[Users] Unexpected slowdown when increasing Carpet::max_refinement_levels (no AMR active)

[Naseri, Mahdi - (mahdinaseri)]

Hello,

I’m seeing an unexpected performance drop in Carpet (ET). I ran two simulations with identical parameter files (attached) except for:

Carpet::max_refinement_levels = 9     (Run A)
Carpet::max_refinement_levels = 12     (Run B)

During the period I conducted this experiment, all the refinement levels were inactive except for the base level. Therefore, I expected to see the same performance in both Runs A and B, because the active grid structure is the same. However, I realized that Run B is almost 4 times slower than Run A; a plot of wall-clock time per simulation time for these two simulations is also attached for comparison.

Is this behavior known/expected (e.g., due to bookkeeping, allocation, or scheduling costs that scale with the allowed number of levels), or does it suggest a configuration issue on my side? Any pointers on what to check, or how to profile this within Carpet/Cactus, to understand the overhead would be greatly appreciated.

Thank you,
Mahdi Naseri
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