CPU Core Temperature DROPS when under load?

Discussion in 'Mac Pro' started by eyup, Jan 12, 2016.

  1. eyup macrumors regular

    eyup

    Joined:
    Aug 21, 2006
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    UK
    #1
    This is odd. I've just upgraded to 8 cores on my 2006 Mac Pro to X5365s.

    I'm using TGPro to measure Core CPU temperatures and when idle the Cores are low 60s, but stress them (eg rendering previews in Lightroom) and they drop below the 30s!

    Heatsink temp increases though.

    Anyone any ideas what's going on with the core temps?



    [​IMG]

    Fan RPM increases a bit but the drop is pretty quick.

    Ramping up the RPMs to 1200 seems to increase the CPU core temp (69°C!)

    (Firmware upgraded to 2,1)
     
  2. ActionableMango macrumors 604

    ActionableMango

    Joined:
    Sep 21, 2010
    #2
    Very strange. Maybe a bug in the software... have you tried any other software to corroborate the results?
     
  3. Derived macrumors regular

    Derived

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    Mar 1, 2015
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    Midwest
    #3
    Something is not right there...definitely seems like a software glitch.
     
  4. eyup thread starter macrumors regular

    eyup

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    #4
    Trying to find another that also does Core CPU temps...
     
  5. ActionableMango macrumors 604

    ActionableMango

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    Sep 21, 2010
    #5
  6. eyup thread starter macrumors regular

    eyup

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    #6
    A haa - iStat!
    - Looks like CPU Die Offset is the same thing.
    And look how that temp drops in that graph when rendering in Lightroom...

    The heatsinks temp rises - which makes more sense. >looks up CPU Die offset<

    [​IMG]
    --- Post Merged, Jan 12, 2016 ---
    Seems die offset temp is arbitrarily set at the factory - also used for overclocking and can be calibrated?
    http://www.tomshardware.co.uk/forum/id-1800828/intel-temperature-guide.html

    It's too late for this...

    Maybe by the morning some guru will simplify it all for us.

    (tho if Heatsink temp is anything to go by 42/45°C is pretty good huh!? 28/30 idle :))

    Manyana!
     
  7. h9826790 macrumors 604

    h9826790

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    #7
    That looks like the same as "xx degree to processor hot" in the 4,1. That means, the higher the better, and if you hit zero, the CPU overheat.
     
  8. eyup thread starter macrumors regular

    eyup

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    UK
    #8

    Aye! Lookee what I found:

    It seems the CPU Die Offset temperature is not an absolute value in degrees Celsius, but it's an offset (distance) from the maximum CPU junction temperature (Tj,max) beyond which it will throttle or shutdown your CPU.

    The greater the offset the better.

    Hence that number lowers when under stress because it’s getting closer to the Tj,max !

    TjMax for a x5365 is 90-95°C

    What's interesting is idle A Die offset + Cpu Heatsink = 90°C and B = 80°C
    and stressed = A 74 B 69

    You would have thought they would always add up to 90-95°C ? Maybe they would if the heatsinks were running at 100% efficiency?
     
  9. h9826790 macrumors 604

    h9826790

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    #9
    The "add up number" is a variable in iStat. I have no idea how this number achieve in iStat, but in my ovservation, that number is higher during idle and will be reduced a bit under stress. Anyway, your temperature looks very good to me. >20C offset temperature should be very safe.

    If the thermal paste apply correctly. The CPU diode temperature (not Tj) usually is just 1-5C higher than the heat sink. But when under stress, the temperature difference will increase to about 15-20C.

    So, in your case. Heat sink + offset + delta T…

    CPU A idle,
    28+62+2 = 92C

    CPU A loaded,
    42+32+16 = 90C

    CPU B idle,
    28+54+2 = 94C

    CPU B loaded,
    24+45+16 = 85C

    They are all very close to spec's max value.
     
  10. eyup thread starter macrumors regular

    eyup

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    #10

    Interesting.

    Hold on - where did you get the +2 and +16 from?
     
  11. h9826790 macrumors 604

    h9826790

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    #11
    During idle, a correctly applied thermal paste + clean heatsink should be very effective, which makes the heatsink temperature very close to the CPU diode temperature (usually around 1-5C), and 2C is the normal delta Temperature I got, so I use this temp.

    When under stress, the thermal paste and heatsink can't really catch up the CPU's temperature increment. So, the delta T usually will increase to something like 15-20C. And I use my average delta T (~16C) as the reference to finish the equation.
     
  12. eyup thread starter macrumors regular

    eyup

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    Aug 21, 2006
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    #12

    Ah haa - cheers bud!
     

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