Why does the 8250U require such a high voltage?

Discussion in 'Hardware Components and Aftermarket Upgrades' started by Che0063, Apr 2, 2018.

  1. Che0063

    Che0063 Notebook Consultant

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    This difference has been bugging me over some time ever since I got this laptop, but why does the 8250U (and presumably other 8th gen U series chips require such a high voltage to be stable?

    I ran a Prime95 Stress Test, set Speedshift EPP to 0, and used TPL-Speedshift Limits in Throttlestop to control the maximum frequency of the CPU, starting to 0.4GHz all the way to the maximum Turbo Frequency of 3.4GHz on all 4 cores. The 8250U turbos to 3.4 regardless whether the load is on 1 core or all 4 cores.

    All the tests were done with a -125mV undervolt.

    Then I graphed the voltages and package power consumption. The graphs are here, and the excel spreadsheet is attached.
    upload_2018-4-3_7-42-26.png

    To me, the most efficient operating speeds for my 8250U was in the blue bars, between 1.6GHz and 2.3GHz. Beyond 2.3GHz, the Core Voltage ramps up significantly and power consumption shoots up.

    The graph also explains how my 8250U could be significantly less efficient than my friend's 7700HQ. We both ran TS Bench 32M with the speedshift value set to 0 and a -125mV undervolt and no throttling. Guess what?

    The 7700HQ completed the 32M Test in 7.3 seconds with an average package power consumption of 18W and average voltage of about 0.97v
    The 8250U completed the 32M Test in 8.0 seconds with an average power of 25W and voltage of around 1.11v
    Yes, both processors just so happened to have an equally stable undervolt amount and equally similar max quad core turbo clock of 3.4GHz. As well, my 8250U just so happened to have a similar Prime 95 power consumption and Passmark CPUMark performance when the TDP Limit was raised to 44W

    I simply couldn't believe this and made my friend send a screenshot. So my 8250U, at times, will use more power yet perform worse under some workloads, probably due to its higher turbo voltage. But sure, in many other day to day workloads, my 8250U is vastly more efficient compared to the 7700HQ, but that all changes when turboboost kicks in. I get it, TurboBoost was never supposed to be continuously utilised, nor was it ever meant to be efficient, but why the big difference between a <strike>high performance<strike> HQ CPU and a "low power" U CPU?

    Also to note, the 7700HQ idles at a lower voltage (0.6v @ 800MHz) whilst my 8250U idles higher (0.72v), both at a -125mV undervolt. Interestingly enough, my dual core i7-6500U would idle at 0.55v and sustain 3.0GHz turbo at 0.97v. But that's a dual core CPU and it may not be related.

    I read online that CPUs use more power when they are hotter... I've never noticed an unstable undervolt that was stable on 40C but locked up at 90C

    If I disable turbo (And thus limiting the CPU to the 1.8GHz TDP Level 2 limit) I can undervolt to -200mV. Doing so results in the performance slightly better than the stock 7500U but with almost half the power consumption

    I understand that CPUs are complex and that I'm comparing CPUs of different generation, but isn't 8th gen supposed to be a refresh of Kabylake? So what is causing the difference between the voltages and power consumption of these mobile chips?
     

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    Last edited: Apr 2, 2018
  2. Falkentyne

    Falkentyne Notebook Prophet

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    8th gen is nothing more than 7th gen with moar cores.
    An 8700K is a 6 core 7700K. They even reach the same clocks with 8700k slightly better since the process is more mature.

    And a HQ is NOT a high performance CPU. Dear God please don't call it that.
    You just made @Papusan vomit even more. Oh dear.....

    5295874_steam.jpg

    P.S.

    HQ CPU=Heavily Qrippled.
    HK CPU= Heavily Kancerous.
     
    Last edited: Apr 2, 2018
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  3. Papusan

    Papusan JOKEBOOKS = That sucks! Dont wast your $ on FILTHY

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    Could you and your friend compare results in same way with Cinebench R15 ? As I understand your BGA manage 3.4GHz who is equal clocks as 7700hq on 4 cores load. Thanks
    Same IPC. Skylake-Kaby and Coffee Lake. All the same for single cores load.
     
    Last edited: Apr 2, 2018
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  4. Che0063

    Che0063 Notebook Consultant

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    I will have to do in in the next coming weeks. It's school holidays and lord knows what my friends do (waste their lives playing games 12hrs a day. that's what everyone at my school does)
    Well you see, back when I was 10 years old, I received my first ever computing device with a DUAL CORE AMD E2-1800 running at a blistering 1.7GHz which took 3 minutes to boot up to Windows 7 and another minute to open Internet Explorer (don't kill me for using it. I was just a child) Then I saw a laptop with the 4700HQ with literally 12x the performance of my E2. I have never owned any device with anything better than a U CPU.
     
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  5. Falkentyne

    Falkentyne Notebook Prophet

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    Not the same!!!
    +1 bin speed needed for BGA CPU compared to LGA CPU!!!

    learn to swim, Poopoosan ! :):)

    facepalms.jpg
     
  6. Papusan

    Papusan JOKEBOOKS = That sucks! Dont wast your $ on FILTHY

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    Who said I compared LGA vs BGA?
    upload_2018-4-3_1-19-18.png
    BGA as well have Skylake, Kaby and soon new Coffee. Would you think Coffee BGA have better IPC than the BGA predecessors:D And have Kaby better IPC than Skylake in the BGA family?

    No I think I can swim:cool:
     
  7. Arrrrbol

    Arrrrbol Notebook Evangelist

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    I suppose its like the old analogy of two cars: a Porsche doing 30mph will use a lot more fuel than a Mini, but a Porsche doing 100mph will use a lot less fuel than a Mini doing the same speed as it doesn't need to work as hard. The U line of CPUs are probably pretty far down the binning process and therefore are poor quality silicon compared to the MID range 7700HQ (@Papusan ;)). I'd assume the silicon is pretty bad as generally lower quality chips need a higher voltage to sustain a given clock speed.
     
  8. Papusan

    Papusan JOKEBOOKS = That sucks! Dont wast your $ on FILTHY

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    Yeah, Intel is smart, they pack in their scam into max boost who will be turned down after a few sec. See... single round Cinebench R15 and the power seeping out as the air from a punctured balloon:hi:
     
  9. Falkentyne

    Falkentyne Notebook Prophet

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    I believe this can be avoided in an unlocked Bios.
    I turned my 7820HK into a 7700HQ by changing EC registers (it locked the TDP at 45W sustained and 62W for 28 seconds, like locked chip).
    Then if you change IMON SLOPE to 25 or 50 (thank you @Vistar Shook , your CPU will report its using half the TDP), then you can pass 45W unlimited time.

    I have not tested this on 7700HQ real chips but I think others can try. But they have to unlock their Bios.

    If they are afraid, maybe they can use "RU" and find the register and change it to 25 or 50 in EFI prompt, but that depends if it accepts a DECIMAL or HEXADECIMAL value. And this may not work, usually the EFI prompt and setup strings are changed with "enabled" and "disabled" values of 0x01 and 0x00. Not sure if you need to convert 25 or 50 to hexadecimal or not then use 0x32 (32 hex=50 decimal).
     
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  10. Vistar Shook

    Vistar Shook Notebook Deity

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    Yes it can be done with the HQ chips and an unlocked BIOS, the hard part is getting the chip to use more than 45W anyways.
     
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