[Qgis-developer] Possible memory leak in QGIS redraws?

[Nyall Dawson]

On 9 October 2015 at 01:45, Régis Haubourg
<regis.haubourg at eau-adour-garonne.fr> wrote:
> Hi all,
> I try to narrow down a production issue here.
> In long sessions with lot's of redraws on complex projects, memory seems to
> increase regularly.
>
> In a more simple environnement with no plugins, just sending multiple
> redraws using F5 key triggers it.
> It grows quite fast on 2.6 up to 4Go sometimes, quite slower in master and
> 2.10. Every 5-6 redraws, QGIS adds 1Mo of RAM in 2.6, less often in recent
> versions.
>
> If confirmed, and if nothing else is to be blamed (windows?, QT?) this is a
> serious issue for enterprise uses, especially if we want to build
> application that run all day long with many canvas redraws.
>

I've done some more testing, using valgrind and massif. Here's what I've found:

- there's a big leak coming from Qt's font database. Fixed in Qt 5.1
(https://bugreports.qt.io/browse/QTBUG-26861). About 50% of the memory
leaked is caused by this one. We could possibly work around it by
having a singleton so we only ever use one QFontDatabase in QGIS, or
just get on with the move to Qt5 and get the proper fix for free. ;)
- I also see a lot of memory used by Python objects. Specifically, a
lot coming from QgsPythonUtilsImpl::evalString and the PyRun_String
call in that function. I don't think these are leaks though (at least,
not within QGIS). But that's the second largest memory allocator I see
reported by massif
- There was a small leak of a GEOS object in labeling... don't think
this would cause too much memory usage though. I'll submit a fix after
giving it some more testing.
- Lastly, the remaining large chunk of allocated memory is used by the
images stored during rendering. I'm 90% sure these are all properly
cleaned, but I still can't throw the niggling suspicion that just
maybe there's some special condition in which these images aren't
properly deleted.

So yeah... no magic fixes to this one unfortunately. (At least, not
until QGIS 3.0!)

Nyall