Introducing sampling method for Kepler and finding reasonable sampling rate

[eklee15]

Due to the high overhead of Kepler/eBPF, we are investigating the sampling method for Kepler.
#823 (comment)

However, because sampling would potentially reduce the accuracy of the model, we would want to find a good sampling rate for Kepler. Of course, this would depend on workload, application, and system. This tradeoff study would be very helpful for Kepler in the longer term.

[rootfs]

#1685