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[Colin Watson <cjwatson@xxxxxxxxxxxxx>]

** Package changed: ubuntu => linux (Ubuntu)

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https://bugs.launchpad.net/bugs/1315619

Title:
  Job with 4 threads not assigned 4 distinct cores

Status in “linux” package in Ubuntu:
  New

Bug description:
  On 12.04, I have reliably found that a job which starts 4 long-running
  threads on an otherwise-idle machine (there are some other threads
  which have been running for some time, but they have been kill
  -STOPped) gets the threads allocated to 4 distinct cores on my
  Haswell.

  On 14.04, I found that the threads were allocated to cores 0, 1, 5, 6
  - that is, two were running on the two hyper-threads 1 and 5 of the
  same core, and performance was 30% less than expected.  I can get
  around this with explicit taskset commands, but for my use case it's a
  serious regression from 12.04.

  I am not willing to disable hyperthreading, because it gives a large
  performance boost on other things I do on the same machine.

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