Going for Power

How to Manage 5.0 GHz Turbo

Intel lists the Core i9-9900KS processor as having a 127W TDP. As we’ve discussed at length [1,2] regarding what TDP means, as well as interviewing Intel Fellows about it, this means that the Core i9-9900KS is rated to require a cooling power of 127W when running at its base frequency, 4.0 GHz. Above this frequency, for example at its turbo frequency of 5.0 GHz, we are likely to see higher than 127W.

Now, I started saying in this review that the length of time that the processor will spend at 5.0 GHz will be motherboard dependent. This is true: Intel does not strictly define how long turbo should be enabled on any processor. It allows the motherboard manufacturer to ‘over-engineer’ the motherboard in order to help push the power behind the turbo higher and enable turbo for longer. The specific values that matter here are called PL2 (Power Limit 2, or peak turbo power limit), and Tau (a time for turbo).

For an Intel processor, each one has a ‘bucket’ of extra turbo energy. As the processor draws more power above its TDP (also called PL1), the bucket is drained to provide this energy. When the bucket is empty, the processor has to come back down to the PL1 power value, and eventually when the processor is less active below PL1, the bucket will refill. How big this bucket is depends on the value of PL1, PL2, and Tau. The bigger the bucket, the longer an Intel processor can hold its turbo frequency. Typically Tau isn’t so much as a time for turbo, but a scalar based on how big that bucket should be.

An example graph showing the effect of implementing Turbo on power/frequency

Motherboard manufacturers can set PL1, PL2, and Tau as they wish – they have to engineer the motherboard in order to cope with high numbers, but it means that every motherboard can have different long turbo performance. Intel even suggests testing processors on high-end and low-end motherboards to see the difference. Users can also manually adjust PL1, PL2, and Tau, based on the cooling they are providing.

For the Core i9-9900KS, Intel has given the PL1 value on the box, of 127 W. PL2 it says should at least be 1.25x the value of PL1, which is 159 W. Tau should be at least 28 seconds. This means, with a given workload (typically 95% equivalent of a power virus), the CPU should turbo up to 159 W for 28 seconds before coming back down to 127 W. A very important thing to note is that if the CPU needs more than 159 W to hit the 5.0 GHz turbo frequency, it will reduce the frequency until it hits 159 W. This might mean 4.8 GHz, or lower.

Despite giving us these numbers for PL1, PL2, and Tau, Intel also stated to us that they recommend that motherboard manufacturers determine the best values based on the hardware capabilities. The values of 127 W, 159 W, and 28 seconds are merely guidelines – most motherboards should be able to go beyond this, and Intel encourages its partners to adjust these values by default as required.

We tested Intel’s guidelines with a 10 minute run of Cinebench R20.

He we can see that at idle, the CPU sits at 5.0 GHz. But immediately when the workload comes on, it has to reduce the average CPU frequency because it goes straight up to the 159W limit – simply put, 159W isn’t enough to hit 5.0 GHz. We see the temperature slowly rise to 92C, but because the power isn’t enough the frequency keeps fluctuating.

By the end of the first Cinebench R20 section, it seems that the majority of it occurred during the turbo period. This means that this run scored almost the same as a pure 5.0 GHz run. However the subsequent runs were not as performant.

Because the turbo budget had been used up, the processor had to sit at 127 W, its PL1 value. At this power, the processor kept bouncing between 4.6 GHz and 4.7 GHz to find the balance. The temperatures in this mode kept stable, nearer 80C, but the performance of Cinebench R20 dropped around 8-10% because the CPU was now limited by its PL1/TDP value, as per Intel’s base configuration recommendation.

Going Beyond

Because motherboard manufactuers can do what they want with these values, we set the task on the motherboard we tested, the MSI Z390 Gaming Edge AC. By default, MSI has set the BIOS for the Core i9-9900KS with a simple equation. PL1 = PL2 = 255 W. When PL1 and PL2 are equal to each other, then Tau doesn’t matter. But what this setting does is state that MSI will allow the processor to consume as much power as it needs to up to 255 W. If it can hit 5.0 GHz before this value (hint, it does), then the user can turbo at 5.0 GHz forever. The only way that this processor will reduce in frequency is either at idle or due to thermal issues.

Here’s the same run but done with MSI’s own settings:

The processor stayed at a constant 5.0 GHz through the whole run. The CPU started pulling around 172W on average during the test, fluctuating a little bit based on exactly which 1s and 0s were going through. The CPU temperature is obviously higher, as we used the same cooling setup as before, and peaked at 92C, but the system was fully table the entire time.

Here was our system setup – a 2kg TRUE Copper air cooler powered by an average fan running at full speed in an open test bed.

But what this means is that users are going to have to be wary of exactly what settings the motherboard manufacturers are using. For those of you reading this review on the day it goes live, you’ll likely see more than a dozen other reviews testing this chip – each one is likely using a different motherboard, and each one might be using different PL2 and Tau values. What you’ve got here are the two extremes: Intel’s recommendation and MSI’s ‘going to the max’. Be prepared for a range of results. Where time has permitted, we’ve tested both extremes.

Test Bed and Setup CPU Performance: System Tests
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  • amnesia0287 - Saturday, November 16, 2019 - link

    Can’t tell if playing stupid or srs.

    The IPC advantage is 10%, the clock speed advantage is >10%.

    Intel chips all the cores are able to boost to the same levels. AMD chips now have varying spec cores and only the best 1-2 hit the advertised speeds and only when there are only a few threads running. If you are doing heavy multitasking, most of the AMD cores will be dramatically slower than those of the 9900ks.

    That said the AMD chips still have a number of other advantages such as more pci bandwidth and no tiny DMI bottleneck. Lower cost, lower power use and generally more cores.

    Right now it really comes down to what you plan to use it for. There is little doubt AMD is in a better spot right now, but it remains to be seen if they can hold it past 2021 as intel transitions to 7nm and MCM.
  • Korguz - Saturday, November 16, 2019 - link

    " The IPC advantage is 10%, the clock speed advantage is >10%." and WHY do you think intel has the lead, even barely, in some cases. because of the clock speed advantage. Intel's cpus NEED this advantage just to keep the performance they have. think about it, a LOWER clocked chip over all is performing about the same,( depending on usage ) and in some cases better, then the higher clocked equivalents clock these chips the same, and i bet the story would be quite different. " Intel chips all the cores are able to boost to the same levels " at quite a bit more power usage too.
    " 2021 as intel transitions to 7nm and MCM. " more like IF intel can transition to 7nm by then, look how long it has taken them to get to 10nm..
  • shaolin95 - Monday, November 11, 2019 - link

    *cough, cough* butthurt AMD fanboy detected *cough, cough*
  • Qasar - Monday, November 11, 2019 - link

    how so ??? seems there are more intel fans but hurt lately...
  • peevee - Friday, November 1, 2019 - link

    Funny, I had exactly the same thought.
    But honestly, for many real world uses (games etc) 4 cores at even faster frequency would be even better. Physically separated on the die as far as possible (in the corners) by huge amount of shared L3.
    I guess 5GHz base/6GHz turbo is not out of the question within the same TDP with liquid cooling.
  • eva02langley - Thursday, October 31, 2019 - link

    A joke of a CPU. How can this cost more than a 3900x?
  • eva02langley - Thursday, October 31, 2019 - link

    Not to mention that with the price gouging, you are almost near the MSRP of the 3950x.
  • prophet001 - Thursday, October 31, 2019 - link

    People want this for clock speed.

    12 slow cores aren't helpful if you need a few fast ones.
  • eva02langley - Thursday, October 31, 2019 - link

    AHAHAHA... okay, we have a blind fanboy here. Do you know anything about IPC? This CPU get destroyed in EVERYTHING except old games running outdated engines at 1080p. SO unless you buy this with a 2080 TI and a 240Hz 1080p monitor, you are not going to benefit from it.

    Basically, with a budget, the 9900KS is a waste of money, period. The money you save buying a 3700x and investing in your GPU will give you some serious gaming performances increase.
  • xenol - Thursday, October 31, 2019 - link

    Honestly anything more than a midrange CPU and GPU is a waste of money for most people.

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