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I verified that saving the edit does not call geometryChanged so many times

Under gdb I can also see that a thread is started for each vertex !! This is serious performance loss

The origin seems to be this one:

#15 0x00007fb20cfdd0a1 in QgsVectorLayerUndoCommandChangeGeometry::undo (this=0x2be0e40)
    at /usr/src/qgis/qgis/src/core/qgsvectorlayerundocommand.cpp:176
#16 0x00007fb20b6bc5b1 in QUndoCommand::undo() () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#17 0x00007fb20b6bcebf in QUndoStack::setIndex(int) () from /usr/lib/x86_64-linux-gnu/libQtGui.so.4
#18 0x00007fb20cfcf61a in QgsVectorLayerEditBuffer::rollBack (this=0x29a4080)
    at /usr/src/qgis/qgis/src/core/qgsvectorlayereditbuffer.cpp:450
#19 0x00007fb20cfbed98 in QgsVectorLayer::rollBack (this=0x2ecae40, deleteBuffer=true)
    at /usr/src/qgis/qgis/src/core/qgsvectorlayer.cpp:2612

And:

#14 0x00007fb20d35ca9b in QgsVectorLayerEditBuffer::geometryChanged (this=0x29a4080, _t1=4, _t2=...)
    at /usr/src/qgis/Quantum-GIS/b/src/core/moc_qgsvectorlayereditbuffer.cxx:164

I believe the problem is the undo stack as QgsVectorLayerEditUtils only exposes a function to delete a single vertex so it makes a geometry clone for each removed vertex and rolls back one vertex at the time.

It would be much much better to expose a "mass vertexes removal" kind of interface, to make it an atomic operation.

Seems to be a work for Martin Dobias :)

Sandro Santilli wrote:

I believe the problem is the undo stack as QgsVectorLayerEditUtils only exposes a function to delete a single vertex so it makes a geometry clone for each removed vertex and rolls back one vertex at the time.

Um, isn't that already in place? The undo stack should contain only one entry for "Delete vertices" for each time you hit "delete". Multiple deletes shouldn't be accumulated as they should still be separately undoable.

Evidently not, "delete vertices" is not even supported by the utility class used to keep track of undo stack (AFAIU):
https://github.com/qgis/Quantum-GIS/blob/master/src/core/qgsvectorlayereditutils.h

Note that the nodeTool contains an hack to avoid multiple "geometryChanged" events by simply disconnecting the hook, deleting each vertex in turn and re-connecting the event, but the method invoked for deleting the single vertex is the one that seems to be filling up the undo stack, with as many geometry copies as vertexes selected. Which explains the fast memory growth for a big geometry (>45k vertexes).

Great job, the memory issue is also fixed by this!

Only, it may be better to comment out the "geometry changes merged" debug line as getting tens of thousand of lines when mass-removing vertices kills CPU ;)