[gdal-dev] Thread-safe raster access

[Deyan Vasilev]

Hi,
I've written a piece of software that fetches tiles out of a single
MBTiles raster done by multiple threads. Tiles go to a common cache
which can be used by a "view" thread that assembles tiles on the
screen in a seamless map.

Each fetcher thread uses GDALOpenEx() and then RasterIO(GF_Read,,,) to
get individual RGB bands out of the stored PNG tiles in the source
MBTiles db. On the lowest level
GDALGPKGMBTilesLikePseudoDataset::ReadTile() does a sqlite3 "SELECT
tile_data FROM tiles..." with a db context which is unique to every
fetcher thread.

Giving a recap, a multithreaded raster read on a single raster source
is doable, eased probably by the sqlite raster container that I use in
my app.

Best regards,
Deyan




On Mon, Jun 3, 2024 at 8:04 PM Even Rouault via gdal-dev
<gdal-dev at lists.osgeo.org> wrote:
>
> Andrew,
>
> what would be the purpose of thread-safe access: just making it thread-safe without any particular requirement on how efficient this would be (1), or hope for true concurrent access with ideally close to linear scalability with the number of threads (2) ?
>
> If (1), then we could add a GDALMutexedDataset class, similarly to https://github.com/OSGeo/gdal/blob/master/ogr/ogrsf_frmts/generic/ogrmutexeddatasource.h which exists on the vector side (just used by the FileGDB driver due to the fact that the underlying SDK is not even re-entrant), which uses the decorator pattern around all public API entry points to call the underlying dataset under a mutex.  One could imagine to have a GDAL_OF_THREADSAFE open flag that GDALOpen() would use to return such instance. Shouldn't be too hard to implement, but probably not that useful IMHO. I can anticipate most users would have higher expectations than a mutex-based implementation.
>
> If (2), it seems to me that it would require a huge effort, and the programming language we use (C++) offers hardly any safety belt to make sure we don't make mistakes, the main one being forgetting to lock things that should be locked, or dead locks situation. If we go into doing that, I'm not even sure how we can reliably identify all parts of the code that must be modified
>
> Neither GDAL raster core nor any driver are designed to be thread-safe. For core, at least gcore/gdalarraybandblockcache.cpp and gcore/gdalhashsetbandblockcache.cpp which interact with the block cache should be made thread-safe, and "just" adding a lock would defeat the aim to achieve linear scalability. The change in GDALDataset::RasterIO() I did in https://github.com/OSGeo/gdal/commit/7f3a0e582eb189744bc7cb8e4a751135edaecaf5 isn't thread-safe either (would be easy to make thread-safe though)
>
> Once GDAL raster code is ready, the main challenge is making drivers themselves thread-safe. Raster drivers may directly read from a VSILFILE* handle, which isn't thread safe when using the standard Seek() + Read() pair. A few VSIVirtualFileSystem have a PRead() implementation, which is thread-safe, but not all). Or they rely on using some instance of a "reader" returned by a third-party library (libtiff, libjpeg, libpng, sqlite3, etc.) (which in most cases also uses a VSILFILE*), none of which are thread-safe (except sqlite3 that can be made thread-safe by passing a flag at sqlite3_open() time, that will basically applies strategy (1) by protecting all calls with a mutex). Perhaps using thread-specific instances of VSILFILE* and third-party "reader" objects could be a way of solving this. But realistically doing a pass in all GDAL drivers would be a multi-month-man to multi-year-man type of effort. A realistic plan should be designed to allow combining (1) and (2): (2) for a few select drivers, and (1) as a fallback for most drivers that wouldn't be updated.
>
> Even
>
> Le 03/06/2024 à 15:44, Andrew Bell via gdal-dev a écrit :
>
> Hi,
>
> I am aware that there isn't thread-safe raster access with the current GDAL interface for various reasons. Given the state of processors, I was wondering if it would be valuable to take a look at providing the ability to do Raster I/O (at least reads) in a thread-safe way. This could be done through a new set of API calls or perhaps by modifications to what currently exists -- I don't know what makes sense at this point. I would be happy to spend some time looking at this if there is interest, but I would also like to learn from existing experience as to what kinds of things that I'm surely not considering would have to be dealt with.
>
> Thanks,
>
> --
> Andrew Bell
> andrew.bell.ia at gmail.com
>
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> --
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