Saving frequency scaling in the data center

Posted May 22, 2020 12:41 UTC (Fri) by ju3Ceemi (subscriber, #102464)
In reply to: Saving frequency scaling in the data center by josh
Parent article: Saving frequency scaling in the data center

Power is part of ones datacenter bill

When you get rack space somewhere, two things are marked on the contract :
- how many rackunits
- how many Watts

Saving frequency scaling in the data center

Posted May 22, 2020 14:27 UTC (Fri) by epa (subscriber, #39769) [Link]

But if you use less than your maximum power allowance over the month, do you get some money back?

Saving frequency scaling in the data center

Posted May 22, 2020 20:23 UTC (Fri) by amarao (guest, #87073) [Link] (2 responses)

Nope, no one charge for energy in DC. The bill is usually is an allowed peak load or 95% percentile of that load. Even the electric companies often charge DC based on contracted power (watts), not the energy (joules or kw*hr, like household bills). The reason is that dc reserves specific power from supplier, and that power is often billed as 'take or pay', as it is not oversubscribed on the supplier capacity.

Saving frequency scaling in the data center

Posted May 23, 2020 11:53 UTC (Sat) by mathstuf (subscriber, #69389) [Link]

This sounds like it's also what allows these cloud companies to basically give away their CPU time to CI infrastructure to FOSS for free. Since the extra CPU usage doesn't actually cost them more in resources, they can instead buy some goodwill with the cycles instead (sure there's some personnel overhead, but once it's built out I suspect that it isn't too much to tack it onto the bottom of the sysadmin team's laundry list).

Saving frequency scaling in the data center

Posted May 23, 2020 18:24 UTC (Sat) by bugfood (subscriber, #124228) [Link]

> Nope, no one charge for energy in DC.

This can vary per colo provider. "Metered power" (which should really be called "metered energy") contracts are available. I can't speak to their relative prevalence, but they are common enough to matter.

https://www.greenhousedata.com/blog/colocation-pricing-ex...

My own experience with being on a "metered power" contract, and from looking at several competitive offers, is that the rent was on the order of 2/3 of the total bill. Since the rent is fixed for the duration of the contract, this limited the potential immediate savings from power reduction to 1/3. That said, being at a lower power level is good when it comes time to get the next contract, though of course colo providers will charge lower $/kW for higher kW allotments, so the savings are not linear. Different contracts from different providers in different locations at different times will vary....

Regarding use of frequency scaling on servers, running at higher frequencies means being able to get each unit of work done more quickly, which means being able to spend more time in higher C states. It's been a while since I measured this, but back on Sandy Bridge servers, using the performance governor resulted in substantially more time in higher C states, which presumably partially offset the increased power usage from higher frequencies. Also, as CPU usage gets higher, the difference between governors becomes lower, and it is good to consolidate workloads until CPUs have a meaningful load anyway, in order to avoid wasted servers. With all that, I don't hesitate to use the performance governor on tiers where latency matters at all; on the other hand, anything asynchronous (e.g. kafka) or high-latency (e.g. email) can generally use ondemand with no real downside.

All of this comes down to a business decision: determine your needs, measure the impact, calculate the cost, and make a decision. Having a less conservative governor would simply be another option to measure, and that doesn't sound like a bad thing.