FULL_PREEMPTION

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Hi,

I have seen a few posts from Andriy as as well as the PC-BSD default
that for desktop systems, kern.sched.preempt_thresh=224 would improve
responsiveness.  Looking at the code, it seems that this is equivalent
to compiling the kernel with FULL_PREEMPTION.

The sys/conf/NOTES says, however:

# FULL_PREEMPTION instructs the kernel to preempt non-realtime kernel
#         threads.  Its sole use is to expose race conditions and other
#         bugs during development.  Enabling this option will reduce
#         performance and increase the frequency of kernel panics by
#         design.  If you aren't sure that you need it then you don't.
#         Relies on the PREEMPTION option.  DON'T TURN THIS ON.

Despite the possibility of exposing race conditions as well as
potentially hurting throughput because of (possibly more) context
switching, is it considered as a goal that we should support it?  If
so, should we enable it on -CURRENT?

Cheers,
- -- 
Xin LI <delphij_at_delphij.net>    https://www.delphij.net/
FreeBSD - The Power to Serve!           Live free or die
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