How does everyone count "Rays per Second"? Recently I'm developing a Monte-Carlo path tracer. To measure my tracer's performance, I decided to implement a simple mechanism to count how many rays can it trace in a second. Then here comes the problem, there are two ways to define a single ray:


  
  A complete ray, i.e., starting from camera and bouncing around the scene until it terminates.
  The ray originated from each intersection point.
  


Although many people use "Rays per Second" to measure ray/path tracer's performance, how they recognize "rays" is unclear to me. Perhaps anybody knows?
I've seen both, unfortunately. I'm not fond of rays per second as meaning exclusively primary rays and I'd suggest "paths per second" or better yet "samples per second" instead. "Complete ray" is not a common term: a ray is a (potentially unbounded) line segment and a sequence of rays is a path.

Rays per second in your second sense of total ray casts is not well-specified for a path tracer: do shadow rays count, for instance? It's a useful metric for evaluating an acceleration structure or intersection testing framework (i.e. Embree or OptiX) but I'd avoid it for a renderer.

Finally, be aware that samples per second still isn't a great metric of actual performance since sample quality will vary wildly depending on implementation details. It's probably the best thing you can do starting out though, as the better solutions involve fairly complex variance estimates.