More documentation on CPU monitoring

[wbjin]

• Check if gpu_indices is empty when syncing execution so it won't try to do it when only measuring CPU
• Add documentation on availability of RAPL, how to specify cpu_index, and how to start a docker container for monitoring CPU

[jaywonchung]

Thanks for your work! In general looks good and I can do a little nitpicking myself, but there is one thing that doesn't quite align with what I experienced in one of our Optane nodes:

$ docker run --rm -it -v /sys/class/powercap:/zeus_sys/class/powercap ubuntu:latest bash
root@03fdfd11b520:/# cat /zeus_sys/class/powercap/intel-rapl:0/energy_uj
cat: '/zeus_sys/class/powercap/intel-rapl:0/energy_uj': No such file or directory

$ docker run --rm -it -v /sys/devices/virtual/powercap:/zeus_sys/devices/virtual/powercap -v /sys/class/powercap:/zeus_sys/class/powercap ubuntu:latest bash
root@4d662a3c13e3:/# cat /zeus_sys/class/powercap/intel-rapl:0/energy_uj
23737296043

The above is what I just ran and copy-pasted. So mounting something from /sys/devices also seemed necessary. Could you look into this?

[jaywonchung]

Offline discussion -- mounting /sys/class/powercap/intel-rapl is enough as Docker will resolve symlinks.

[jaywonchung]

@wbjin Pushed some changes -- could you take a look?

[wbjin]

LGTM!