Why does the RAPL sampling interval have to be >= 50ms?

[JaykumarPatel4802]

tracer.py logs a critical error if rapl_interval_sec < 0.05.

[HongyuHe]

Hi @JaykumarPatel4802, thanks for asking! The RAPL MSRs update approximately every millisecond, but the actual measurement period is not precisely 1 ms, leading to variations in sample periods.

In fact, RAPL provides energy consumption data rather than power consumption, and lacks timestamps for each counter update [1]. This limitation makes it impractical to achieve rates exceeding 20 samples/s if the goal is to keep systematic error below 5%. Also, continuously polling the RAPL registers not only consumes processor core resources but also distorts the accuracy of the measurements [2], which might increase the cost of energy tracing.

Hope this explanation helps a bit. Feel free to reach out if you have any other questions!

[1] Khan, Kashif Nizam, et al. "RAPL in Action: Experiences in Using RAPL for Power measurements." ACM Transactions on Modeling and Performance Evaluation of Computing Systems (TOMPECS) 3.2 (2018): 1-26.
[2] Hackenberg, Daniel, et al. "Power measurement techniques on standard compute nodes: A quantitative comparison." 2013 IEEE International Symposium on Performance Analysis of Systems and Software (ISPASS). IEEE, 2013.