Inconsistent values of perfevent hardware counters when measuring power consumption of DRAM using RAPL

[Pradyumna Kaushik]

Hi,

I recently noticed a large inconsistency in the data retrieved from
the perf_event hardware counters pertaining to the power consumption
of DRAM.
This inconsistency was noticed upon a kernel upgrade from 4.4.0 to 4.14.20.

Machine specs,
Dual Socket.
Processor specs: Intel(R) Xeon(R) CPU E5-2650 v3, 2.30 GHz (10 cores
with 2 threads per core).
So, overall it's a 20 core, 40 thread machine.
TDP of the machine -- 210 Watts (105 Watts for each Xeon processor)

In my research lab, we use performance co-pilot to measure power
consumption of the CPU and DRAM through the use of perfevent hardware
counters and RAPL.

Old results
Ubuntu Xenial
Kernel Version: 4.4.0
idle power consumption of CPU -- 33.34 Watts
idle power consumption of DRAM -- 14.76 Watts

New results (after kernel upgrade):
Ubuntu Xenial
Kernel Version: 4.14.20
Idle power consumption of CPU -- 33.23 Watts
Idle power consumption of DRAM -- 59.15 Watts

Can anyone give me some insight as to why this is the case?

-- 
Thanks and Regards
Pradyumna Kaushik

Graduate Student
Department of Computer Science
Thomas J. Watson School of Engineering and Applied Science
Binghamton University