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)