[Qgis-developer] QGIS Multi-threaded Rendering

[Even Rouault]

Selon Martin Dobias <wonder.sk at gmail.com>:

> Hi Even
>
> thanks for your thoughts.
>
> On Thu, Dec 12, 2013 at 8:30 PM, Even Rouault
> <even.rouault at mines-paris.org> wrote:
> >> - rasters - currently we do not have API to cancel requests for raster
> >> blocks. This means that currently we need to wait until the raster
> >> block is fully read even when we cancel the rendering job. GDAL has
> >> some support for asynchronous requests - anyone has some experience
> >> with it?
> >
> > Indeed there's an API in GDAL for asynchronous requests :
> > http://trac.osgeo.org/gdal/wiki/rfc24_progressive_data_support
> > Note that it has a "real" implementation only in the JPIPKAK driver
> > (driver that implement the JPIP protocol - JPEG2000 trough the network -
> > with the Kakadu library). For other drivers that don't have a dedicated
> > implementation, a default implementation will just turn the async requests
> > as a sync request (more precisely GetNextUpdatedRegion() will just do
> > a RasterIO(), other methods mentionned in the RFC are no-op).
> > Real cancellation / async request is generally hard to implement since the
> > low level libraries used by drivers must support it. The analysis must be
> > done on a per-driver basis (determine which drivers are important for you,
> > which ones are slow, see if it can be remedied in the general synchronous
> > case, otherwise imagine how async request could be implemented, ...)
> > We could imagine that the default GetNextUpdatedRegion(), instead of
> > issuing a single RasterIO() could split it up into several RasterIO()
> > if the request window crosses several tiles (in the case of a tiled
> > dataset) to allow cancellation between the calls to GetNextUpdatedRegion().
> > But that's no always a beneficial strategy. Drivers can use the fact that
> > a RasterIO() is done on a "big" window to optimize things a bit, w.r.t
> > issuing several RasterIO() on smaller windows. In GDAL 1.10, I've for
> example
> > implemented multi-threaded block decoding in the OpenJPEG driver : if the
> > window of a RasterIO() call spans over several tiles and you have several
> > cores, then each tile will be decoded in parallel.
> > Regarding the current API of asynchronous requests, I feel there's perhaps
> > a lack of a way of signaling that data is ready. GetNextUpdatedRegion()
> > works on a pulling startegy. Perhaps a callback mechanism could be usefull.
> > That topic needs some thinking...
>
> So obviously adding async API support for other common formats would
> take a lot of time... I was secretly hoping that the remark in rfc24
> mentioning only jpipkak driver was just a temporary state of affairs
> :-)
>
> However I realize that actually I do not really need to have truly
> asynchronous requests. The requests from QGIS are being made from
> within a worker thread, so it is not a problem if GDAL block that
> thread. The only thing I need is to be able to tell GDAL to stop
> reading data as soon as possible once the rendering gets cancelled.
> API-wise, I would like to have a method in GDALDataset, using which I
> could register my custom function returning cancellation status of the
> request (just true/false). The drivers would be responsible for
> calling that function whenever appropriate during RasterIO to check
> whether the request has been cancelled or not. It would be probably
> much less hassle to implement this functionality in drivers - although
> I understand that still many drivers using custom libraries won't be
> able to provide that functionality.

Yes, that's a reasonable possibility. The callback could also take
as parameter a percentage of completion. That could actually be a
GDALProgressFunc (
http://trac.osgeo.org/gdal/browser/trunk/gdal/port/cpl_progress.h )


>
> Regards
> Martin
>