Martin Dobias of Lutra Consulting explaining some of the performance enhancements coming in the next releases of QGIS including multi-threaded processing.

1. QGIS Performance Improvements
Scottish QGIS User Group
March 19th 2014
Martin Dobias
Lutra Consulting

2. Who are we?
● A group of consulting engineers specialising in:
● GIS (Open Source GIS)
● Software Development
● Numerical Modelling (in the water engineering sector)
Introduction to Python

3. About me
● With Lutra Consulting since autumn 2013
● Background in computer science
● Self-taught in GIS
● QGIS developer since 2005
● Python support
● Advanced symbology and labeling support
● Expressions
● Under the hood improvements (libraries, vectors, canvas, ...)
QGIS Performance Improvements

4. Performance Problems in QGIS
● DEMO
● Slow rendering
● Every map refresh can take several seconds with complex projects
● Rendering freezes user interface
● User has to wait with any actions until the rendering has finished
QGIS Performance Improvements

5. Rendering in QGIS (1)
QGIS Performance Improvements

6. Rendering in QGIS (2)
QGIS Performance Improvements

7. Parallelization
● Algorithms vary significantly in how parallelizable they are
● Embarrassingly parallel problems
● e.g. rendering of 3D scenes, serving static files from web server
● Inherently serial problems
● e.g. iterative numerical methods
QGIS Performance Improvements

8. Parallel Rendering in QGIS (1)
● Each layer rendered to a separate image, then composed together
● N-times faster!
● Framework to handle scheduling (QtConcurrent)
● Use all available CPU cores
● Automatic load balancing
QGIS Performance Improvements

9. Parallel Rendering in QGIS (2)
● Real world
● Speed improvement depends on data
● One complex layer may dominate total time → smaller speed improvement
● Lower waiting time with remote services → higher speed improvement
QGIS Performance Improvements

10. User Interface Freeze
● Application waits for events and handles them
● When busy, events need to wait to be processed
● Move the rendering to the background!
QGIS Performance Improvements

11. Rendering in Background
● Users may do some changes that affect rendering while rendering is still in progress
● e.g. change layer style, delete a layer
● Possible solutions:
● Do nothing
● Do not allow such actions
● Make sure all such actions are safe
QGIS Performance Improvements

12. Rendering in Background - Problems
● Data used for rendering must be protected from simultaneous access
● Complex task, needs to be addressed rigidly
● Locking
● Waiting
● If not done properly: dead-locks, crashes
● Copying
● May add overhead
● QGIS uses both locking and copying when appropriate
QGIS Performance Improvements

13. DEMO
QGIS Performance Improvements

14. Project History
● Summer 2010: Started as a Google Summer of Code (GSoC) project
● Spring 2013: Updates to QGIS infrastructure before the 2.0 release (API changes)
● Winter 2013: Implementation of the parallel/background rendering
● Spring 2014: Merged into QGIS master branch, will be in 2.4 release
QGIS Performance Improvements

15. Looking into the Future...
● Cancellation of raster data requests (needs also work in GDAL)
● Vector caching
● Faster labeling and vector reprojection
● Rendering with OpenGL
QGIS Performance Improvements

16. Summary
● QGIS 2.4 will be
● More interactive when browsing maps
● Able to use all available CPUs for rendering
● Q & A
QGIS Performance Improvements

17. Tips for Better Performance
● Vector layers: use spatial index
● Raster layers: use overviews
● Rendering configuration
● Turn on render caching
● Turn off anti-aliasing
● Vector styling
● Thin lines
● Polygons without border
● Without data-defined properties
● Data store specific features
● PostGIS – disable SSL mode on local network
QGIS Performance Improvements