1. A graphics processing system configured to track event counts in a tile-based architecture, the graphics processing system comprising: a screen-space pipeline comprising: a count memory, a first unit configured to detect an event type and increment the count memory in response, and an accumulating memory associated with the first unit; a tiling unit configured to: cause the screen-space pipeline to increment a value stored at an external memory address by a first value stored in the count memory when the first unit completes processing of a first set of primitives that overlap a first cache tile, and cause the screen-space pipeline to update the accumulating memory to reflect a second value stored in the count memory when the first unit completes processing of a second set of primitives that overlap the first cache tile, wherein at least one primitive in the first set of primitives and at least one primitive in the second set of primitives are configured to cause the first unit to detect the event type and increment the count memory.