Parallel threads in query

I've tried porting some of PostGIS algorithms to utilize multiple cores via OpenMP to return faster.

Question is, what's the best policy to allocate cores so we can play nice with rest of postgres?

What I'd like to see is some function that I can call and get a number of threads I'm allowed to run, that will also advise rest of postgres to not use them, and a function to return the cores back (or do it automatically at the end of query). Is there an infrastructure for that?