GPU out of memory error when directly evaluating magnet properties from returned magnet

[ilateur]

It's easier to show than to explain. When a magnet is returned from a function and some property is immediately evaluated, the program gives an error. When that magnet is first given a name it is fine.

from mumaxplus import World, Grid, Ferromagnet

def make_magnet():
    world = World((1e-9, 1e-9, 1e-9))
    magnet = Ferromagnet(world, Grid((2, 2, 2)))
    magnet.msat = 1
    magnet.aex = 1
    return magnet

# This works
magnet = make_magnet()
print(magnet.exchange_energy())

# This Gives the error `GPUassert: out of memory`
print(make_magnet().exchange_energy())

I guess world instantly gets destroyed in the compact version?

[lamoreel]

My guess is the same. The world property of Magnet does not hold a direct reference to the real world, but it creates a new one using

World._from_impl(self._impl.world)

This means that Pythons garbage collector destroys the world pointer when the function goes out of scope. A fix would be to let Magnet hold the actual reference, but I'm sure we made the choice above for a reason I can't recall right now 😊 .