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From: John Baldwin <jhb@freebsd.org>
To: freebsd-hackers@freebsd.org
Cc: Dieter BSD <dieterbsd@gmail.com>
Subject: Re: Realtime process CPU starvation
Date: Tue, 04 Aug 2015 12:55:47 -0700
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On Saturday, June 20, 2015 11:20:38 AM Dieter BSD wrote:
> For those who care, I'm running
> kern.sched.name: ULE
> 
> My (feeble) understanding is that the scheduler mostly looks at cpu
> time, and processes doing i/o actually get a bump *up* in priority,
> since because of the way hardware worked in the late 1970s-early 1980s
> (PDP-11, VAX, ...) the i/o got useful work done while using very little
> cpu time.  As a result, jobs that do a lot of i/o can receive more
> than their fair share of cpu time.  And nice(1) (even rtprio and idprio)
> may not have much effect on jobs that are i/o bound.  Some form of
> ionice is sorely needed.

Realtime priorities in 8.<ish> and later are higher than the I/O bump.
Only ithreads are higher than realtime in modern versions.

-- 
John Baldwin