Is undervolting dangerous?

Discussion in 'Hardware Components and Aftermarket Upgrades' started by michealscott, Jul 18, 2018.

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  1. michealscott

    michealscott Notebook Consultant

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    Is undervolting a Skylake Mobile i7 dangerous in the sense that there could be CPU calculation errors?
    Say its calculating 2+1 and it answers 4 there for corrupting data (?)

    Oh and what Undervolts are you guys running on 6820HK/HQ because 98 deg. C isn't to good for my laptop I assume..
     
  2. bennyg

    bennyg Notebook Virtuoso

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    Dangerous? Just causes a WHEA error (not usually visible unless you monitor for them), an app crash or bsod/reboot. Just don't do anything important like install programs or let windows do updates before you're sure it's completely stable, where the calculation errors could mess things up (so have a system image or a full System Restore point or other backup)

    The tolerances are pretty reliable. Run a 100% load benchmark (aida, XTU cpu stress test, TSBench, real bench encode, Intel burn test etc), slowly drop the core and cache offset until it crashes, dial it back 20mV from that minimum and you'll probably never have to touch it ever again.
     
    Last edited: Jul 18, 2018
  3. Danishblunt

    Danishblunt Guest

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    As bennyg already said, it's unlikely you'll damage anything. The worst that can happen is that it will bluescreen. When you install something and it bluescreens it could happen that you need to reinstall whatever you tried to install later again.
     
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  4. hmscott

    hmscott Notebook Nobel Laureate

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    Well, yes and no. When you are tuning the undervolt, finding the lowest stable undervolt - reducing the voltage to the CPU until it crashes and then backing off +5mV, you might need to back off a bit more.

    Prime95 will start threads independent of each other and run them while checking results - if it finds a math error it will exit the thread with an error code. That's why running prime95 is a good test for undervolting - but you don't need to run it for hours, 15 minutes max is enough.

    Then when you have a stable undervolt at 100% load testing, you do idle testing. When the laptop CPU is idle it will drop the voltage lower and lower, and if the undervolt is too low - it will crash or otherwise doing something odd - so you back off the undervolt another +5mV.

    The combination of those 2 +5mV back off's should give you an idle/100% stable undervolt.

    Note: I meant +5mV *increments*, keep tuning until stable. Some CPU's will be stable with +5mV once for 100% vs Idle, some will need more, increment and test.

    If you are in a hurry I guess +20mV from 100% load undervolt might nail it right off, but you could also be giving up perfectly stable undervolt at +5mV or +10mV, if you don't have time to test right now go for +20mV and get back to having fun, and come back to see if you can get a little more undervolt when you have testing time, and patience.

    That's why in the past I've said start at -100mV undervolt, and if you are stable and don't have time for more right now, stop there and you'll get that 10c temperature reduction at load, and then come back later for more undervolt testing when you have time. So far I've seen -150mV to -180mV 8th gen CPU undervolt range reported, so -100mV should be aok,.

    Once you have that stable undervolt setting for idle/100%, you shouldn't get errors.

    So, yes undervolted too far and you could get "math" errors, or crashes, or programs exiting oddly, but once you have a stable idle/100% undervolt, everything should run aok.
     
    Last edited: Jul 18, 2018
  5. Danishblunt

    Danishblunt Guest

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    ehhhhh +5mv is maybe a little to much on edge, I'd say safe margin is like bennyg said +20mv. Sometimes the CPU will first bluescreen after 1 hour even if it ran stable with prime95 @ 15mins.
     
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  6. hmscott

    hmscott Notebook Nobel Laureate

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    I'll update my post since that wasn't clear. I'm not saying stop there after backing off +5mV, I'm saying back off +5mV at a time until stable, and until you are not getting "math" errors - or other symptoms of being unstable.

    If you started at -100mV and incremented -5mV or -10mV at a time until unstable, backing off in +5mV or +10mV increments should quickly get you to the 100% load stable value. I haven't seen a need to back off +20mV when incrementing in 5mV/10mV increments.

    It's a long term thing too. I've found undervolt can be increased slowly over time, -1mV at a time, something to do when everything is running right and you want to induce instability - go back and try to undervolt some more. I took a -15mV CPU to -45mV stable in 18 months. Every little mV helps. ;)
     
    Last edited: Jul 18, 2018
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  7. Danishblunt

    Danishblunt Guest

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    Seems a little time consuming for my taste, but yeah, you could do that.
     
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  8. hmscott

    hmscott Notebook Nobel Laureate

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    Yeah, time consuming :)

    Compared to re-pasting, unlocked BIOS OC'ing, and other extreme tuning - undervolting is a very brief exercise. Once you've done it a few times, it doesn't take long at all.

    Start at -100mV test for stable, then I'd jump up to -180mV for 8th gen, -150mV 7th gen, and for 6th gen starting at -100mV I'd start incrementing -5mV/-10mV, test until unstable, then back off +5mv at a time until 100% load stable.

    That shouldn't take long, maybe 15-30 minutes if you are taking your time. You could leisurely take longer, it's up to you.

    Then back off +5mv or +10mV, or even +20mV to avoid idle BSOD, and you are done for the initial undervolt.

    When you have time to play with tuning for a closer tune come back with smaller increments, but it's not really necessary if you are already running without thermal throttling or power throttling and are happy with temps and performance.

    Take whatever pace or increment you want to keep it fun. :)
     
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  9. Danishblunt

    Danishblunt Guest

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    I would argue that isn't entirely true. As stated, sometimes the bluescreen will happen after some hours, it can take days to find the right calue.
     
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  10. hmscott

    hmscott Notebook Nobel Laureate

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    You shouldn't have that happen if you are dropping back +20mV from 100% load stable undervolt, you should be stable right then and there.

    For myself and most others I've helped / seen, +5mV / +10mV backoff from 100% load stable undervolt is enough for idle stable undervolt.

    If you didn't do enough stability testing at the 100% load undervolt to make sure it is stable then I can see how it could BSOD later. Make sure you use a serious CPU load for 100% load stability testing. Do some gaming on that setting, run other apps, work on testing that 100% load undervolt before declaring it stable.

    If you crash hours or days later at idle, then the 100% load undervolt wasn't really stable, so the additional backoff for idle undervolt won't be enough either.

    Take your time going through the undervolting process, especially if this is new to you. It's not a race. Just because someone else can do it quickly due to experience doesn't mean you are expected to run through the process that quickly too.

    The quicker you can determine an undervolt setting is unstable the sooner you can reduce the undervolt to test at a lower undervolt and reach a truly stable undervolt. You want to stress test it and make it fail, not find all the ways you can make it work.

    I have seen some people declare a large undervolt over -220mV stable and then report they needed to drop back to around -180mV, so their initial CPU load testing wasn't adequate - don't use XTU Stress / Benchmark and declare stable, use Prime95 for final 100% load stability test.

    If you declare -220mV stable due to inadequate CPU load testing, and then drop back +20mV to get idle stable, that's not going to be enough and you will be BSOD'ing down the road. :)
     
    Last edited: Jul 18, 2018
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