Is that normal CPU speed drops lower than its labelled speed when using handbrake?

Discussion in 'iMac' started by zhaoxin, Dec 23, 2016.

  1. zhaoxin macrumors regular

    Joined:
    Jan 28, 2015
    #1
    iMac 5k 2014 late i7 4.0 GHz.

    When using handbrake to convert 1080p mkv to mp4, CPU speed will drop to 3.7 to 3.8 after 2 minutes until all converting finish.
     
  2. bernuli macrumors 6502

    Joined:
    Oct 10, 2011
    #2
    Maybe it is running out of electricity as it has to run the fans on turbo?
     
  3. zhaoxin thread starter macrumors regular

    Joined:
    Jan 28, 2015
    #3
    Fan speed is 26xx by macOS controlling, the fan's max speed is 2800 in iStat menus. The temperature is over 95 centigrade.
     
  4. bernuli macrumors 6502

    Joined:
    Oct 10, 2011
    #4
    I don't use Handbrake but it looks to me like the CPU is being slowed down to prevent overheating.

    You might try lowering the ambient temperature of the room to get a lower internal temperature with the same fan speed.
     
  5. JesperA macrumors 6502a

    Joined:
    Feb 10, 2012
    Location:
    Sweden
    #5
    95c sounds a bit high so the CPU is maybe thermal throttling (lowering the CPU speed to keep the temperature under control)
     
  6. wlossw macrumors 65816

    wlossw

    Joined:
    May 9, 2012
    Location:
    Montreal, Quebec, Canada
    #6
    This is a known issue. It is thermal related throttling. Lowering the ambient temp may help, alternatively, raise fan speed (if it can go over 2800) or point a desk fan at the machine. You can also try resetting the SMC https://support.apple.com/en-ca/HT201295
     
  7. elf69 macrumors 68020

    elf69

    Joined:
    Jun 2, 2016
    Location:
    Cornwall UK
    #7
    I have a laptop cooling try under my mac.

    I was using it with an old toshiba before and i the toshiba it dropped the temp by 3 degrees when really using it hard.

    Not sure about the mac but feels cooler at the exhaust vents of the mac.

    But your machine does seem bit hot.
    try tucking something under the back to tilt it forward a little.
    This allow air to flow under the laptop as well.
     
  8. zhaoxin thread starter macrumors regular

    Joined:
    Jan 28, 2015
    #8
    Is that normal that the max speed of fan is only 2800? My Mac mini's can be over 4000.
     
  9. wlossw macrumors 65816

    wlossw

    Joined:
    May 9, 2012
    Location:
    Montreal, Quebec, Canada
    #9
    Fans in the iMac are different than MacBooks or mac minis but they should still be capable of more than 2800rpm... I suspect OP has limited fan speed with istat menus.
     
  10. CWallace macrumors 603

    CWallace

    Joined:
    Aug 17, 2007
    Location:
    Seattle, WA
    #10
    You're encountering thermal throttling, which is by design. The 4GHz base frequency is with all four cores operating within the TDP limits, which I fully expect is at below 100% load per core considering the maximum single-core speed is 4.2GHz. So with all four cores at 100% (which is common with Handbrake), the TDP is being exceeded so the CPU is slowing the clock down to keep it within the TDP. And that it can hold 3.7-3.8GHz is a pretty good sign, IMO. My friend just put a new cooler on his i7 that is the size of a car radiator (I kid, but the Noctus NH-U14S is almost ridiculously large) and even it struggles to keep each core under 40° at full-load with rendering video in a full-size case with a half-dozen fans.


    The fans are larger on the iMac so they can spin slower while still moving the same or more volume of air. I think 2800 might very well be the maximum speed for them. Barefeets did some temp and fan speed tests and maximum fan speed they saw was under 2500RPM with the CPU showing 100°C / 212°F.
     
  11. zhaoxin thread starter macrumors regular

    Joined:
    Jan 28, 2015
    #11
    Thanks. I was thinking that an i7 4.0 GHz meant when needed, it should at least run at 4.0 GHz. I do agree the temperature is high enough for CPU to limit its speed. But I still very disappointed at Apple as the 4.0GHz could only last for 2 minutes. 4.0 GHz is a designed speed, not from overclocking, it should last as it should with a good cooling system, not like it is now. The cooling system designed by Apple just didn't work well.
     
  12. CWallace macrumors 603

    CWallace

    Joined:
    Aug 17, 2007
    Location:
    Seattle, WA
    #12
    It works well enough, IMO. 3.7GHz is 92% of the rated speed. I had the original 5K with an even higher-clocked i7 and under Handbrake the fans were very audible trying to keep that thing from melting. The new smaller process Intel now uses has improved the thermal headroom of the CPUs (along with the lower maximum clock) to allow the iMac to still run quiet while providing almost full performance.
     
  13. zhaoxin thread starter macrumors regular

    Joined:
    Jan 28, 2015
    #13
    By original, you mean prototype for testing? As i7 4.0 GHz was already the highest speed that could be customized in Apple Store? Are you a hardware engineer?

    Unfortunately, mine iMac was picked up by Apple for repairing yesterday. It was not because of the CPU speed problem, but for randomly power shutdown problem. Mine and some other iMac 5k 2015 late users encountered random shutdown problem recently. When using iMac, its screen went black suddenly and all power were gone. We couldn't start the Mac by pressing power button.

    The only working process was to reset SMC. Unplug power cable, wait and reconnect, then pressing power button.

    I found this CPU speed dropping issue by accident. I was actually trying to find why my iMac lost power at that time. Since I suspected the power supply. I wanted the iMac consumed more power. I didn't have games in my mac, so I chose to run a video converting.
     
  14. joema2 macrumors 65816

    joema2

    Joined:
    Sep 3, 2013
    #14
    I just tested HandBrake transcoding a large MKV file to MP4 on my top-spec 2015 iMac 27. After 15 minutes, it never hit thermal limiting, as monitored by Intel Power Gadget.

    I then ran Prime95 on both my 2015 iMac 27 and my Windows PC with a 3.8Ghz i7-875K, and they both hit thermal limiting after a few minutes. The Windows PC has a Noctua NH-D14 cooler, plus additional 200mm top and side case fans, plus the CPU uses the Indigo Xtreme thermal interface: http://indigo-xtreme.com/page-bab.shtml

    Supposedly thermal management was improved from the 2014 to the 2015 iMac 27. This video examines the difference:



    As a video editor I transcode a lot of material and I don't see my 2015 iMac 27 hitting thermal limiting frequently, if ever. You can probably drive most PCs *or* Macs into thermal limiting with a dedicated stress utility.

    For normal production work I don't see any thermal problems on my iMac, and when running stress utilities, the huge cooler on my PC doesn't keep it from eventually hitting thermal limiting.
     
  15. CWallace macrumors 603

    CWallace

    Joined:
    Aug 17, 2007
    Location:
    Seattle, WA
    #15
    The 2014 iMac 5K could be configured with an i7-4790K which had a single-core speed of 4.4GHz compared to the 4.2GHz of the current i7-6700K, though both had a base clock speed of 4.0GHz.
     
  16. zhaoxin thread starter macrumors regular

    Joined:
    Jan 28, 2015
    #16
    Mine is i7 4790K.
    --- Post Merged, Dec 25, 2016 ---
    Thanks.
     
  17. CWallace macrumors 603

    CWallace

    Joined:
    Aug 17, 2007
    Location:
    Seattle, WA
    #17
    Ah. Yeah those seem to run hotter than the i7-6700K. My fans were much higher / louder with the i7-4790K than the i7-6700K. The 14nm process used in the i7-6700K results in lower thermal levels than the 22nm process of the i7-4790K even though the i7-6700K has a higher TDP.
     
  18. kschendel macrumors 6502a

    Joined:
    Dec 9, 2014
    #18
    I think you're being a bit unreasonable. 4.0 Ghz is not a designed continuous speed, at least not without a seriously heroic cooling system (massive heat sinks+noisy fans, or liquid cooling) that would be far more objectionable than losing a few percentage points of clock. It's a top limit burst speed; in a sense, it really is an overclock speed, just one that's controlled internally by the chip rather than externally by a tinkerer. Feel free to blame both Intel and Apple marketers for not making that clear, but don't blame the engineers for coming up with what seems to be a pretty good compromise cooling solution.
     
  19. cynics macrumors G4

    Joined:
    Jan 8, 2012
    #19
    I tend to disagree. In a PC build I can keep a 6700K in the 70-80c range under full load without much effort at all using a 30 dollar cooler. Even less with a beefy cooler.

    I just helped a friend build a PC for VR gaming and with Cooler Master Hyper something (model escapes me) during a his 24 hour stress test the 6700k didn't exceed 78c and it's barely audible, at least compared to an iMac running above 2000RPM. Not even a comparison really. No throttling what so ever. GTX 1080 was a different story but that's besides the point.

    Granted an iMac has space restrictions however you should be willing to compromise in space constraint to accommodate for the parts inside. 92% of 100 is A LOT if you are encoding video for hours on end. I've had encodes last 5+ hours, that can be 30 minutes extra just because the CPU is using its last line of defense before self destruction.

    Does it over heat? Screen tends to go blank, fans blast at max and requires an SMC reset. Hmm strange the OP is experiencing that..... Don't fault the engineers? I actually agree with that because they were likely told to do the impossible and it's as good as it can probably be without compromising the case design or CPU choice or cost or some other variable. Doesn't make it right though.
     
  20. joema2 macrumors 65816

    joema2

    Joined:
    Sep 3, 2013
    #20
    That is not my experience with my PC using a Noctua NH-D14 and multiple 200mm case fans. If doing an intense dedicated stress test it gets hot (over 90C) and eventually hits thermal throttling.

    I'd be interested if you ran Prime95 with option #2 (In-place FFTs) for several hours while also continuously running the FurMark GPU burn-in test -- does that stay cool, barely audible and never approaches thermal throttling?
     
  21. cynics macrumors G4

    Joined:
    Jan 8, 2012
    #21
    This weekend I'll have him try it and I'll post screenshots. We didn't do that but I'm sure he'll be happy to, since he's looking for excuses to max it out anyway and he's anti-apple.

    We'll be custom water cooling before this is over. Lol
     
  22. CWallace macrumors 603

    CWallace

    Joined:
    Aug 17, 2007
    Location:
    Seattle, WA
    #22
    Yes, but consider how much more space there is in a full-size case and how many more (and larger) fans they tend to have. It's more equivalent to the volume and cooling system of the old "cheese-grater" Mac Pro. That an iMac can do as well as it can with the small volume it has and without the fans running into the five figure RPMs is, at least to me, impressive (and welcomed).
     
  23. cynics macrumors G4

    Joined:
    Jan 8, 2012
    #23
    Don't get me wrong. I'm not saying what they've accomplished isn't impressive or that I could do any better.

    Just if I'm paying the premium Apple charges I want every inkling of performance from it.

    I like Apples ecosystem too much to leave and do a ton of video encoding so regardless my next computer will be a Mac. Hopefully a quad core mini if not a quad core iMac.

    In my 2013 I have an i5 which even though I do a ton of video encoding I may get again since typically I start it and walk away. Currently it RARELY gets to 95c so the fan never exceeds 1400rpm. I would prefer that then what sounds like a vacuum running in my office while I sit in the living room.

    Anyway I'm patiently waiting for an update to the Mini and iMac then reviews on them to see what I should do.
     

Share This Page