[gdal-dev] Slow GDALComputeRasterMinMax on nc grids

Joaquim Manuel Freire Luís jluis at ualg.pt
Fri Jan 3 09:27:19 PST 2020

Hi Even,

New Year, new mysteries. I'm having quite strange slow times in computing the min/max of netCDF files.

In GMT we can read files via GDAL by appending =gd to the file name. So both of these do a similar job
grdinfo grav_29_img.nc=gd
gdalinfo grav_29_img.nc -mm

and it takes around 28 sec in a new laptop running Windows and master GDAL. However in WSL, same laptop but GDAL 2.2.3, they take about 5:30 MINUTES to run. On a OSX it takes about 3:30 minutes (not me who run this, and a GDAL 3.0.3 MacPorts)

On Windows for files of similar size it run faster when data is of type float (the grid in this example is a short int).

Now, perhaps the weirdest thing is that if I run the GMT command via our Julia wrapper it only takes ~8 sec, whilst the same command via the Matlab wrapper took 1:18 min

The ~8 sec time is close to what I get with a pure GMT (i.e. not using GDAL to read the nc file  (grdinfo grav_29_img.nc -M))

>From the GMT side all happens in the call

        GDALComputeRasterMinMax(hBand, false, adfMinMax);

And I guess that the same holds from the pure GDAL call.

If you want to test with the grid used in these testings, it's here

Happy new year

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