How to monitor swap activity on desktop in real time (preferrably separately for each swap device)

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http://askubuntu.com – It would be informative to know in real-time the swap activity (number of reads and writes per minute or similar measure). I intend to use it with conky. I can gather similar statistics by vmstat or agregate output of iotop. Unfortunately I cannot learn about the activity specifically on each swap device. The swapon command doesn't provide such answer and neither do /proc/swaps. (I use zram swap, so the moment when swapping occurs is not immediately obvious. And since zram swap makes penalty for cpu I would like to know if it happens when I start long computations. ) (HowTos)