RT_DPTBL(4)RT_DPTBL(4)NAMErt_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 pol‐
icy, used to schedule processes within that class. Associated with each
scheduling class is a set of priority queues on which ready to run pro‐
cesses are linked. These priority queues are mapped by the system con‐
figuration into a set of global scheduling priorities which are avail‐
able 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 depen‐
dent 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 struc‐
tures (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 parame‐
ter 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 pri‐
ority level. The rt_globpri values cannot be changed with
dispadmin(1M).
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 pri‐
ocntl system call.
In the high resolution clock mode (hires_tick set to 1),
the value of hz is set to 1000. Increase quantums to
maintain the same absolute time quantums.
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(1M) 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 ta‐
ble 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 proce‐
dure 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(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 configura‐
tion file used for input to dispadmin must conform to the specific for‐
mat described below.
Blank lines are ignored and any part of a line to the right of a # sym‐
bol 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 exam‐
ple, 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 resolu‐
tion.
The remaining lines in the file are used to specify the rt_quantum val‐
ues for each of the real-time priority levels. The first line specifies
the quantum for real-time level 0, the second line specifies the quan‐
tum for real-time level 1. There must be exactly one line for each con‐
figured real-time priority level. Each rt_quantum entry must be either
a positive integer specifying the desired time quantum (in the resolu‐
tion 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 num‐
bers indicate priority within the real-time class, and the mapping
between these real-time priorities and the corresponding global sched‐
uling priorities is determined by the configuration specified in the
RT_DPTBL loadable module. The level numbers are strictly for the conve‐
nience 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 num‐
bers 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 load‐
able 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 sys‐
tem(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);
}
SEE ALSOpriocntl(1), dispadmin(1M), priocntl(2), system(4)
System Administration Guide: Basic Administration
Programming Interfaces Guide
Oct 15, 2002 RT_DPTBL(4)
