Memory read stall rate

The ports are indeed separate, but they are all connected to the same bus and they will be all competing with each other for the memory bandwidth. The writes in your kernel are already coalesced over x (all the ports in your image are actually 512-bit ports). What you should do is to disable unrolling over y since that is the reason for all the non-coalescable ports you are getting. Please check my reply in the other thread.