Massive slowdown at Merge Node

[rlvelocity]

Hello,

My scene consists of an asteroid field and a "hero" asteroid together inside a geometry node. Both are slowly revolving around a planet (the planet is in a separate geo node).

I've been rendering Flipbooks to test the revolve/rotation speeds of the asteroids.

If I run a flipbook of just the asteroid field, it takes about 2.5 minutes. A flipbook of just the hero asteroid takes about 1 minute. If I add a merge node so that I can flipbook BOTH the asteroid field and the hero asteroid, it takes well over 20 minutes.

So my question is, what fundamental aspect of the merge node am I not understanding? The only clue I see about something weird is that the merge node indicates a "mis-match of attributes on the inputs" error but I don't think that would cause the slowdown (I'm assuming it's some UV thing). I suppose a better way to build my project would be to break the hero asteroid out into it's own separate geometry node but why would the merge inside the same geo node cause nearly an exponential slowdown? I have 96 GB of RAM so I can't believe it's some hardware limitation. Thank you.

[Masoud]

Hi;

I think: because each group of your objects in your merge node inputs are too far away from eachother, merging them you will create a huge bounding box in memory, so  Houdini have to handle  a big cluster of memory...

[rlvelocity]

Thank you, Masoud. That's as good of an answer as any I've gotten. Which is... the only one  

I ended up solving the issue by placing the hero asteroid into its own geometry node instead of merging them in the same geo node. I'd still like to dig deeper into why the merge node would behave like that when I'm not running any simulations. It's all just simple translation/rotation keyframes. _shrug_

[symek]

MergeSOP effectively creates new geometry object consisting of primitives of its inputs. Houdini can usually avoid actual copy, but I suspect that depending on a state of input geometry it might be forced to do even more. Copying attribute requires hardening it, which means baking all intermediate stages, removing holes, sorting etc. It can take a while. I don't know how Houdini handles copying packed prims, but it might be yet another story.

Whatever it your case, you shouldn't merge geometry for rendering. People usually do the opposite: so even if peaces are kept together for some reason (like sim), they are at the very end separated into many objects for rendering. It's cleaner and easier to handle, it's better performance-wise both for exporting to renderer and for rendering itself.