When is swap triggered or how to calculate swap_tendency?

I'm trying to use Redis for production services and trying to avoiding swapping, which is bad for performance. I had learn that swap is triggered by swap_tendency which is depending on > swap_tendency = mapped_ratio/2 + swappiness + distress How can I get mapped_ratio/distress from /proc/meminfo for my monitor script? Or anything parameter that can info me that system is going to swap pages?