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...@gmail.com
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