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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.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Tags: bot-comment
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Job with 4 threads not assigned 4 distinct cores
https://bugs.launchpad.net/bugs/1315619
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