Question Ryzen 9 7900 CPU not boosting to advertisted frequency unless PBO is enabled

[ClementNerma]

Hi there!

I recently built myself a PC with a Ryzen 9 7900 in it.

After running some benchmarks, I found out I was getting very low scores compared to others (like 25% below median).

After updating the BIOS and installing chipset drivers, it reduced to about a 10~15% loss. Which is still quite a bit, and I noticed during benchmarks that the CPU was never going past 4.47 GHz on all cores. On single-core benchmark, I get up to 5.29 GHz one one core.

I tried to enable PBO in the BIOS, and now I can get up to ~5.05 GHz on all cores during benchmark, and the score significantly improved, reaching the medians I've seen online.

Question is: are you actually supposed to enable PBO in order to get advertised boost frequencies? Enabling it means that my CPU is now running a lot hotter and drawing a lot more power, which is a problem for me as I value silence a lot.

I'll disable PBO anyway as without it my PC run without any amount of noise even in benchmarks, which is really nice. With PBO temps stay fine (never exceeding 72°C) but I can clearly hear the fan.

Anyway, I mainly want to make sure if this behaviour is normal?

Oh and by the way, since the BIOS update (on a MSI A620M-E) my 5600 MHz RAM cannot run above 5200 MHz anymore, otherwise the PC won't start and the EZ Debug LED indicates that's something wrong the RAM and the CPU. No additional infos. 4800 MHz works fine, 5200 too, 5400 and 5600 make it crash (and the only solution is then to connect the two dedicated "BAT" pins on the motherboard), A-XMP also makes it crash. Where before it ran absolutely fine and I correctly had 5600 MHz shown in Windows' task manager. Don't know if it's related.
I rolled back to the previous BIOS version and everything's fine, I can re-enable A-XMP or set the frequency to 5600 MHz and it works fine.

Thanks in advance for your help

 

[helpstar]

advertised boost is only one single core max. frequency (single threaded)

 

[hotaru.hino]

PBO is not supposed to be necessary to get the advertised boost speed, however, you're not guaranteed it anyway.

Given the chip is a 65W TDP one and it's a three chiplet total processor, I wouldn't be surprised if it had aggressive throttling mechanisms in play. And the A-series chipset motherboards I'd argue tend to not be designed around a Ryzen 9 processor. Especially considering the one you picked doesn't have a heat sink on the VRM, which is going to limit power delivery performance when things really get going.

It also seems like this review of the Ryzen 9 7900 couldn't even get their sample up to 5.4GHz: https://www.techpowerup.com/review/amd-ryzen-9-7900/24.html

In any case, missing 200MHz in this context means you're only losing about 4% clock speed, which doesn't necessarily translate to 4% loss in performance.

 

[ClementNerma]

advertised boost is only one single core max. frequency (single threaded)

Yeah I know, but 4.4 GHz on all cores seems pretty low to me?

PBO is not supposed to be necessary to get the advertised boost speed, however, you're not guaranteed it anyway.

Given the chip is a 65W TDP one and it's a three chiplet total processor, I wouldn't be surprised if it had aggressive throttling mechanisms in play. And the A-series chipset motherboards I'd argue tend to not be designed around a Ryzen 9 processor. Especially considering the one you picked doesn't have a heat sink on the VRM, which is going to limit power delivery performance when things really get going.

It also seems like this review of the Ryzen 9 7900 couldn't even get their sample up to 5.4GHz: https://www.techpowerup.com/review/amd-ryzen-9-7900/24.html

So that means the motherboard could potentially limit the performance of the CPU? I wouldn't have guessed, especially since it's a 65W CPU and not a >100W one like the 'X' variant.

Also if I understand correctly TechPowerUP's charts, they get an all-core boost of about 5.0 GHz right? Which is miles ahead what I get (and approximately what I get with PBO enabled).

 

[helpstar]

Yeah I know, but 4.4 GHz on all cores seems pretty low to me?

Then you should have gotten the 7900X

If you wanna compare, you would need to use the same benchmark "This test uses a custom-coded application that mimics real-life performance—it is not a stress test like Prime95", which is not a stresstest benchmark like this:
" On CCD 0, the frequency stabilizes at 4350 MHz and on CCD 1 at 4250 MHz. So, to put it simply, 6 cores at 4350 MHz and the other 6 at 4250 MHz. The CCD 0 is therefore more powerful"

Review: Ryzen R9 7900, R7 7700 and R5 7600 - Page 4 of 10 - Overclocking.com

Today is the official launch of the non-X Ryzen 7900, 7700 and 7600 which have a lower TDP than their big brothers

en.overclocking.com

EDIT:
Which BIOS version did you try and failed to get the 5600MHz?

 

[ClementNerma]

It failed on version 1.40 (7E28v14) but works on the previous stable version (which was the one u got when I received the motherboard): v.130 (7E28v13)

The thing that surprises me is the low scores I get in cinebench given it should be about 15% higher on multicore, which is quite a lot.

EDIT: just took a look at the article and indeed they mention clock frequencies similar to mine. So I don't where these low scores come from. It's definitely not the cooler given the temperatures without PBO never exceed 62 °C.

And I didn't go for the X variant as I value silence above all and read the performance was only slightly below for the non X (something like 5 to 10 % IIRC)

 

[Misgar]

You might be able to run at 5600MT/s with the latest BIOS if you manually tweak CL (CAS) on your DIMMs and relax the setting by one or two clock cycles, e.g. if the normal value is CL38 at 5600MT/s, increase it to CL39 or CL40.

 

[ClementNerma]

My RAM is 5600 MHz @ CL40 stock, I don't know if I can go further than CL40 but I'll check.
But wouldn't this hurt performances a bit?