RT_DPTBL(5) File Formats and Configurations RT_DPTBL(5)

NAME

rt_dptbl - real-time dispatcher parameter table

DESCRIPTION

The process scheduler (or dispatcher) is the portion of the kernel
that controls allocation of the CPU to processes. The scheduler
supports the notion of scheduling classes where each class defines a
scheduling policy, used to schedule processes within that class.
Associated with each scheduling class is a set of priority queues on
which ready to run processes are linked. These priority queues are
mapped by the system configuration into a set of global scheduling
priorities which are available to processes within the class. The
dispatcher always selects for execution the process with the highest
global scheduling priority in the system. The priority queues
associated with a given class are viewed by that class as a
contiguous set of priority levels numbered from 0 (lowest priority)
to n (highest priority--a configuration dependent value). The set of
global scheduling priorities that the queues for a given class are
mapped into might not start at zero and might not be contiguous,
depending on the configuration.


The real-time class maintains an in-core table, with an entry for
each priority level, giving the properties of that level. This table
is called the real-time dispatcher parameter table (rt_dptbl). The
rt_dptbl consists of an array (config_rt_dptbl[]) of parameter
structures (struct rtdpent_t), one for each of the n priority levels.
The structure are accessed via a pointer, (rt_dptbl), to the array.
The properties of a given priority level i are specified by the ith
parameter structure in this array ( rt_dptbl[i] ).


A parameter structure consists of the following members. These are
also described in the /usr/include/sys/rt.h header file.

rt_globpri
The global scheduling priority associated with this
priority level. The rt_globpri values cannot be changed
with dispadmin(8).


rt_quantum
The length of the time quantum allocated to processes
at this level in ticks (hz). The time quantum value is
only a default or starting value for processes at a
particular level as the time quantum of a real-time
process can be changed by the user with the priocntl
command or the priocntl system call.

In the default high resolution clock mode (hires_tick
set to 1), the value of hz is set to 1000. If this
value is overridden to 0 then hz will instead be 100;
the number of ticks per quantum must then be decreased
to maintain the same length of quantum in absolute
time.


An administrator can affect the behavior of the real-time portion of
the scheduler by reconfiguring the rt_dptbl. There are two methods
available for doing this: reconfigure with a loadable module at boot-
time or by using dispadmin(8) at run-time.

rt_dptbl Loadable Module
The rt_dptbl can be reconfigured with a loadable module which
contains a new real time dispatch table. The module containing the
dispatch table is separate from the RT loadable module which contains
the rest of the real time software. This is the only method that can
be used to change the number of real time priority levels or the set
of global scheduling priorities used by the real time class. The
relevant procedure and source code is described in the Examples
section.

dispadmin Configuration File
The rt_quantum values in the rt_dptbl can be examined and modified on
a running system using the dispadmin(8) command. Invoking dispadmin
for the real-time class allows the administrator to retrieve the
current rt_dptbl configuration from the kernel's in-core table, or
overwrite the in-core table with values from a configuration file.
The configuration file used for input to dispadmin must conform to
the specific format described below.


Blank lines are ignored and any part of a line to the right of a #
symbol is treated as a comment. The first non-blank, non-comment line
must indicate the resolution to be used for interpreting the time
quantum values. The resolution is specified as

RES=res


where res is a positive integer between 1 and 1,000,000,000 inclusive
and the resolution used is the reciprocal of res in seconds. (For
example, RES=1000 specifies millisecond resolution.) Although very
fine (nanosecond) resolution may be specified, the time quantum
lengths are rounded up to the next integral multiple of the system
clock's resolution.


The remaining lines in the file are used to specify the rt_quantum
values for each of the real-time priority levels. The first line
specifies the quantum for real-time level 0, the second line
specifies the quantum for real-time level 1. There must be exactly
one line for each configured real-time priority level. Each
rt_quantum entry must be either a positive integer specifying the
desired time quantum (in the resolution given by res), or the value
-2 indicating an infinite time quantum for that level.

EXAMPLES

Example 1: A Sample dispadmin Configuration File

The following excerpt from a dispadmin configuration file illustrates
the format. Note that for each line specifying a time quantum there
is a comment indicating the corresponding priority level. These level
numbers indicate priority within the real-time class, and the mapping
between these real-time priorities and the corresponding global
scheduling priorities is determined by the configuration specified in
the RT_DPTBL loadable module. The level numbers are strictly for the
convenience of the administrator reading the file and, as with any
comment, they are ignored by dispadmin on input. dispadmin assumes
that the lines in the file are ordered by consecutive, increasing
priority level (from 0 to the maximum configured real-time priority).
The level numbers in the comments should normally agree with this
ordering; if for some reason they don't, however, dispadmin is
unaffected.


# Real-Time Dispatcher Configuration File
RES=1000

# TIME QUANTUM PRIORITY
# (rt_quantum)LEVEL
100# 0
100# 1
100# 2
100# 3
100# 4
100# 5
90 # 6
90 # 7
.. .
.. .
.. .
10# 58
10# 59

Example 2: Replacing The rt_dptbl Loadable Module

In order to change the size of the real time dispatch table, the
loadable module which contains the dispatch table information will
have to be built. It is recommended that you save the existing module
before using the following procedure.


1. Place the dispatch table code shown below in a file called
rt_dptbl.c An example of an rt_dptbl.c file follows.

2. Compile the code using the given compilation and link
lines supplied.

cc -c -0 -D_KERNEL rt_dptbl.c
ld -r -o RT_DPTBL rt_dptbl.o


3. Copy the current dispatch table in /usr/kernel/sched to
RT_DPTBL.bak.

4. Replace the current RT_DPTBL in /usr/kernel/sched.

5. You will have to make changes in the /etc/system file to
reflect the changes to the sizes of the tables. See
system(5). The rt_maxpri variable may need changing. The
syntax for setting this is:

set RT:rt_maxpri=(class-specific value for maximum \
real-time priority)


6. Reboot the system to use the new dispatch table.


Great care should be used in replacing the dispatch table using this
method. If you don't get it right, the system may not behave
properly.


The following is an example of a rt_dptbl.c file used for building
the new rt_dptbl.


/* BEGIN rt_dptbl.c */
#include <sys/proc.h>
#include <sys/priocntl.h>
#include <sys/class.h>
#include <sys/disp.h>
#include <sys/rt.h>
#include <sys/rtpriocntl.h>
/*
* This is the loadable module wrapper.
*/
#include <sys/modctl.h>
extern struct mod_ops mod_miscops;
/*
* Module linkage information for the kernel.
*/
static struct modlmisc modlmisc = {
&mod_miscops, "realtime dispatch table"
};
static struct modlinkage modlinkage = {
MODREV_1, &modlmisc, 0
};
_init()
{
return (mod_install(&modlinkage));
}
_info (struct modinfo *modinfop)
{
return (mod_info(&modlinkage, modinfop));
}
rtdpent_t config_rt_dptbl[] = {

/* prilevel Time quantum */

100,100,
101,100,
102,100,
103,100,
104,100,
105,100,
106,100,
107,100,
108,100,
109,100,
110,80,
111,80,
112,80,
113,80,
114,80,
115,80,
116,80,
117,80,
118,80,
119,80,
120,60,
121,60,
122,60,
123,60,
124,60,
125,60,
126,60,
127,60,
128,60,
129,60,
130,40,
131,40,
132,40,
133,40,
134,40,
135,40,
136,40,
137,40,
138,40,
139,40,
140,20,
141,20,
142,20,
143,20,
144,20,
145,20,
146,20,
147,20,
148,20,
149,20,
150,10,
151,10,
152,10,
153,10,
154,10,
155,10,
156,10,
157,10,
158,10,
159,10,

};
/*
* Return the address of config_rt_dptbl
*/ rtdpent_t *
rt_getdptbl()
{
return (config_rt_dptbl);
}

SEE ALSO

priocntl(1), priocntl(2), system(5), dispadmin(8)


System Administration Guide: Basic Administration


Programming Interfaces Guide

October 15, 2002 RT_DPTBL(5)