[vtkusers] VTK memory management

[David Gobbi]

No, the tp_traverse function for PyVTKObject does not traverse the
object's observers, so observer reference cycles will not be detected
by the python garbage collector.  This might be fixed in a later
version of VTK, i.e. VTK six point something (or sooner if someone
provides a patch).

 - David

On Wed, Aug 29, 2012 at 4:28 PM, José M. Rodriguez Bacallao
<jmrbcu at gmail.com> wrote:
> with respect to reference cycles, some time ago I was using vtk
> Messenger from TVTK to get rid of reference cycles when adding
> observers to any vtk object, this is the reasoning from the source
> code documentation in this file:
>
> """
> Implements a simple, robust, safe, Messenger class that allows one to
> register callbacks for a signal/slot (or event/handler) kind of
> messaging system.  One can basically register a callback
> function/method to be called when an object sends a particular event.
> The Messenger class is Borg.  So it is easy to instantiate and use.
> This module is also reload-safe, so if the module is reloaded the
> callback information is not lost.  Method callbacks do not have a
> reference counting problem since weak references are used.
>
> The main functionality of this module is provided by three functions,
> `connect`, `disconnect` and `send`.
>
> Here is example usage with VTK::
>
>     >>> import messenger, vtk
>     >>> def cb(obj, evt):
>     ...  print obj.__class__.__name__, evt
>     ...
>     >>> o = vtk.vtkProperty()
>     >>> o.AddObserver('ModifiedEvent', messenger.send)
>     1
>     >>> messenger.connect(o, 'ModifiedEvent', cb)
>     >>>
>     >>> o.SetRepresentation(1)
>     vtkOpenGLProperty ModifiedEvent
>     >>> messenger.connect(o, 'AnyEvent', cb)
>     >>> o.SetRepresentation(2)
>     vtkOpenGLProperty ModifiedEvent
>     vtkOpenGLProperty ModifiedEvent
>     >>>
>     >>> messenger.send(o, 'foo')
>     vtkOpenGLProperty foo
>     >>> messenger.disconnect(o, 'AnyEvent')
>     >>> messenger.send(o, 'foo')
>     >>>
>
> This approach is necessary if you don't want to be bitten by reference
> cycles.  If you have a Python object holding a reference to a VTK
> object and pass a method of the object to the AddObserver call, you
> will get a reference cycle that cannot be collected by the garbage
> collector.  Using this messenger module gets around the problem.
>
> Also note that adding a connection for 'AnyEvent' will trigger a
> callback no matter what event was generated.  The code above also
> shows how disconnection works.
> """
>
> right now, has vtk solved this problem?
>
> On Wed, Aug 29, 2012 at 4:38 PM, David Gobbi <david.gobbi at gmail.com> wrote:
>> In general python's memory management is fairly deterministic because
>> it relies on reference counting (i.e. most objects are deleted as soon
>> as they aren't needed anymore).  But if reference cycles are present,
>> the behavior can become non-deterministic, because it depends on when
>> the garbage collector is run.  You can always call gc.collect()
>> yourself to force the garbage collection to occur at certain times in
>> your program.
>>
>>  - David
>>
>>
>> On Wed, Aug 29, 2012 at 2:03 PM, Thiago Franco Moraes
>> <tfmoraes at cti.gov.br> wrote:
>>> Hi,
>>>
>>> I'm doing some tests about memory management in VTK with Python. I
>>> wrote this little python script [1]. It takes as parameter a filename
>>> to a vtkPolydata saved in a xml file and do some processings in it. In
>>> this script I'm taking care of freeing the memory used by each filter
>>> and intermediate vtkPolydata. I'm using an tool called memory profiler
>>> (very interesting by the way) [2] to measure the memory used along the
>>> script execution. The problem is the memory measurement in the end of
>>> script execution is somewhat strange, because the memory measurement
>>> is very fluctuating, ranging from 170MB to 500MB. Is this expected?
>>> I'm freeing memory from vtk objects in the right way? It's a problem
>>> with Python?
>>>
>>> Thanks!
>>>
>>> [1] - http://pastebin.com/iYUK1jxk
>>> [2] - https://github.com/fabianp/memory_profiler
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