FX8350 turbo is weird...

[Ramses]

I've had this 8350 on a Sabertooth 990FX R2 for awhile now, never overclocked it or messed with it much other than just using it. I was playing with the Overdrive software, mostly just observing, today with Prime 95 and the CPU freq's are weird. The turbo core and CnC and all that are on, it's bone stock and I use it for work, but a few observations.

2 workers in Prime95 engage all eight "cores" up to the first step of the turbo freq (4100).
Even though it's set to only use 4 cores at a time for turbo freqs.
Even though there are only 2 workers active in P95.
Huh.

8 workers in P95, still all eight "cores" are up to 4100 (I'm yet to see any hit stage 2 turbo of 4200), but two or three are always bouncing down to 3400 or so and back up.
I'm monitoring with HWinfo64, verified with CPUZ and AMD's overdrive software.

If I turn off Turbo in Overdrive, I get a steady full load of 8 workers in P95 at 4ghz, same for 2 workers.

What I expected to see, was with two workers, two "cores" hit the second stage of turbo, which is 4200, never saw that. Also not sure why it's bouncing some of the 8 "cores" down to 3400. Temps are not an issue at all, NH-D14 35C core temp tops.

I have a hard time believing it just don't work, is this normal behavior?
I can swallow the randomly lowering clock speed sorta, but the 8 cores at 4100
rather than the 4 cores it's set to turbo-boost don't make sense. Nor does P95 having
all 8 cores ramped up to 4100mhz with just two workers active. Is that Windows delegating?

I think I'm going to disable turbo and just set it to 4.2 or 4.4 or whatever it's happy with, preferably with the ability to throttle down when idle still intact.

 

[sandorski]

I'm not exactly sure how sophisticated the Turbo is, but perhaps all 8 Cores are on Turbo due to there being enough Thermal/Power headroom. Meaning, that the load on the Cores is light enough that it allows Turbo to kick in. That said, like you, I thought it only worked on fewer Cores and not all of them.

As for Cores downclocking, that's common and part of Power Saving. Even on my old Phenom2 1055T many of my Cores would idle at 800mhz(IIRC) or some other increment between that and 2.8ghz(Stock). I haven't really monitored the speeds of my 8320 to see what Idle speeds are like.

 

[Ramses]

I was aware of the downclocking at idle/low load, but don't remember seeing it do so under full tilt use like Prime 95 on my Phenom x4 or the 1090T I had briefly. Further testing shows I can get it a lot harder if I try and that it no longer throttles under load with Turbo core disabled, everything else is still turned on/auto.

So while under load on all cores by P95, with turbo enabled, it's bouncing through the cores, lowering some for a second or two to 3400ish, raising some to 4100ish (still have yet to see it hit the second higher stage of turbo), and runs dramatically cooler in the process. Huh..

soooooo how much of a performance hit is this freq bouncing and temp lowering?

 

[WhoBeDaPlaya]

I saw similar odd behavior with my 8320s on both the ASUS M5A97 R2 and M5A99FX Pro R2.
If I disabled PowerNow, C1E and C6, but left Turbo enabled, core speeds would stick to 3.7GHz but keep dipping down to 1.4GHz under full load (eg. Cinebench),
despite more than adequate thermal headroom for the CPU and VRM (BIOS was set to Current-based rather than Temp-based limit for the VRM).

Disabling Turbo kept the speed solidly at 3.5GHz, with no difference in the resulting Cinebench score.

 

[Ramses]

http://www.ronwoods.us/2014/02/what-does-amd-application-power.html Related to this maybe.

Some quickie benchmarking turns up better scores leaving it as AMD intended.
Not counting turning it off, then overclocking the crap out of it.
I got 4.3 stable at 1.4 but beyond that seemed to either want more juice or to turn off all the power saving stuff that I'm not willing to do. Might pick up a 9590 and forget about it again for awhile.

Interesting though, this stuff.