My machine specs:

iMac (Retina 5K, 27-inch, 2017)

Processor: 3.4 GHz Quad-Core Intel Core i5

Memory: 16 GB 2400 MHz DDR4

Graphics: Radeon Pro 570 4 GB

Very recently I have started noticing that all cores suddenly report idle for a moment and then go back to full use. Quite similar to garbage collection in a JVM. Please see the attached CPU History picture below. Note how at one place all cores were being fully used but they suddenly black out:

CPU History

This is a recent phenomenon on my machine. Can someone confirm that it happens on their machine too?

If not, would someone help me identify the cause behind it.

  • If you still have AppleCare, why not just bring it in for service? – nohillside Jun 23 '20 at 20:07
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    @nohillside: Apple stores are not open where I am. Also it is quite a big machine to bring in for nothing. If someone has already experienced this, it would be better to know. – displayName Jun 23 '20 at 20:17
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    What is your Mac doing during this time? What tasks are you running? Do you notice any performance drop? – benwiggy Jun 23 '20 at 21:42
  • @benwiggy: Sometimes a code build is underway. Sometimes I’m playing CS:GO. I don’t notice any perform drop because both the tasks are already very time consuming. – displayName Jun 24 '20 at 0:59
  • Let me know if I mis read your post. I’m assuming you are concerned with the lack of red/blue bars in some sample times in the cascading display. What is the boot filesystem type and is it all flash / NVMe storage? – bmike Jun 27 '20 at 17:55

We see that CPU usage pattern all the time. I wouldn’t worry since that’s what cores should do.

I would focus on one specific app at a time in Instruments if you feel it’s not idling when it should be idle and busy when it should be busy.

Missing metrics from time to time isn’t something that concerns me in general absent other much more telling issues like crashes or stalls. Again, in that case I look at what’s stalling and what’s crashing and not the overall CPU graph to identify what next.

In the context of needing a hardware swap or repair, this is very likely software and if the CPU were having issues, you would have thousands of crashes and issues - not that one graph has missed updates. But, I would absolutely open a case with Apple 2 to 4 weeks before warranty ends to have them run diagnostics of offer you to bring in in for a formal hardware diagnostic. MRI would only show something massively wrong, and you clearly don’t have that, so the only way to test this would be in the shop Apple custom diagnostics that they have, plus getting an experienced tech to load test your Mac.

  • I wanted to add - when you get the repair or talk with Apple - ask them to quote logic board out of warranty repair costs. Now you can know worst case, if it fails right out side warranty - Apple had the chance to fix it, but didn’t. They clearly would want to pay for that repair 7 days after end of coverage. A month, three months - you would certainly be asked to pay if it’s not broken now. Say it’s $800, you can weigh if your chances are slim it needs a repair at all (since CPU fail in dramatic ways - they don’t slowly degrade) to self-insure or seek other coverage. – bmike Jun 27 '20 at 18:11
  • TY for your comprehensive answer. I too feel it is another issue shipped with the OS. As you mentioned, if it was hardware, it would be showing in applications rather than just the cores usage graph. – displayName Jun 28 '20 at 2:37
  • Absolutely, @displayName the odds it’s hardware are slim but I so appreciate the detailed technical question. These were some of the most interesting cases I handled when escalating to engineering. In my current role we have hundreds of these iMac in 24x7 production and they are absolute workhorses. – bmike Jun 28 '20 at 3:53

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