know, this is not optimal but for my application, it's simplifying the job).
My code is like this :

    for (int i = 0; i < nbx1 ; i+=step)
    {
        for (int j = 0; j < nby1 ; j+=step)
        {
            z1 = z2 = 0.0;
            eErr = band1->RasterIO( GF_Read, i , j , 1 ,1 , &z1 ,1 ,  1 
, GDT_Float32, 0,0);
            if (eErr != CE_None)
                printf("Problem reading band1  %d %d\n",i,j);
            eErr = band2->RasterIO( GF_Read, i , j , 1 ,1 , &z2 ,1 ,  1 
, GDT_Float32, 0,0);
            if (eErr != CE_None)
                printf("Problem reading band2  %d %d\n",i,j);
        }
    }
When I only read a single dem, the test program takes 0.95 s to run. If 
I read a second dem at the same time (as above), the program takes 50 s. 
Why this difference of performance ? (With a writting process performing 
the same test, the program reach 12min !!)

Thanks

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