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["Guilherme G. Piccoli" <1724614@xxxxxxxxxxxxxxxxxx>]

** Changed in: linux (Ubuntu Xenial)
       Status: New => Fix Released

** No longer affects: linux (Ubuntu Groovy)

** Changed in: linux (Ubuntu Xenial)
   Importance: Undecided => Medium

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https://bugs.launchpad.net/bugs/1724614

Title:
  [KVM] Lower the default for halt_poll_ns to 200000 ns

Status in linux source package in Xenial:
  Fix Released
Status in linux source package in Zesty:
  Won't Fix

Bug description:
  [Environment]

  Distributor ID:	Ubuntu
  Description:	Ubuntu 16.04.3 LTS
  Release:	16.04
  Codename:	xenial

  Linux porygon 4.4.0-112-generic #135-Ubuntu SMP Fri Jan 19 11:48:36
  UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

  [Description]

  We've identified a constant high (~90%) system time load at the host level
  when a VCPU in a KVM guest remains or switches/resumes in/from halt/idle state
  in a constant frequency, usually for a slightly smaller time than the default polling
  period.

  The halt polling mechanism has the intention to reduce latency in the cases
  on which the guest is quickly resumed saving a call to the scheduler.

  We've performed some testing by adjusting the /sys/module/kvm/parameters/halt_poll_ns
  value which defines the max time that should be spend polling before calling the
  scheduler to allow it to run other tasks (which defaults to 400000 ns in Ubuntu).

  With the default value the tests shows that the load remains nearly on 90% on a
  VCPU that has a single task in the run queue.

  We've also tested altering the halt_poll_ns value to 200000 ns and the results
  seems to drop the system time usage from 90% to ~25%.

  root@porygon:/home/ubuntu# echo 200000 > /sys/module/kvm/parameters/halt_poll_ns
  root@porygon:/home/ubuntu# mpstat 1 -P 6 5
  Linux 4.4.0-112-generic (porygon) 	01/24/2018 	_x86_64_	(64 CPU)

  02:06:08 PM  CPU  %usr  %nice  %sys %iowait  %irq   %soft  %steal  %guest  %gnice   %idle
  02:06:09 PM    6  0.00  0.00   4.85  0.00    0.00    0.00    0.00   16.50    0.00   78.64
  [...]
  Average:       6  0.00  0.00   4.26  0.00    0.00    0.00    0.00   17.83    0.00   77.91

  
  root@porygon:/home/ubuntu# echo 400000 > /sys/module/kvm/parameters/halt_poll_ns
  root@porygon:/home/ubuntu# mpstat 1 -P 6 5
  Linux 4.4.0-112-generic (porygon) 	01/24/2018 	_x86_64_	(64 CPU)

  02:06:20 PM  CPU  %usr  %nice  %sys %iowait   %irq   %soft  %steal  %guest  %gnice   %idle
  02:06:21 PM    6  0.00  0.00   87.13  0.00    0.00    0.00    0.00   11.88    0.00    0.99
  [...]
  Average:       6  0.00  0.00   89.59  0.00    0.00    0.00    0.00    8.45    0.00    1.96

  [Reproducer]

  1) Configure a KVM guest with a single pinned VCPU.
  2) Run the following program (http://pastebin.ubuntu.com/25731919/) at the KVM guest.
  $ gcc test.c -lpthread -o test && ./test 250 0
  3) Run mpstat at the host on the pinned CPU and compare the stats
  $ sudo mpstat 1 -P 6 5

  [Fix]

  Change the halt polling max time to half of the current value.

  In some fio benchmarks, halt_poll_ns=400000 caused CPU utilization to
  increase heavily even in cases where the performance improvement was
  small.  In particular, bandwidth divided by CPU usage was as much as
  60% lower.

  To some extent this is the expected effect of the patch, and the
  additional CPU utilization is only visible when running the
  benchmarks.  However, halving the threshold also halves the extra
  CPU utilization (from +30-130% to +20-70%) and has no negative
  effect on performance.

  Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

  *
  https://github.com/torvalds/linux/commit/b401ee0b85a53e89739ff68a5b1a0667d664afc9

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