[QGIS-Developer] The virtual QgsRasterInterface block method is too slow

[René-Luc Dhont]

Hi devs,

A customer found that reading a lightweight GeoTIFF takes 1 second in 
QGIS Server needs less than 200 milliseconds.

With timer I found somethings strange around the virtual 
QgsRasterInterface block method and it's reimplation in QgsGdalProvider.
Here is part of my logs:

../src/core/raster/qgsrasteriterator.cpp:87 : 
(readNextRasterPartInternal) [2ms] Entered
../src/core/providers/gdal/qgsgdalprovider.cpp:2641 : (initIfNeeded) 
[1ms] GdalDataset opened
../src/core/providers/gdal/qgsgdalprovider.cpp:317 : (crsFromWkt) [0ms] 
+proj=aea +lat_1=43 +lat_2=62 +lat_0=30 +lon_0=-30 +x_0=0 +y_0=0 
+datum=WGS84 +units=m +no_defs
../src/core/providers/gdal/qgsgdalprovider.cpp:2836 : (initBaseDataset) 
[0ms] GDALGetRasterNoDataValue = -3.4e+38
../src/core/providers/gdal/qgsgdalprovider.cpp:647 : (block) [10ms] 
QgsGdalProvider::block
../src/core/providers/gdal/qgsgdalprovider.cpp:718 : (readBlock) [0ms] 
QgsGdalProvider::readBlock
../src/core/providers/gdal/qgsgdalprovider.cpp:965 : (readBlock) [2ms] 
QgsGdalProvider::readBlock completed in (seconds): 0.002
../src/core/providers/gdal/qgsgdalprovider.cpp:684 : (block) [0ms] 
QgsGdalProvider::block completed in (seconds): 0.002
../src/core/raster/qgsrasteriterator.cpp:137 : 
(readNextRasterPartInternal) [643ms] Raster iterator read next block 
completed in (seconds): 0.656
../src/core/raster/qgsrasteriterator.cpp:87 : 
(readNextRasterPartInternal) [3ms] Entered
../src/core/providers/gdal/qgsgdalprovider.cpp:647 : (block) [6ms] 
QgsGdalProvider::block
../src/core/providers/gdal/qgsgdalprovider.cpp:718 : (readBlock) [0ms] 
QgsGdalProvider::readBlock
../src/core/providers/gdal/qgsgdalprovider.cpp:965 : (readBlock) [0ms] 
QgsGdalProvider::readBlock completed in (seconds): 0
../src/core/providers/gdal/qgsgdalprovider.cpp:684 : (block) [0ms] 
QgsGdalProvider::block completed in (seconds): 0
../src/core/raster/qgsrasteriterator.cpp:137 : 
(readNextRasterPartInternal) [283ms] Raster iterator read next block 
completed in (seconds): 0.289
../src/core/raster/qgsrasteriterator.cpp:87 : 
(readNextRasterPartInternal) [0ms] Entered


The qgsrasterinterator.cpp ligne 130 [1] tooks almost 300 times longer 
than it's reimplementation.

Do you know a way to enhance this ?

Regards,
René-Luc

1 
https://github.com/qgis/QGIS/blob/master/src/core/raster/qgsrasteriterator.cpp#L130