On Sat, Feb 28, 2004 at 01:06:35PM +0800, Zhang Weiwu wrote:
> Erik Trulsson wrote:
> >On Sat, Feb 28, 2004 at 11:37:51AM +0800, Zhang Weiwu wrote:
> > 
> >
> >>Hello. I thought scheduling priority is the kind of absolute priority, 
> >>that is only when the higher priority process don't ask for resource, can 
> >>the lower priority process gets resource. If the higher priority process 
> >>sucks, the lower priority process starvs.
> >>
> >>Now I have a old Pentium-mmx 166 box, running mpg321 fine. I wish to 
> >>listen to music when recompile the kernel, so turn on the music, do:
> >>#nice make;
> >>I thought "nice make" use the resouce mpg321 left to it, but actually the 
> >>music process is seriously disturbed, it begins to sound like .... 
> >>terrible.
> >>
> >>So I don't realy understand the scheduling priority mechnism in FreeBSD?
> >>   
> >>
> >
> >No, you dont quite understand the scheduling.
> >There are actually two different priorities that influence scheduling.
> >If you look at the output from top(1) you can see them in the columns
> >labeled 'PRI' and 'NICE'.  The first one ('PRI') is the one that is
> >actually used to determine when a process gets to run. This is
> >dynamically adjusted by the system to make sure that all processes get
> >at least *some* CPU-time (so no process can be completely starved.) The
> >second ('NICE') is what is changed by the nice/renice commands and
> >affects how the actual priority is changed by the system. (Processes
> >with a high nice-value essentially gets their priority raised slowly
> >and lowered quickly .)
> > 
> >
> I don't understand. I watched top(1) for some time, it seems the mpg321 
> process, having NICE=-20, keep constant PRI of 108~110, but other 
> processes like 'yacc', 'cc', although running at NICE=4, comes out even 
> at PRI=120, and almost keep that high until it finish its work in 
> several seconds and quit, meanwhile the speaker keep producing broken 
> music. If NICE=-20 processes cannot have more PRI than NICE=4, what's 
> the use of NICE?

Try comparing two CPU-hungry processes that both run for a long time
and you will see the difference.
Processes which do a lot of I/O and don't use much CPU tends to get
fairly high priority. Processes also tend to start with relatively high
priority so a short-lived process will not have had time to get its
priority adjusted much regardless of its nice-level.

> >I run at a very similar box, but use mpg123 rather than mpg321, and I
> >can play music without significant disturbance.
> >You might wish to try mpg123 instead, in my experience it needs less
> >CPU-power than mpg321.
> > 
> >
> My mpg123 hangs after copyright notice. It's another story though.
> >You could also try using a higher nice value for the make process. (i.e
> >run it with 'nice -19 make' or something like that.)
> > 
> >
> I tried 'nice -n 19 make' which gives 'badly formed number' error. 'nice 
> -19 make' made 'make' even greedy.

Let me guess: You are using [t]csh as your shell, right?
That has 'nice' as a built-in command with a slightly different syntax
than /usr/bin/nice  (which is what is documented in the nice(1)

<Insert your favourite quote here.>
Erik Trulsson
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