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Electromigration: Why AMD Ryzen Current Boosting Won't Kill Your CPU
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[Image: Carou2a_678x452.png]

Where there is a will to get extra performance out of a CPU, there is often a way. Either through end-user overclocking or motherboard vendors tweaking settings to improve their stock performance, at the end of the day everyone wants better performance, and for a multitude of reasons. This insatiable drive for peak performance, however, means that some of these tweaks and adjustments can start to skirt the lines of what is ‘in specification’. And as a result, we sometimes see methods of increasing processor performance that clearly deliver on their promises, but perhaps at the expense of thermals or longevity.

To this end, it has recently come to light that motherboard vendors have been taking advantage of a setting on AMD motherboards to misrepresent the current delivered to the CPU. By doing so, they are able to increase the processor's power headroom, and ultimately allowing for higher performance at the cost of higher thermals. To be sure, this kind of tweaking isn’t new, but recent events have lead to no shortage of confusion over what exactly is going on, and what the ramifications are for AMD Ryzen processors. So to try to clarify matters, here’s our take on the situation.

The Old Fashioned Way: Spread Spectrum, MultiCore Enhancement, PL2

One of the common themes I've noticed throughout my time at AnandTech as our motherboard editor and now our CPU editor is the lengths to which motherboard vendors will go to in order to get increased performance over the competition. We were the first outlet to break out features such as [Only registered and activated users can see links Click here to register], way back in August 2012, which led to higher-than-specified all-core frequencies, or in some cases, outright overclocks. But the history of motherboard vendors adjusting and tweaking features for performance goes further back than that, such as variations with the base frequency from 100 MHz to 104.7 MHz with [Only registered and activated users can see links Click here to register], leading to increased performance on systems that can support it.

More recently, on Intel platforms, we’ve seen vendors increase their turbo power limits so that the motherboard can sustain the highest turbo for as long as the world remains in existence, just because the [Only registered and activated users can see links Click here to register] in order to support it. In the past couple of weeks, we have also found examples of motherboards ignoring Intel’s new Thermal Velocity Boost requirements, which is something we'll be delving into more in a future article.

In short, motherboard vendors want to be the best, and that often means pushing the limits of what is considered the ‘base specification’ of the processor. As we’ve regularly discussed on topics like this with Intel’s turbo power limits, the differentiation between a ‘specification’ and a ‘recommended setting’ can get quite blurred – for Intel, the turbo power listed in the documents is a recommended setting, and any value the motherboard is set to is technically ‘in specification’. The point at which Intel considers it overclocking it seems is if the peak turbo frequency is increased.

Tweaking AM4 Above and Beyond

So now we move on to the news of the day, with motherboard manufacturers now attempting to tweak AMD based Ryzen motherboards in order to drive higher performance. As thoroughly explained over on the HWiNFO forums by [Only registered and activated users can see links Click here to register] and summarized here, AM4 platforms typically have three defined limiters:

Package Power Tracking (PPT), which indicates the power threshold that is allowed to be delivered to the socket; Thermal Design Current (TDC), which is the maximum current delivered by the motherboards voltage regulators under thermal limits; and Electrical Design Current (EDC), which is the max current at any time that can be delivered by the voltage regulators. Some of these values are compared to metrics derived internally in the CPU or externally in the power delivery, to see if these limits have been triggered.

In order to calculate the software-based power measurement for which PPT is compared to, the power management co-processor takes the value of current from the voltage regulator management controller. This isn’t an actual value of current, but a dimensionless value (0 to 255) designed to represent 0 = 0 amps, and 255 = peak amps that the VRMs can handle. The power management co-processor on the CPU then performs its power calculation (power in watts = voltage in volts multiplied by current in amps).

The dimensionless value range has to be calibrated on a per-motherboard layout, based on the componentry used (VRMs, Controllers) as well as the tracing, the board layers, and the quality of the design. In order to get an accurate scaler value for this dimensionless range, a motherboard vendor should accurately probe the correct values and then write the firmware to use that look-up table in the system power calculations.

This means that there is a potential way to fiddle with the way that the system interprets the peak power value of the processor. Motherboard vendors can reduce this dimensionless value of current in order to make the processor and the power management co-processor think that there is less power going to the CPU, and as a result, the package power tracking (PPT) limiter has not been yet achieved, and more power can be supplied. This allows the processor to turbo further than was originally intended by AMD.

This has knock-on effects. The processor will be consuming more power, mostly in the form of increased amps, leading to more heat being generated and increased thermals. Because the processor is turboing further (by being allowed to draw more power than what the software is reporting) the processor will also perform better in benchmarks.

As The Stilt points out, if you are running a CPU with a base TDP of 105 W and a PPT value of 142 W, under normal circumstances you should expect to see 142 W power being reported by the CPU at stock settings. However, if the dimensionless current value is only 75% of its real-world current, then the real world power consumption is actually ~190 W, which is the 142 W value divided by the 0.75 factor. Assuming that none of the other limits have been hit (TDC, EDC), the processor will only report 75% of the original PPT power, causing a lot of the confusion.
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