[gdal-dev] Multi-threading GDALRasterBand::RasterIO() ?

[Simon Eves]

You can ignore this.

I have rather belatedly found the documentation that says that one must
open a GDALDataset per thread, even if it's on the same file.

The multi-threading now works just fine.

Interestingly, we're not actually doing that with our existing geo
importer. I guess it's OK because we're pulling the OGRFeatures out with
the process thread, and only converting and loading them with the child
threads. I guess I really ought to rewrite that code too now. Sigh.

As you were...

Simon

On Sun, Oct 31, 2021 at 4:27 PM Simon Eves <simon.eves at omnisci.com> wrote:

> We are writing a raster importer, and finding that
> GDALRasterBand::RasterIO() is unexpectedly slow for some GRIB2 files.
>
> We have a file which is about 1800x1000 pixels, with 49 bands of type
> DOUBLE. The file is about 47MB on disc.
>
> Reading all the bands of a single scanline from this file takes about
> 1300ms, which is about 26ms per band, hence the entire file takes around 20
> minutes to import. All the time seems to be spent in the RasterIO() call,
> even though it's not doing any raster rescaling or data format conversion
> (1:1 pixels, fetching as GDT_Float64).
>
> So, I figured we'd try multi-threading it, but evidently the call is not
> thread-safe. Here is just one of various stack traces it will throw.
>
> libc.so.6!raise (Unknown Source:0)
> libc.so.6!abort (Unknown Source:0)
> libc.so.6![Unknown/Just-In-Time compiled code] (Unknown Source:0)
> libgdal.so.28!GRIBRasterBand::UncacheData(GRIBRasterBand * const this)
> (/build/scripts/gdal-3.2.2/frmts/grib/gribdataset.cpp:948)
> libgdal.so.28!GRIBRasterBand::LoadData(GRIBRasterBand * const this)
> (/build/scripts/gdal-3.2.2/frmts/grib/gribdataset.cpp:730)
> libgdal.so.28!GRIBRasterBand::LoadData(GRIBRasterBand * const this)
> (/build/scripts/gdal-3.2.2/frmts/grib/gribdataset.cpp:697)
> libgdal.so.28!GRIBRasterBand::IReadBlock(GRIBRasterBand * const this, int
> nBlockYOff, void * pImage)
> (/build/scripts/gdal-3.2.2/frmts/grib/gribdataset.cpp:803)
> libgdal.so.28!GDALRasterBand::GetLockedBlockRef(int bJustInitialize, int
> nYBlockOff, int nXBlockOff, GDALRasterBand * const this)
> (/build/scripts/gdal-3.2.2/gcore/gdal_priv.h:963)
> libgdal.so.28!GDALRasterBand::GetLockedBlockRef(GDALRasterBand * const
> this, int nXBlockOff, int nYBlockOff, int bJustInitialize)
> (/build/scripts/gdal-3.2.2/gcore/gdalrasterband.cpp:1238)
> libgdal.so.28!GDALRasterBand::IRasterIO(GDALRasterBand * const this,
> GDALRWFlag eRWFlag, int nXOff, int nYOff, int nXSize, int nYSize, void *
> pData, int nBufXSize, int nBufYSize, GDALDataType eBufType, GSpacing
> nPixelSpace, GSpacing nLineSpace, GDALRasterIOExtraArg * psExtraArg)
> (/build/scripts/gdal-3.2.2/gcore/rasterio.cpp:149)
> libgdal.so.28!GDALRasterBand::RasterIO(GDALRasterBand * const this,
> GDALRWFlag eRWFlag, int nXOff, int nYOff, int nXSize, int nYSize, void *
> pData, int nBufXSize, int nBufYSize, GDALDataType eBufType, GSpacing
> nPixelSpace, GSpacing nLineSpace, GDALRasterIOExtraArg * psExtraArg)
> (/build/scripts/gdal-3.2.2/gcore/gdalrasterband.cpp:372)
> import_export::Importer::<lambda(size_t, int)>::operator()(size_t, int)
> const(const import_export::Importer::<lambda(size_t, int)> * const
> __closure, const size_t thread_id, const int y)
> (/home/simon.eves/work/omniscidb-internal/ImportExport/Importer.cpp:5721)
> ...
>
> All of the parameters to the call are either constant or uncontended
> simple variables, and obviously there is a unique data buffer (pData) per
> thread.
>
> Is there anything we can do to make this work?
>
> I was intending to look into the lower level block-based API, in the hope
> that it will be faster, but have not yet done so.
>
> This is all with a local static build of GDAL 3.2.2 on Ubuntu 20.04 with
> GCC 9.
>
> Yours,
>
> Simon Eves
>
> --
> <http://www.omnisci.com/>
> Simon Eves
> Senior Graphics Engineer, Rendering Group
> 100 Montgomery St (5th Floor), San Francisco, CA 94104, USA
>
>
> Email: simon.eves at omnisci.com | Cell:  +1 (415) 902-1996
>
>

-- 
<http://www.omnisci.com/>
Simon Eves
Senior Graphics Engineer, Rendering Group
100 Montgomery St (5th Floor), San Francisco, CA 94104, USA


Email: simon.eves at omnisci.com | Cell:  +1 (415) 902-1996
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