After moving from High-Sierra to Mojave I noticed my Mac (23", fusion-drive 3.4GHz) has become much slower. In particular, I noticed that just after the startup I have a significant amount of disk activity.

I investigated this disk activity quite a bit and have come to understand that it's mainly related to the process kernel_task. I read in many posts about kernel_task taking the CPU to prevent it from becoming too hot, but I don't think this is my case:

  • I have disk activity, not CPU activity
  • This disk activity is related to my user (from 500MB to 2GB disk activity at each startup) while other users on the same Mac don't have (about 30MB at each startup).

I tried with multiple tools to understand what may be the source behind this kernel_task writing:

  • Activity Monitor: I clearly see the kernel_task as the main contributor to disk activity
  • log show: Not able to see any indication on a specific process
  • sudo fs_usage -f filesys: Sometimes I see kernel_task in the list, but without specific indication, just indicating /dev/disk2
  • ps axv: to show startup processes
  • sudo syscallbypid.d: to list the system calls, with their process
  • sudo lsof: to list open files
  • iosnoop and iota cannot be applied because of System Integrity Protection

So basically despite all my efforts shown above, I'm not able to say why in my user, at the startup, kernel_task writes and reads at least (but even more) few GB.

Are there any other hints to help me understanding what is kernel_task reading or writing after the startup?

  • Sounds exactly like your issue - discussions.apple.com/thread/8445009. – slm Apr 19 at 19:42
  • Some other things to try/investigate - makeuseof.com/tag/fix-mac-kernel-task-high-cpu-usage. – slm Apr 19 at 19:53
  • @slm That doesn't sound like his issue at all. – jksoegaard Apr 19 at 19:57
  • @mp1 This sounds like an XY-problem. Do you want to fix the slowness of your computer, or do you want to know why kernel_task reads/writes files at startup? I would advise you to ask the first question instead of jumping to a conclusion that this is related to kernel_task reading stuff at startup. If you rather ask the latter question, please indicate how you have established that the two are related. – jksoegaard Apr 19 at 20:00
  • @mp1 In general I would note that kernel_task doesn't read/write stuff per se, it only acts on behalf of other programs. Have you checked that your system isn't swapping memory out to disk? – jksoegaard Apr 19 at 20:01

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