[gdal-dev] RFC 47 and Threading

[Blake Thompson]

Even and Andre,

> I want to start off by saying a big thanks to Blake for taking his time
> > to tackle what can only be a very difficult problem.
> >  From what I can observe, the current discussion seems to be around the
> > boundary of who should be responsible for ensuring thread safety around
> > the block cache. The core of GDAL versus the individual drivers.
>
> The core will necessarily have to know about thread-safety because the
> block
> cache is there. The discussion is more whether the drivers must also
> necessary
> be thread aware, or if core mechanisms are sufficient to hide this detail
> to the
> drivers. And potentially offering to drivers a mechanism to deal
> themselves with
> thread-safety if they can have a more optimized implementation than the
> default
> one.


Agreed, most specifically how to hide the detail of the protection of the
pointer to the block cache's data that is passed through IReadBlock,
IWriteBlock, and IRasterIO.


> > While I
> > see why such a conversation is important, as far as I am concerned, the
> > most important part should be how it affects users of GDAL at the
> > interface level.
>
> Agreed.
>
> > That is, if an application that is threaded is trying
> > to use GDAL, how does it ensure thread-safety? What you have to keep in
> > mind is that having some parts of the library not thread-safe basically
> > just pushes the mutexing/locking to the calling applications.
>
> Not necessarily. That's what I suggested in my previous email : if the
> costs of
> the mutex are not too expensive for non-threaded usage, then the API could
> systematically return thread-safe versions, that are potentially wrapped by
> GDALDatasetThreadSafe
>

So in the scope of my RFC, I am not certain how we could have a thread safe
cache and a non-threadsafe cache in any simple manner. I know that you are
specifically talking about the possiblity of thread safe datasets, which I
feel is necessarily part of this discussion but wanted to separate the two.
If the thread-safe cache is too expensive I feel like that is a major issue
however, and I am doing my best to avoid any performance hits for this
change.


>
> >
> > Also, while it is important to document thread-safety limitations, might
> > I suggest adding thread-safe related capabilities (TestCapability),
> > especially if all drivers do not end-up having the same thread-safety
> > constraints.
>
> That might be a solution, although not ideal from a usability point of
> view (if
> we come with something more complex that non-thread-safe vs thread-safe,
> that
> might be difficult to understand by users of GDAL API), and from a
> GDAL-developer point of view as well (need to assess thread-safety for each
> driver).
> There can have subtelties : imagine that the VRT driver is made to be
> thread
> safe, but uses sources of drivers that might be not thread safe...
>
> >
> > I personally do not see a GDALDatasetThreadSafe wrapper as adding much
> > complexity. For instance, if you were to add a capability that indicates
> > if a driver is inherently thread-safe, you could add a new open method
> > to open a dataset in a thread-safe way with something like the following
> > pseudocode:
> >
> > DataSet OpenThreadSafe(GDALOpenInfo openInfo)
> > {
> >      DataSet dataSet = Open(openInfo);
> >
> >      if (!dataSet.TestCapability(THREAD_SAFE))
> >      {
> >           dataSet = wrapWithThreadSafeWrapper(dataSet);
> >      }
> >
> >      return dataSet;
> > }
>
> Yes, that's similar to what I imagined with my idea of GDAL_OF_THREADSAFE
> open
> flag.


On the topic of the thread safe wrapper, I have spent more time thinking
about it and I think this is probably the best solution to the problem of
making all Datasets read safe, and I am willing to champion another RFC to
implement this. However, the scope of this is even larger because it should
be required to work with the OGR datasets as well.

Thanks,

Blake
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