Date:	Fri, 5 Jun 2009 16:12:53 +0200
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Linda Walsh <lkml@...nx.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: ionice priority "none: prio 0" v. "none: prio 1" v. best-effort 
	v. idle?

Hi Linda,
the ioprio class 'none' is the default class in which all processes
are put when created, if not specified otherwise (this is in contrast
with what I read in the man page, where it says Best Effort is the
default).
For CFQ (other io schedulers just ignore it), the 'none' class has a
special meaning, in fact, looking at:
http://git.kernel.org/?p=linux/kernel/git/aegl/linux-2.6.git;a=blob;f=block/cfq-iosched.c;h=a55a9bd75bd1baf616a3a1b7118acaeee328759f;hb=HEAD#l1579
you will see that for processes with class 'none', the class and
priority will be inherited from CPU scheduling (including RT
scheduling & nice levels).

HTH,
Corrado

On Fri, Jun 5, 2009 at 5:22 AM, Linda Walsh<lkml@...nx.org> wrote:
> <-- vim: se sts=4 sw=4 ts=8 nosi sc ai: /--> I was looking at the output of
> ionice on the various processes
> running.
>
> Other than one I set for 'best-effort' (-c2), the rest all had
> priorities of
>   1) "none: prio 0"
> OR
>   2) "none: prio 4"
>
> Out of 183 process:
>
>  79 had "none: prio 0"   and
> 103 had "none: prio 4"
> (1 had "best effort, prio 4").
>
> Where does priority class  'none' fit in?  above or below 'idle'?
>
> Or is 'none' equal to 'best effort' (which is logical in once sense, but
> strictly, I could argue 'none' is at least below 'best effort', and possibly
> below idle, as 'idle' at least has been assigned a scheduling priority
> (vs. processes that have not -- BUT, but definition, idle is clearly meant
> to be lower precedence, so that would argue, logically, that 'none' is
> above idle and below 'best_effort' (since processes with no assigned
> priority would logically be below those assigned as 'best effort'.
>
> Within 'besteffort', 0=high, and 7=low.
>
> Is the same true in 'none', or are the values 'meaningless'?
> (In which case, why do they all exist at either 4 or 0?)
>
> Since the iopriority DOES NOT correlate with either
> the cpu priority nor 'nice' value, then how are different
> processes assigned different priority values?
> They *seem* to be mostly fixed, but very rarely I'll see maybe
> 1 process toggle from 0 to 4 or back (but its usually fixed).
>
> So why is everything in the 'none' class at either "highest level" (prio
> 0), OR the mid level of prio=4?  Um...found exception on a 32-bit i386
> based kernel, all prio's are "none: prio 0". But two different x86_64 bit
> kernels have "a split", majority in none:prio 4, and a minority of
> 25-44% in 'none:prio 4'.
>
> I'm using the cfq scheduler on all 3 machines.
>
> So what is it with 'schedclass'=none?  Is it lower than 'best effort'?  (I'd
> hope so, or like to see it that way, but wants are nice...:-))...
>
> If they *ARE* the same, why are 44% running at highest priority
> (regardless of cpu prio, 'nice' value, user-id, and the rest at 'mid')?
>
> Why so random, but worse, why put any at 'highest' (unless they
> ask for it).  Wouldn't 'mid' priority be consistent with 'mid-cpu-nice'
> value of 0 (out of +/-19)?
>
> FWIW -- I thought once the priorities varied dynamically based on
> cpu-nice levels (for cfq, anyway)...it would be VERY nice to see that
> reflected in the readable ionice data for those processes.
>
> Thanks for clarification/enlightenment...
> Linda
>
>
>
> --
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-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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