AMD’s Turbo

With AMD introducing Turbo after Intel, as has often been the case in their history, they've had to live in Intel's world. And this has repercussions for the company.

By the time AMD introduced their first Turbo-enabled processors, everyone in the desktop space ‘knew’ what Turbo meant, because we had gotten used to how Intel did things. For everyone, saying ‘Turbo’ meant only one thing: Intel’s definition of Turbo, which we subconsciously took as the default, and that’s all that mattered. Every time an Intel processor family is released, we ask for the Turbo tables, and life is good and easy.

Enter AMD, and Zen. Despite AMD making it clear that Turbo doesn’t work the same way, the message wasn’t pushed home. AMD had a lot of things to talk about with the new Zen core, and Turbo, while important, wasn’t as important as the core performance messaging. Certain parts of how the increased performance were understood, however the finer points were missed, with users (and press) assuming an Intel like arrangement, especially given that the Zen core layout kind of looks like an Intel core layout if you squint.

What needed to be pushed home was the sense of a finer grained control, and how the Ryzen chips respond and use this control.

When users look at an AMD processor, the company promotes three numbers: a base frequency, a turbo frequency, and the thermal design power (TDP). Sometimes an all-core turbo is provided. These processors do not have any form of turbo tables, and AMD states that the design is not engineered to decrease in frequency (and thus performance) when it detects instructions that could cause hot spots.

It should be made clear at this point that Zen (Ryzen 1000, Ryzen 2000) and Zen2 (Ryzen 3000) act very differently when it comes to turbo.

Turbo in Zen

At a base level, AMD’s Zen turbo was just a step function implementation, with two cores getting the higher turbo speed. However, most cores shipped with features that allowed the CPU to get higher-than-turbo frequencies depending on its power delivery and current delivery limitations.

You may remember this graph from the Ryzen 7 1800X launch:

For Zen processors, AMD enabled a 0.25x multiplier increment, which allows the CPU to jump up in 25 MHz steps, rather than 100 MHz. This bit was easy to understand: it meant more flexibility in what the frequency could be at any given time. AMD also announced XFR, or ‘eXtreme Frequency Range’, which meant that with sufficient cooling and power headroom, the CPU could perform better than the rated turbo frequency in the box. Users that had access to a better cooling solution, or had lower ambient temperatures, would expect to see better frequencies, and better performance.

So the Ryzen 7 1800X was a CPU with a 3.6 GHz base frequency and a 4.0 GHz turbo frequency, which it achieves when 2 or fewer cores are active. If possible, the CPU will use the (now depreciated in later models) eXtended Frequency Range feature to go beyond 4.1 GHz if the conditions are correct (thermals, power, current). When more than two cores are active, the CPU drops down to its all-core boost, 3.7 GHz, and may transition down to 3.6 GHz depending on the conditions (thermals, power, current).

Turbo in Zen+, then Zen2

AMD dropped XFR from its marketing materials, tying it all under Precision Boost. Ultimately the boost function of the processor relied on three new metrics, alongside the regular thermal and total power consumption guidelines:

PPT: Socket Power Capacity
TDC: Sustained VRM Capacity
EDC: Peak/Transient VRM Capacity

In order to get the highest turbo frequencies, users would have to score big on all three metrics, as well as cooling, to stop one being a bottleneck. The end result promised by AMD was an aggressive voltage/frequency curve that would ride the limit of the hardware, right up to the TDP listed on the box.

This means we saw a much tighter turbo boost algorithm compared to Zen. Both Zen+ and Zen2 then moved to this boost algorithm that was designed to offer a lot more frequency opportunities in mixed workloads. This was known as Precision Boost 2.

In this algorithm, we saw more than a simple step function beyond two threads, and depending on the specific chip performance as well as the environment the chip was in, the non-linear curve would react to the conditions and the workload to match hit the total power consumption of the chip as listed. The benefit of this was more performance in mixed workloads, in exchange for a tighter power consumption and frequency algorithm.

Move forward to Zen2, and one of the biggest differences for Zen2 is how the CPUs are binned. Since Zen, AMD’s own Ryzen Master software had been listing ‘best cores’ for each chip – for every Ryzen CPU, it would tell the user which cores had performed best based on internal testing, and were predicted to have this best voltage frequency curve. AMD took this a step further, and with the new 7nm process, in order to get the best frequencies out of every chip, it would perform binning per core, and only one core was required to reach the rated turbo speed.

So for example, here is a six-core Ryzen 5 3600X, with a base frequency of 3.8 GHz and a turbo frequency of 4.4 GHz. By binning tightly to the silicon maximums (for a given voltage), AMD was able to extract more performance on specific cores. If AMD had followed Intel’s binning strategy relating to turbo here, we would see a chip that would only be 4.2 GHz or 4.1 GHz maximum turbo – by going close to the chip limits for the given voltage, AMD is arguably offering more turbo functionality and ultimately more immediate performance.

There is one thing to note here though, which was the point of Paul’s article. In order to achieve maximum performance in a given workload, AMD had to adjust the Windows CPPC scheduler in order to assign a workload to the best core. By identifying the best cores on a chip, it meant that when a single threaded workload needed the best speed, it could be assigned to the best core (in our theoretical chip above that would be Core 2).

Note that with an Intel binning strategy, as the binning does not go to the per-core limits but rather relies on per-chip limits, it doesn’t matter what core the work is assigned to: this is the benefit of a homogeneous turbo binning design, and ultimately makes the scheduler algorithm in the operating system very simple. With AMD’s solution, that single best core is frequency scheduled that work, and as such the software stack in place needs to know the operation of the CPU and how to assign work to that specific core.

Does this make any difference to the casual user? No. For anyone just getting on with their daily activities, it makes absolutely zero difference. While the platform exposes the best cores, you need to be able to use tools to see it, and unless you uninstall the driver stack or micromanage where threads are allocated, you can’t really modify it. For casual users, and for gamers, it makes no difference to their workflow.

This binning strategy however does affect casual overclockers looking to get more frequency – based on AMD’s binning, there isn’t much headroom. All-core overclocks don’t really work in this scenario, because the chip is so close to the voltage/frequency curve already. This is why we’re not seeing great all-core overclocks on most Ryzen 3000 series CPUs. In order to get the best overall system overclocks this time around, users are going to have to play with each core one-by-one, which makes the whole process time consuming.

A small note about Precision Boost Overdrive (PBO) here. AMD introduced PBO in Zen and Zen+, and given the binning strategy on those chips, along with the mature 14/12nm process, users with the right thermal environment and right motherboards could extract another 100-200 MHz from the chip without doing much more than flicking a switch in the Ryzen Master software. Because of the new binning strategy – and despite what some of AMD's poorly executed marketing material has been saying – PBO hasn't been having the same effect, and users are seeing little-to-no benefit. This isn’t because PBO is failing, it’s because the CPU out of the box is already near its peak limits, and AMD’s metrics from manufacturing state that the CPU has a lifespan that AMD is happy with despite being near silicon limits. It ends up being a win-win, although people wanting more performance from overclocking aren’t going to get it – because they already have some of the best performance that piece of silicon has to offer.

The other point of assigning workloads to a specific core does revolve around lifespan. Typically over time, silicon is prone to electromigration, where electrons over time will slowly adjust the position of the silicon atoms inside the chip. Adjusting atom positioning typically leads to higher resistance paths, requiring more voltage over time to drive the same frequency, but which also leads to more electromigration. It’s a vicious cycle.

With electromigration, there are two solutions. One is to set the frequency and voltage of the processor low enough that over the expected age of the CPU it won’t ever become an issue, as it happens at such a slow rate – alternatively set the voltage high enough that it won’t become an issue over the lifetime. The second solution is to monitor the effect of electromigration as the core is used over months and years, then adjust the voltage upwards to compensate. This requires a greater level of detection and management inside the CPU, and is arguably a more difficult problem.

What AMD does in Ryzen 3000 is the second solution. The first solution results in lower-than-ideal performance, and so the second solution allows AMD to ride the voltage/frequency limits of a given core. The upshot of this is that AMD also knows (through TSMC’s reporting) how long each chip or each core is expected to last, and the results in their eyes are very positive, even with a single core getting the majority of the traffic. For users that are worried about this, the question is, do you trust AMD?

Also, to point out, Intel could use this method of binning by core. There’s nothing stopping them. It all depends on how comfortable the company is with its manufacturing process aligning with the expected longevity. To a certain extent, Intel already kind of does this with its Turbo Boost Max 3.0 processors, given that they specify specific cores to go beyond the Turbo Boost 2.0 frequency – and these cores get all the priority programs to run at a higher frequency and would experience the same electromigration worries that users might have by running the priority core more often. There difference between the two companies is that AMD has essentially applied this idea chip-wide and through its product stack, while Intel has not, potentially leaving out-of-the-box performance on the table.

Defining Turbo, Intel Style A Short Detour on Mobile CPUs
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  • peevee - Friday, September 20, 2019 - link

    Sunny Cove is about just as new as Zen2 compared to Zen+, if not more.

    Only not "architecture" but "microarchitecture", architecture is the same Von Neumann prevalent since 1940s, or its Harvard variant prevalent since 1980s.
    Reply
  • Jovec - Tuesday, September 17, 2019 - link

    Might go to credibility and past behavior. Bulldozer couldn't maintain stock speeds under load (although the lawsuit was for something else IIRC). Reply
  • evilspoons - Tuesday, September 17, 2019 - link

    Thanks for the informative article. The modal frequency being so close to the rating on all those 3000-series chips, BEFORE the +25-50 MHz fix, means it's not even worth worrying about. As consumers it's good to be informed and keep an eye on the companies we buy from, but there's no conspiracy here. Just confusion. Reply
  • Karmena - Wednesday, September 18, 2019 - link

    Companies have to be held accountable on what they write on the boxes. Even if that is off by 25 out of 4400. Just be honest and write 4375. Reply
  • Atari2600 - Wednesday, September 18, 2019 - link

    I'm sure your bright enough to be able to round up to 1 decimal place.

    What does 4.375 become when rounded to 1 decimal place?
    Reply
  • ianisiam - Wednesday, September 18, 2019 - link

    Except for the fact that 4400 isn't guaranteed. Like evilspoons said, it's just confusion. Reply
  • eva02langley - Wednesday, September 18, 2019 - link

    You didn`t read the article obviously... these are not guaranteed. If you apply thermal paste and HSF pressure like an amateur, your results will not be in line with AMD numbers, but of course you will scream rip-off... Reply
  • Karmena - Monday, September 23, 2019 - link

    Will not scream, just it would be nice to have every little bit. With ABBA, that is the case, in the end I need good mobo, good cooling, some memory OC prooves and I do not have to worry about CPU OC as that is brought to maximum performance by itself already. Reply
  • limitedaccess - Tuesday, September 17, 2019 - link

    I feel there is one more aspect to this that wasn't fully addressed nor have seen it very addressed in discussions.

    According to your article (which matches previous understanding) is that Intel (even if they won't guarantee it the boost speeds) will bin CPUs so that every core is capable of running at the listed turbo speeds. In your example every core can reach 4.6ghz for the 9600k.

    While AMD with Zen is only binning one core as capable of reaching the boost speeds. Although you list a more likely typical example in your article does this not in theory mean that a 3600x which can only reach 4.4ghz on a single core, while all other 7 can only hit 3.8ghz will pass (even if statistically such a CPU is extremely unlikely)? Or is their binning actually tighter then this but not disclosed anywhere?

    This has been one of my concerns with respect the this type of situation going forward. It seems like there needs to be more disclosure/data points on binning requirements going forward.
    Reply
  • ajlueke - Tuesday, September 17, 2019 - link

    Thanks Ian!

    Could you highlight what the nebulous "limits of the silicon" are? I have noticed, that in low current (core) work loads with Zen 2, the performance seems to be fixed. I'm not reaching any temperature, EDC, TDC or PPT boundary, but changing the scalar, increasing fmax, or increasing the aforementioned limits. It seems something else is limiting the processor's ability to boost, but there is no clear indication what that is. I have observed the Stilt mention the Fitness Monitoring Tool (FIT) as a voltage limit baked into the silicon. Zen 2, tend to be at this limit, and it simply will not go higher regardless of what you set PPT, TDC and EDC at. Do you have any additional information on this limiter?
    It does seem fundamentally different for overclockers. The voltage limit on boosting seems to be reached before thermal limits, meaning that Zen2 CPUs with the stock cooler boost the same as those with far better heat dissipation, at least in lightly threaded workloads. I guess this is a good thing for users overall, as everyone gets the same performance out of the box, but jarring for enthusiasts who are used to seeing much high numbers on their systems. I noticed the delta between the bottom 5th percentile and upper 95th percentile of the 3900X on userbenchmark was 12.4%. Incredibly tight, and demonstrates that there likely isn't much to ring out of these systems.
    Reply

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