Hmm, interesting question.
Radix sorts, by their nature, execute the same code regardless of the contents of their elements. ( I think, I haven't thought about this before).
However you will see some variation in execution time based on varying memory access patterns.
No, it's not constant time. Depending on implementation details, you end up putting different elements in different buckets in different cache lines and that creates side channels.
Radix sort will perform differently for an input that is 1,1,1,... and 1,2,3,...,n.
I have not read up on how djbsort deals with this issue, but it's the problem it's trying to solve.
