An online parallel algorithm for finding top K elements
Here is one naive solution:
Let the number of parallel processes be $m$. Then, keep a separate buffer/heap/balanced tree for each of the $m$ processes. Divide the range of input into $m$ sections. Whenever you read a number, insert it into the appropriate process' heap. This way, each process only works on its own heap/tree. When you are done with the input, and want to output the top $k$ elements, do a partial mergesort to find the overall top-k.

This will require $O(km)$ space and $O(n\log k + km\log k)$ time.