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I have a mid-2010 iMac running High Sierra. It has started getting very sluggish several times a week. I have tracked it down to the memory pressure and the process kernal_task. Shown below is a screenshot of the activity monitor when this occurs.

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As you can see the process kernal_task is using 9.43 GB of memory. This usually lasts 20 or 30 minutes. The kernal_task will occasionally drop to 5.46 GB and then sometimes down to 1.6 GB at which time the memory pressure is small and it turns green and my machine will return to normal.

The next image is of the Activity Monitor showing CPU usage. Kernal_task is not using a lot of CPU but has been running for quite a while.

enter image description here

Does anyone have an idea of what is happening?

I've tried restarting my computer and quitting all open apps when it gets sluggish. Restarting the computer worked but eventually it gets sluggish again. Restarting every time this happens is a real pain. Quitting the open apps seems to have no effect. The final image below shoes the memory pressure when things return to normal.

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    Are you using an SSD in that iMac? As crazy as it sounds an old(er) spinning drive that's on it's last legs will exhibit symptoms like this as your computer "waits" to read/write to the drive. If you're not using an SSD, spring for for one - they're under $50USD now and do a fresh install. – Allan Jul 2 at 23:45
  • As for "new" iMac announcements at WWDC, I'm with you on that one. I've got my eye on an 27" iMac Pro and I know that exactly 31 days from the moment the charge goes through on my credit card, new models will be announced and the price will drop to make way for new ones. – Allan Jul 2 at 23:47
  • lol. I hear you. I just hate putting money into my old computer. Hopefully, in a month or two, I'll have a new computer. And yes my HDD is dying. The SMART status is Failed but it keeps grinding away for now. I've backed it up for the inevitable. Thanks for the answer. if you want to write it up i'll accept it. – jmh Jul 2 at 23:56
  • My iMac (27 inch, 2011, 3.4GHz i7, 16GB ram, 2TB harddisk) is still performing well, still runs handbrake for editing videos and kids use it so often for homework tasks `(word, excel, research etc). Will consider an SSD soon but rarely gets restarted and still flies. – Solar Mike Jul 3 at 6:55
  • I’m in the same boat with my 2017 iMac. Should have got an SSD when I bought it (and / or more memory). Hard to justify spending on it when I’ve now finished university! I’m hoping for the new iMac to be ARM based and I can go for that as well. – Tim Jul 3 at 7:48
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A dying a hard drive will actually cause this symptom as the system "waits" for read/write operations to complete. It'll eventually get there, but often times, it will crash.

From the comments, you've confirmed this with the SMART status saying "Failed" which means that drive literally has one foot in the grave and slowly putting the other one in now.

I just hate putting money into my old computer.

I can't agree more. However, this one might be worth an SSD upgrade. You can get a 120GB for under $20USD and a 250GB for under $40. Why go through the trouble?

Here's a little teaser....

This is a little side project I've been hacking away on... It's a white 2007 MacBook.

2007 White MacBook FreeBSD

(Forgive the bezel, I'm dealing with an unscrupulous vendor in NJ who sent me a dead LCD and refused to exchange it due to COVID-19 and I'm waiting on a resolution from AMEX). I'm into this for about $40 (a little horse trading on ebay helps) and everything works - Ethernet, Bluetooth, external display, and even WiFi! It's utterly useless, from a macOS perspective, but not for FreeBSD with the MATE DE. Even if all else fails, I still have a good SSD that I got for $20 that can go into another machine...

Your iMac is a tank, and I'd hate to see it meet an untimely demise. ;-)

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  • Maybe I will look into a new SSD. At least my machine could be used a while longer. And then if iMacs don't come out till December I'm covered. – jmh Jul 3 at 0:41
  • Second the SSD suggestion. Did that on a 2011 MBP and it flew afterwards. Huge difference. – JL Peyret Jul 3 at 6:01
  • How did you get FreeBSD to boot? I burned the i386 image on a DVD but my MacBook from 2006 doesn‘t find a kernel when booting from it?! – nohillside Jul 3 at 22:19
  • I know the 11-1 RELEASE boots with no issue via DVD. For another install, I manually parititioned the drive, copied over all the kernel files, put the drive back in the Mac, booted macOS from USB then blessed the boot partition. I have to get my notes on that one, but I'm planning on doing a write up (and maybe a video) shortly, I really want to see these Macs continue to be useful. – Allan Jul 3 at 22:32
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Relatively high memory usage for helpd in your screenshot is a hint that this might be the

helpd indexing problem.

It is supposed to index help files when a new contents appears (that is, rarely, like when you install a new application), which is detected by monitoring the /Applications folder. Something goes wrong and it confuses launching the application with changing the folder contents, and triggers the indexing process without the real reason. (the implementation is broken anyway, because obviously taking most of the RAM for a background process and effectively killing the performance can't be intentional. But you can consider yourself lucky with 16 GB of RAM - my mac mini was routinely being halted for an hour or two whenever the mac os decided it is time to take 12GB(!) of its 8GB for the pointless indexing task.)

How did I solve the problem? Unfortunately, I can no longer find the website that helped me save my machine last year, but if I recall correctly, I have deleted the contents of helpd cache, like this:

rm ~/Library/Caches/com.apple.helpd/*

...then restart. If I am mistaken, perhaps it might have been some other cache, but at least you have a new hypothesis to investigate. Deleting the cache files should be safe no matter what, because they are, well, just caches so they are recreated when needed.

Similar questions, not necessarily helpd related:

helpd uses 100% CPU after installing any application with documentation

MacOS Mojave: Always indexing, the whole OS is too slow

https://discussions.apple.com/thread/3209466

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Keep in mind that HDs slow down as they fill, so Keep it down around 50-60% full for best performance. Never go above about 80% full, or things tend to really crawl. You need to clean your drive or expand the storage space.

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