Cpuspeed Govenors on Intel Westmere/Sandy-Bridge CPUs

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I have been looking into the pros and cons of the "performance" and "ondemand" cpuspeed govenors on some of our machine types. On all machines I can see a measurable performance drop-off when using ondemand.

However, when I look at the power draw from the relevant lights out management cards I can't see any power saving to be found on Westmere or Sandy-Bridge CPU.

My testing setup...

Machine 1 - HP ProLiant DL385 G6, 2x 2.4GHz 6-core AMD Opteron 2431, running RHEL 6
Machine 2 - Fujitsu Primergy RX600 S6, 4x 2.67GHz 8-core Intel E7-8837, running RHEL 5
Machine 3 - HP Proliant DL380p G8, 1x 2.4GHz 4-core Intel E5-2609, running RHEL 6

Reported energy usage from LOM GUI under 0% CPU load

         ondemand       performance

Machine 1 160W 236W
Machine 2 624W 624W
Machine 3 64W 64W

I can see that all three machines do appear to be throttling down the CPU speed as I would expect via /proc/cpuinfo. Machines 2 & 3 only seem to be throttling down to 50% of their rated speed compared to the older AMD one that drops to 800MHz.

I switched between govenors by editing /etc/sysconfig/cpuspeed and then issuing a '/etc/init.d/cpuspeed restart'. I confirmed the govenor change by looking at the scaling_govenor values found under /sys/devices/system/cpu

Am I missing something or is there no point in using the ondemand govenor on these latest CPUs?

Responses

Nathan,

This thread may be of interest to you:
https://plus.google.com/+TheodoreTso/posts/2vEekAsG2QT

Additionally (and may not be at all helpful), my personal experience when measuring power consumption has been mixed when using lights out management and I have instead resorted to using the data from metered PDU's.

That thread is very interesting, thank you.

Not sure I completely trust the LOM readouts myself, but they did seem to display a power increase well when I performed some CPU-load-only tests. Plus, two manufacturers LOMs reporting the same thing suggests to me they are reporting accurate enough figures.

Looks like disabling "ondemand" on anything with Intel Nehalems or newer may be beneficial.

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