One of the more significant changes in the 7.0 kernel release is to use the
lazy-preemption mode by default in the CPU
scheduler.  The scheduler developers have wanted to reduce the number of
preemption modes for years, and lazy preemption looks like a step toward
that goal.  But then there came this report
from Salvatore Dipietro that lazy preemption caused a 50% performance
regression on a PostgreSQL benchmark.  Investigation showed that the
situation is not actually so grave, but the episode highlights just how
sensitive some workloads can be to configuration changes; there may be
surprises in store for other users as well.

https://lwn.net/Articles/1067029/
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