There is a lot of discussion about processor power recently. A lot of the issues stem around what exactly that TDP rating means on the box, and if it relates to anything in the real world. A summary of Intel’s official declaration boils down to TDP as the sustained processor power at long periods, however almost zero motherboards follow that guideline. As a result users will usually see much higher sustained power, although with much higher performance. Some small form factor systems rely on setting these limits, so we tested a Core i9-9900K with a 95W limit to see what would happen.

Intel and TDP

We recently published a sizeable analysis on what Intel officially means by TDP, and the associated values of PL1, PL2, and Tau. You can read it all here, although what it boils down to is this diagram:

When a processor is initially loaded, it should enter a state where PL2 describes the maximum power for a time of Tau seconds. When in this PL2 state, the CPU follows Intel’s per-core Turbo table rules, which reduces the frequency based on the number of cores loaded.

After Tau seconds, the CPU should drop down to a PL1 maximum sustained power value, which is usually identical to TDP. Depending on the CPU, this may reduce the frequency to the base frequency, or well below the all-core turbo frequency.

Technically PL2 is obtained over a moving average window, Tau, such that any low power moments on the processor will 'give budget back' to the turbo mode, however the graph above is the easiest way to see the high turbo mode on a fully loaded processor.

So while Intel defines a value for PL1, PL2, and Tau, almost zero consumer motherboard manufacturers actually follow it. There are many reasons why, mostly relating to overengineering the motherboards and wanting users to have the best performance at all times. The only times where these values follow any form of Intel guidance is in small form factor PCs.

For example, I tested an MSI Vortex G3 small form factor desktop at an event last year. It was using a processor normally rated for 65W TDP, and in a normal desktop that processor would push over 100W because the motherboard manufacturer in that system did not put any limits on the power, allowing the power to fall within Intel’s per-core turbo values. However, in this Vortex system, because of the limited thermal capabilities, the BIOS was set to run at 65W the whole time. This made sense for this form factor, but it meant that anyone looking for benchmarks of the processor would be misled – the power profile set in the BIOS was in no-way related to how that CPU would run in a standard desktop.

A Core i9-9900K with a 95W Limit

To put this into perspective, for this review we are using a Core i9-9900K which has a sustained TDP rating of 95W. When we compare the per-core frequencies of a 95W limited scenario and a normal ‘unrestricted scenario’, we get the following:

When a single core is loaded, the CPU is in 5.0 GHz mode as we are well under the power limit. There’s a slight decrease of 200 MHz in the 95W at two cores, but this disappears when 3-6 cores are loaded, with both setups being equal. The major difference happens however when we are at 7-8 cores loaded: because of the power consumption, the Core i9-9900K in 95W mode drops down to 3.6 GHz, which happens to be its base frequency.

This arguably means that we should see a correlation in most benchmarks between the two parts, but not if maximum load is ever required.

This Review

For this review, we’re putting the Core i9-9900K at a 95W power limit (as measured by the internal registers of the system) and running through our CPU test suite to see if how large the performance deficit is between the Core i9-9900K in a thermally unlimited scenario compared to a small form factor system deployment.

Pages In This Review

  1. Analysis and Competition
  2. Test Bed and Setup
  3. 2018 and 2019 Benchmark Suite: Spectre and Meltdown Hardened
  4. CPU Performance: System Tests
  5. CPU Performance: Rendering Tests
  6. CPU Performance: Office Tests
  7. CPU Performance: Encoding Tests
  8. CPU Performance: Legacy Tests
  9. Conclusions and Final Words
Test Bed and Setup
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  • duploxxx - Thursday, November 29, 2018 - link

    Interesting in a way that there are so many people that always believe in benchmarking and that in real world all cores are always idle....

    The world of wonders. Artificial TDP, turbo modes and decreased frequency when running multiple cores. All to fool consumers and benchmark believers.

    Very nice review. Now the question:
    can this also be tested on a Ryzen 2700 and a 8700K and a 9900. Put all 3 albeit in a different setup on a stock or even reduced cooling device and see how they behave....
    Reply
  • olde94 - Thursday, November 29, 2018 - link

    I see why you are intested, but both 2700 and 8700k are actually quite close in power use to their rated TDP. The issue was that the 9900k wasn't at all. If you see the power/performance graph on the last page, i think you have your answer ;) Reply
  • notashill - Thursday, November 29, 2018 - link

    It's almost a very nice graph but could really stand to have a few more CPUs labeled. I mean even the literal headlining CPU that the entire article is about isn't labeled.

    And trying to compare to the POV-Ray results earlier in the article either a bunch of the CPUs are missing or the scale on the chart does not actually match the labels.
    Reply
  • duploxxx - Thursday, November 29, 2018 - link

    according anandtech measurements:

    2700x 105w rated buring 117.18
    8700k 95w rated buring 145.71
    9900k 95w rated burning 168.45

    so no i ma not kidding. even the 8700k will have reduced performance with real tdp limit vs glorious benchmarking with best of best mobo and cooling.
    Reply
  • 4800z - Thursday, November 29, 2018 - link

    No the 9900k and 8700k would have no lower performance on games. This only comes up when maxing out all cores for things like cinibench. Reply
  • TheinsanegamerN - Thursday, November 29, 2018 - link

    Unless a game pushed those TDPs up. Games that can use many cores at once, like CIV and battlefield. You know, two minor franchises nobody would notice..... Reply
  • rhysiam - Friday, November 30, 2018 - link

    There's a big difference between starting to use 6-8 cores (like Civ & BF) and hitting all those cores with a heavy load for a sustained period. Show me a game benchmark that has the 9900K literally doubling the performance of a 7700K and then you'll have a game that can push the 9900K well past its 95W tdp.

    Game streaming from a single PC would certainly do that, but I'd hopefully streamers are doing some research and choosing hardware carefully.

    To be clear, I'm not defending Intel here, the tdp figure has become a joke, but we're a long way from this being a widespread issue for gaming workloads.
    Reply
  • mr_fokyou - Thursday, November 29, 2018 - link

    not if you are streaming while gaming than you are very much bottlenecking 9900k if u force TDP limits Reply
  • bananaforscale - Saturday, December 01, 2018 - link

    You are assuming no game uses all the cores (or enough that they go above TDP). The assumption is incorrect now and it will become more incorrect as quad core becomes the minimum. Reply
  • Samus - Saturday, December 01, 2018 - link

    I think it's totally insane a CPU can use 25-27% more power than its advertised rating. Sure, that includes more performance, but as a system builder this has got to be a liability if you are putting together, say, a little 1U rack for video encoding security camera feeds. You would use a specified CPU based on its performance AND advertised TDP rating, only to find out to GET that performance, it needs to go well beyond its TDP rating, which likely wont be possible in a tiny rack with a 1U cooler (I don't believe they make 1U coolers rated beyond 105W - and those are incredibly rare, most are 73w-88w) Reply

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