[dpdk-dev] librte_power w/ intel_pstate cpufreq governor

Matthew Hall mhall at mhcomputing.net
Thu Jan 14 08:03:55 CET 2016

On Tue, Jan 12, 2016 at 03:17:21PM +0000, Zhang, Helin wrote:
> Hi Matthew
> Yes, you have indicated out the key, the power management module has changed or upgraded.
> Could you help to try the legacy one to see if it still works, as indicated in your link?

I can do this, but according to the documents I am reading, the old Power 
Management module is secretly stubbed out / no-opped inside of the Skylake CPU 
core, and the core manages its own clockrate internally every 1 msec instead 
of every 30 msec with input from the OS (Intel Speed Shift technology).

If this is true, then I suspect there is no point to getting it to work again 
with either the old frequency driver or the new driver, because the chip would 
not listen to it. So then it seems like it makes sense to skip the clock 
adjustment callbacks on Skylake and take extra stuff out of the fastpath code.

> Taking control of the governor from kernel to user space, might need one 
> more checks before that. But it is actually not a big issue, as user can 
> switch it back to anything via 'echo'.

I think it's a bit bigger issue, as it leaves the chip in full-power mode 
without really warning anybody, instead of the standard default adaptive mode. 

> Yes, it seems that librte_power is out of date for a while. It is not easy 
> to track all the kernel versions. Now we have good chance to do that, as you 
> have reported issues. Let's have a look on the new power management 
> mechanism and then see if we can do something.

Yes, let me know how I could help. I don't know very much yet. My machine is 
Skylake Core i7-6700k. Unfortunately I think I am in trouble here, because 
there is no whitepaper on the Intel website for Intel Speed Shift technology 
at all.

> Really thanks to your questions!

I am looking forward to getting some answers figured out together.

> Regards,
> Helin


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