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From: Fred Pedrisa <fredhps10@hotmail.com>
To: "'Ian Lepore'" <ian@FreeBSD.org>
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Subject: RES: Thread Scheduler Priority
Date: Fri, 30 May 2014 23:59:14 -0300
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Hello,

This is 'min' and 'max' for the default policy :

Min : 0, Max : 103

In my system.

-----Mensagem original-----
De: owner-freebsd-current@freebsd.org
[mailto:owner-freebsd-current@freebsd.org] Em nome de Ian Lepore
Enviada em: sexta-feira, 30 de maio de 2014 23:50
Para: Fred Pedrisa
Cc: 'freebsd-current'
Assunto: Re: Thread Scheduler Priority

On Thu, 2014-05-29 at 02:12 -0300, Fred Pedrisa wrote:
> Hi, Guys.
> 
>  
> 
> How can I adjust a certain thread to have the maximum system priority 
> in the scheduler ?
> 
>  
> 
> I've tried doing it this way :
> 
>  
> 
>                                                /* Set thread priority. 
> */
> 
>                                                if 
> (pthread_getschedparam(ts[gnThreadID], &police, &param[gnThreadID]) != 
> 0)
> 
>                                                {
> 
>                                                                error 
> ("Unable to get priority");
> 
>                                                                return 
> 1;
> 
>                                                }
> 
>  
> param[gnThreadID].sched_priority = 99;
> 
>                                                if 
> (pthread_setschedparam(ts[gnThreadID], police, &param[gnThreadID]) != 
> 0)
> 
>                                                {
> 
>                 error("Unable to set priority");
> 
>                 return 1;
> 
>                                                }
> 
>  
> 
> However, in 'top', I don't see the process threads switching to -92 
> priority, like other threads in the system, is something I did wrong 
> or maybe I might be missing something ?

You can't just set the priority to any number you want... per the man page
for pthread_setschedparam() the value has to fall within the ranges returned
by sched_get_priority_min() and sched_get_priority_max() for the given
scheduling class.  On freebsd those ranges are 0-31.

I suspect from your statement of wanting "maximum system priority" maybe
what you need to do is change the scheduling class from SCHED_OTHER to
SCHED_RR, that should give you realtime priority.  Be aware that a realtime
thread that is compute-bound will take over the system (or one core on an
SMP system); it will get all cycles if it is always runnable.

If what you're looking for is the thread equivelent of using the nice
command, so that you give a boost to a thread over other threads in the
timeshare (SCHED_OTHER) scheduling class, there is currently no way to do
that in freebsd.

Last year for $work I about went crazy trying to figure out the mapping
between pthread scheduling classes and priorities and freebsd's idea of
thread prorities.  I eventually gave up on the pthread API and used the
freebsd native function rtprio_thread() instead.

-- Ian


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