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.

The rt_quantum values in the rt_dptbl can be examined and modified on a running system using the dispadmin(1M) 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.

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

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(4). 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);
}