[gdal-dev] Multithreading with OGR

[Martin Dobias]

Hi all,

in my GSoC project [1] I'm trying to employ multithreading to make
rendering in QGIS both faster (on multi-core machines) and more
pleasant to use (not blocking GUI thread while rendering). I've been
successful with making that work in "ideal" conditions (no concurrent
reads/writes) and now the hard part comes: to make the implementation
thread-safe. OGR is an important data provider, so I'd like to ask you
for some hints how to do the things correctly.

I've seen there have been some efforts to make OGR thread-safe - in
RFC 16 [2]. In the RFC it is not clear in what parts have been
implemented, but from the code I guess it ended up with only making
driver registrar and data source classes thread-safe. In QGIS
everything basically runs in the main thread, worker threads are
spawned only when rendering the map. A worker thread for rendering
only reads the features from data source and I can make sure that main
thread doesn't do any write operations while the threaded rendering is
in place. So my only problem is to avoid concurrent reads from both
main thread and worker thread. How can be this best accomplished
without serializing the access?

Currently I think of creating a temporary clone of the OGRLayer for
purposes of reading features in worker thread.
Is that feasible - are the drivers reentrant - at least within their
reading routines?
Is it generally costly to create another instance of the same layer?
When a write operation happens in main thread, will be the change
visible in the cloned layer for worker thread or is it necessary (for
some drivers) to recreate the cloned layer?

[1] http://blog.qgis.org/node/144
[2] http://trac.osgeo.org/gdal/wiki/rfc16_ogr_reentrancy

Thanks in advance for any hints.

Regards
Martin