BTW, when the system is in this weird state, CPU fans are running at the
lowest speed (but running) and never go to their max speed, even under
heavy CPU load like compiling stuff.

Is there a hidden energy saving thing that tries to keep my CPU clock
low / voltages low at high load?

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https://bugs.launchpad.net/bugs/1797802

Title:
  Erratic behavior of intel pstate CPU frequency control

Status in linux package in Ubuntu:
  Incomplete

Bug description:
  I noticed that sometimes my Ubuntu 18.10 system feels sluggish. The
  other times everything works fine. I don't know what triggers this
  weird state. Suspend / resume? Maybe.

  At first I thought this was maybe just a "perception issue", but then
  when the sluggishness happened again, I fired up i7z and it looks that
  CPU frequency almost never got over ~1600 MHz, But my CPU is perfectly
  capable of going up to 4000 MHz!

  I tried to fix it by setting 
  echo 80 > /sys/devices/system/cpu/intel_pstate/min_perf_pct
  echo 80 > /sys/devices/system/cpu/intel_pstate/max_perf_pct

  I hoped I would force the CPU into almost max performance state this
  way. And indeed, once I ran these commands, i7z showed steady 3100+
  MHz on all cores.

  Then something weird happened - when I run some mild load on the CPU,
  like just starting the IDE, during the startup the frequency **
  dropped back to about 800-1600 MHz ** and returned to 3100 MHz after
  the load was gone (IDE loaded).

  The CPU core temperatures shown by sensors / i7z are ok and typically
  at about 40-50 C when this slowdown happens, so this doesn't look like
  thermal throttling.

  Any ideas?

  ProblemType: Bug
  DistroRelease: Ubuntu 18.10
  Package: linux-image-4.18.0-10-generic 4.18.0-10.11
  ProcVersionSignature: Ubuntu 4.18.0-10.11-generic 4.18.12
  Uname: Linux 4.18.0-10-generic x86_64
  ApportVersion: 2.20.10-0ubuntu13
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC0:  pkolaczk   3281 F.... pulseaudio
   /dev/snd/pcmC0D0c:   pkolaczk   3281 F...m pulseaudio
  CurrentDesktop: ubuntu:GNOME
  Date: Sun Oct 14 22:16:50 2018
  DistributionChannelDescriptor:
   # This is a distribution channel descriptor
   # For more information see 
http://wiki.ubuntu.com/DistributionChannelDescriptor
   canonical-oem-somerville-xenial-amd64-20160624-2
  HibernationDevice: RESUME=UUID=4836842e-0116-43c4-98b4-7a56427f81f1
  InstallationDate: Installed on 2017-04-12 (549 days ago)
  InstallationMedia: Ubuntu 16.04 "Xenial" - Build amd64 LIVE Binary 
20160624-10:47
  MachineType: Dell Inc. Precision 5520
  ProcEnviron:
   TERM=xterm-256color
   PATH=(custom, no user)
   XDG_RUNTIME_DIR=<set>
   LANG=pl_PL.UTF-8
   SHELL=/bin/bash
  ProcFB: 0 inteldrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.18.0-10-generic 
root=UUID=7f76b1f0-8fef-41cc-86a2-99e554cb4d40 ro acpi_rev_override quiet 
splash iwlwifi.power_save=1 crashkernel=384M-:128M crashkernel=384M-:128M 
vt.handoff=1
  RelatedPackageVersions:
   linux-restricted-modules-4.18.0-10-generic N/A
   linux-backports-modules-4.18.0-10-generic  N/A
   linux-firmware                             1.175
  SourcePackage: linux
  UpgradeStatus: No upgrade log present (probably fresh install)
  dmi.bios.date: 07/24/2018
  dmi.bios.vendor: Dell Inc.
  dmi.bios.version: 1.11.0
  dmi.board.name: 06X96V
  dmi.board.vendor: Dell Inc.
  dmi.board.version: A00
  dmi.chassis.type: 10
  dmi.chassis.vendor: Dell Inc.
  dmi.modalias: 
dmi:bvnDellInc.:bvr1.11.0:bd07/24/2018:svnDellInc.:pnPrecision5520:pvr:rvnDellInc.:rn06X96V:rvrA00:cvnDellInc.:ct10:cvr:
  dmi.product.family: Precision
  dmi.product.name: Precision 5520
  dmi.product.sku: 07BF
  dmi.sys.vendor: Dell Inc.

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