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PC Processors

MihawkCola
Journeyman III

Undervolting PPT Lower but EDC still at 100%?

Hi, I wanted to undervolt my CPU with PBO. I noticed during testing that if the PPT is reduced, the EDC is still at 100%. My question wouldn't have to reduce the EDC value if I reduce the PPT? I tested cinebench 23.

my undervolting Settings:

  • PPT: 110w TDC:85A EDC:140A
  • all Core -26 

default Setting:

  • PPT: 142w TDC:95A EDC:140A


Setup:
Graphics card: GeForce RTX™ 3080 Ti EAGLE 12G

CPU: AMD Ryzen 9 5900x
Mainboard: Gigabyte X570 Aorus 
Ram: Corsair DIMM 16 GB DDR4-3200 * 2
SSD: SAMSUNG 980 1 TB SSD M.2 * 2
Power supply: 850 Watt Corsair RM Series RM850 Modular 80+ Gold

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7 Replies
ax1700OCer
Adept I

Undervolting would allow clocks under multi core load to run higher, therfore needing more EDC.

You can test this yourself and see what the maximum EDC value is when you run a cbr23 multicore test with an unconstrained EDC and compare EDC maximums with all core undervolt vs no undervolt. Then set the new EDC limit with whatever peak value you observed during a cbr23 test at -26 all core offet.

johnnyenglish
Big Boss

Have just one question? 

Did your undervolt procedure led to a score increase on Cinebench?

The Englishman
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t3chl0rd
Adept III

If running on newer bios, EDC should be left at 140, and lowering the PPT - TDC values will get decreased scores.

Ryzen 5900X PBO/OC, Gigabyte x570 Aorus Elite F37 bios, Arctic Freezer II 420 AIO, Fans: 3x140mm 2x120mm, 24 GB G.Skill DDR4-3600 18-20-20-40 1T, CM MWE V2 1050w, Gigabyte G32QC A 1440p, PowerColor RedDevil RX 6950XT OC.
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So this isn't really an undervolt but more like limiting your CPU, seems fairly similar to EcoMode. 

I asked that because, undervolting occurs with more focus on vCore. I have set a negative Offset and manual PBO. With the lower temps of the undervolt proccess, the CPU boosts for longer, thus more performance out of it. 

The Englishman
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igel
Journeyman III

This is quite normal. However, there may be something funny with the scalar the board VRM uses to report current, and it may be reporting higher current than what is actually flowing though. VRMs are current limited devices. 

P=V.I (power equals voltage times current)

So at 142W PPT (power) and 140A EDC (current), you would be able to go up to 1.0142V (voltage) to saturate the Power limit, and we know that the CPU cores can utilize upwards of 1.2V when boosting, so even at the default PPT, the EDC is the limiting factor for you as you are well under the possible core voltage under full load. But let's say your load only uses half of your cores and the other half is parked and say that configuration uses  85A. (Current flows though physical space, voltage does not (it is a field property), the properties of the silicon and the traces of chip, the size of the elements limit the current). So now, half the cores can go to 1.670V which is obviously too high, and other factors will become the limiting factor in the scenario (most likely the FIT). 

You have decreased the PPT to 110W, and kept the EDC limit at 140A, so you can go up to 0.7857 V at most when drawing full current. Yes, ultimately you have "undervolted" you CPU, but you have not actually changed what is the limiting factor. 

What is your ultimate goal? Why are you trying to undervolt your CPU?

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I enjoyed the way you explained it, but in the "modding community" this process is not seen as the "regular undervolting"

The "regular undervolting" is more like throwing the system out of balance, imagine a 1:1 ratio and then you untick the option "keep ratios" and want a square to be like 0,9:1 and it just looks odd.

If you change the PPT and all those values it will result in a lower voltage to the CPU but at the same time it changes its maximum clock and the CPU scores lower.

When I dial back these values I get lower clocks on both the 7950X and 2700X.

But if I change vCore and leave PPT alone, what I get is less voltage applied in my own terms, it will then self adjust wattage&amp resulting in less heat. (you can't bend physics afterall)

This is when the "magic" occurs, at the same time the PBO is enabled and it goes as far as there is headroom to maintain it, with less temperature it results in prolonged boosting or higher clocks before coming back down again.

Conclusion: With PPT dialed back, I get less performance because I've limited the CPU with a rule. With offset undervolt I get more performance on CineBench because I didn't limited the CPU, I just said, apply -0.1v to vCore of what you usually would apply.

The Englishman
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Correct, this is not he modding "undervolting".

Despite CPUs being devices that work with digital signals, the physical device still deals with analog processes. For something to be a 1 or a 0 the analog signal must be below or above a certain threshold. During the transition from low to high the signal raises over time based on the resistance, current and voltage. The higher the field strength (the voltage) that drives that change, the steeper (closer to vertical) the signal will move in time domain, the sooner (higher clock speed) you can sample the signal and determine if it is above or below the threshold needed to say whether it is a zero or 1. So, higher voltage - higher clocks. There are tables in the CPU that tell the VRMs what voltage the CPU needs to run at a certain speed. These tables have a healthy margin overprovisioned for stability. The better the silicon etching (less resistance/leakage), the actual margin can be even higher.  

The "undervolting" by the modding community is offsetting the demanded voltage stored in the CPU clock/voltage tables by some amount (decreasing the stability margin), so you supply less, hoping that the silicon is good enough. that the signal still raises fast enough so when sampled at the time needed it does not produce an ambiguous or a false reading (should be 1 but reads 0, or should be zero but reads indeterminate). As established before P=V.I so less voltage equals less power (less heat). If you are running against any of the thermal points where the CPU will start suppressing boost, then the lower temperature will help to keep the higher clocks for longer, but again, this as the cost of your stability safety margin.  If you got a well etched CPU on a wafer that didn't have inclusions - it works. If you got a not so well etched CPU on a wafer that wasn't so perfect - the system will crash. AMD has something called "clock stretching" that tries to ensure stability even if you run into situations where the signal isn't clear what it is - clocks apparently stay high, but actual performance is worse as the internals need to wait more than what they should at the given speed to be able to sample the signal reliably.

If you are just playing and learning - good for you, but if you are making money by using these CPUs just make them stable and go on with your business. Time wasted monkeying around with PBO is time you are not billing a customer for. 

If you know you have a not so good CPU sample, or only care about temperatures (e.g. live in a particularly hot climate - 45C ambient in some part of the day without AC, no space for a bigger cooler, etc.) - just go into PBO and change the thermal limit only and call it a day. I have a particularly bad 3600 sample that needs 1.38 to run at its regular boost clocks, does not take any negative voltage offset without crashing (practically non-existing margin), and as a result is very hot. I just put 68C as thermal limit in PBO and kept the rest of the values at default. 

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