[Bug 81021] AMD CPUs w/ Integrated Graphics (APUs) And Turbo Core Only Boost If "fglrx" Module Is Loaded

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=81021

--- Comment #9 from LiNuxXer <andre.d at gmx.net> ---
(In reply to Kertesz Laszlo from comment #3)
> [...]
> Additionally, even if fglrx eables turbo core boost, sometimes something
> weird happens and even the nominal speed is reported as maximum, in effect
> you can have even as low as 1.8 GHz (3.2GHz CPU) after an initial turbo
> boosted speed which lasts a few seconds. This happened to me sometimes
> (almost 50% of times) when i was compiling with 4 threads on my older
> A8-5500. I suppose its some internal downclocking mechanism kicking in at
> the wrong time because the reported temperatures were ~55C (default cooler). 
> 
> In contrast this never happened with radeon and if one thread was used it
> could have 4 GHz for hours (runaway flash in browser for example). 4 threads
> (compile) usually settled all cores at a stable ~3.7-3.8 GHz (temps up to
> 60C, default cooler). These are values on the A8-6500 (3.5GHz nominal/4.1
> GHz turbo speed from which never exceeded 4.0 as long as i monitored it).

I can confirm your findings. It looks like using fglrx will lead to longer
periods of exceeding the TDP significantly, and then suddenly it takes drastic
measures to compensate for that. From my observations I believe that it even
manipulates core affinity if only two cores are busy (having them on the same
module will save a lot of power if the whole other module can be slowed down).

With a bapm-patched radeon driver, nothing similar can be observed. Besides
saving up to 12W in console mode as compared to fglrx, the cores are able to
run at high frequencies forever with no abrupt measures to be seen.

Thank you for pointing this out. I have posted my detailed results at
http://unix.stackexchange.com/a/148918/79761 in case you're interested.

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