Widget memory leak when removed

After removing widgets, memory does not appear to be reclaimed (tested on iOS)

from kivy.app import App
from kivy.lang import Builder

class TestApp(App):
    def on_pause(self, *l):
        return True

    def build(self):
        return Builder.load_string('''
#:import Button kivy.uix.button.Button

BoxLayout:
    orientation: 'vertical'
    FloatLayout:
        id: container
    BoxLayout:
        size_hint_y: None
        height: 100
        orientation: 'horizontal'
        Button:
            text: 'add'
            on_press: container.add_widget(Button())
        Button:
            text: 'remove'
            on_press: container.clear_widgets()
''')

TestApp().run()

Click add several times (memory consumption will increase). Click remove and wait, no memory seems to be reclaimed.

[Kovak]

Because of the way python GC works just because you have removed the Widget does not mean it is time for it to be garbage collected. The default thresholds for Garbage Collection are (700, 10, 10), this means a gc.collect() will only be run once the number of allocations - number of deallocations (throughout the whole codebase) is greater than 700. In addition once an object survives one round of garbage collection it will be moved into generation 2, in which case it will take 10 separate triggerings of the generation 1 garbage collection to trigger the first collection of this object. Meaning that you must move through 700*10 allocations and deallocations at a bare minimum (although in practice it will probably be much larger as many allocations will not get deallocated) to trigger this collect. Likewise if the object survives its first collection in this round it won’t be up for collection again until 10 triggerings of the generation 2 gc and thus 100 triggerings of the 700 alloc difference. This type of examples run into a problem I think because they seem big enough, but they fall in-between collection cycles resulting in a large amount of garbage. For such a use case, matham’s suggestion of manually calling gc.collect() is probably good to keep in mind.

[matham]

I believe we had a discord discussion about this?

The way to test if memory is not reclaimed is creating a weakref to the widget and seeing if it’s not released when the widgets are cleared. Anything beyond that is related to how the system/python handles memory and is not something kivy can control.