How important is the accurate specification of nnz for spalloc?

How important for performance is the accurate specification of nnz for spalloc? And is undershooting worse than overshooting?

Answer 1

[James Tursa]

In general, undershooting is much worse than overshooting. With undershooting, if you go over the allocated amount by even 1 element, the entire data blocks (value & indexing) must be copied over into newly allocated data blocks. But if you overshoot by 1 element, you simply have a few bytes of extra storage allocated and it will not impact performance at all. As to how bad the undershooting will hurt you, that will depend entirely on how large those data blocks are and how often you end up copying them to newly allocated memory blocks.

[Ulrik William Nash]

Thank you, James, for this answer. This will help me quite a bit, I think.

[James Tursa]

FYI, if you add in one element beyond current capacity, MATLAB does not allocate room for just one more element ... it is smarter than that. It will allocate more space using an internal formula based on the current size of the blocks. But still, it is best to limit how often this happens if you can.