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[Tim Gardner <tim.gardner@xxxxxxxxxxxxx>]

** Also affects: linux (Ubuntu Saucy)
   Importance: Medium
     Assignee: Colin King (colin-king)
       Status: In Progress

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https://bugs.launchpad.net/bugs/1233681

Title:
  context switching on 3.11 kernel costing CPU and power

Status in “linux” package in Ubuntu:
  In Progress
Status in “linux” source package in Saucy:
  In Progress

Bug description:
  Heavy context switching is consuming a lot of extra power with the
  current saucy kernel.    Context switching intensive tasks are
  consuming much longer to complete than on 3.8 raring kernels, for
  example:

  dd if=/dev/zero | cat | cat | dd of=/dev/null

  See http://reports.qa.ubuntu.com/power/hardware/arch/amd64/

  The reason is that CONFIG_CONTEXT_TRACKING_FORCE is turned on because
  CONFIG_NO_HZ_FULL is enabled and the former config option adds a lot
  of overhead to context switching.

  See attached LibreOffice spreadsheet with tests with the original
  config and the new fixed config.  Endurance tests take less time to
  run to completion and the overall coulombs used is less with
  CONTEXT_TRACKING_FORCE and NO_HZ_FULL disabled, so this is an overall
  win.   Some tests, such as the dd copy take way less time to complete,
  showing massive overhead of CONTEXT_TRACKING_FORCE.

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