FX_DPTBL(5) Standards, Environments, and Macros FX_DPTBL(5)


fx_dptbl - fixed priority dispatcher parameter table




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.

Processes in the fixed priority class are scheduled according to the
parameters in a fixed-priority dispatcher parameter table (fx_dptbl). The
fx_dptbl table consists of an array (config_fx_dptbl[]) of parameter
structures (struct fxdpent_t), one for each of the n priority levels used
by fixed priority processes in user mode. The structures are accessed by
way of a pointer, (fx_dptbl), to the array. The properties of a given
priority level i are specified by the ith parameter structure in this
array (fx_dptbl[i]).

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

The global scheduling priority associated with this
priority level. The mapping between fixed-priority priority
levels and global scheduling priorities is determined at
boot time by the system configuration. fx_globpri can not
be changed with dispadmin(8).

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 fixed priority process can
be changed by the user with the priocntl(1) command or the
priocntl(2) 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 fixed
priority portion of the scheduler by reconfiguring the
fx_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.

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

dispadmin Configuration File
The fx_quantum values in the fx_dptbl can be examined and modified on a
running system using the dispadmin(8) command. Invoking dispadmin for the
fixed-priority class allows the administrator to retrieve the current
fx_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 as follows:

o Blank lines are ignored and any part of a line to the right of
a # symbol is treated as a comment.

o 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:


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 you can specify very fine (nanosecond)
resolution, the time quantum lengths are rounded up to the
next integral multiple of the system clock's resolution.

o The remaining lines in the file are used to specify the
fx_quantum values for each of the fixed-priority priority
levels. The first line specifies the quantum for fixed-
priority level 0, the second line specifies the quantum for
fixed-priority level 1, and so forth. There must be exactly
one line for each configured fixed priority priority level.
Each fx_quantum entry must be a positive integer specifying
the desired time quantum in the resolution given by res.

See Examples for an example of an excerpt of a dispadmin configuration

Replacing the fx_dptbl Loadable Module
To change the size of the fixed priority dispatch table, you must build
the loadable module that contains the dispatch table information. Save
the existing module before using the following procedure.

1. Place the dispatch table code shown below in a file called
fx_dptbl.c. See EXAMPLES, below, for an example of this file.

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

cc -c -0 -D_KERNEL fx_dptbl.c
ld -r -o FX_DPTBL fx_dptbl.o

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

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

5. Make changes in the /etc/system file to reflect the changes to
the sizes of the tables. See system(5). The variables affected
is fx_maxupri. The syntax for setting this is as follows:

set FX:fx_maxupri=(value for max fixed-priority user priority)

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

Exercise great care in using the preceding method to replace the dispatch
table. A mistake can result in panics, thus making the system unusable.


Example 1: Configuration File Excerpt

The following excerpt from a dispadmin configuration file illustrates the
correct format. Note that, for each line specifying a set of parameters,
there is a comment indicating the corresponding priority level. These
level numbers indicate priority within the fixed priority class; the
mapping between these fixed-priority priorities and the corresponding
global scheduling priorities is determined by the configuration specified
in the FX_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. The dispadmin command assumes
that the lines in the file are ordered by consecutive, increasing
priority level (from 0 to the maximum configured fixed-priority
priority). For the sake of someone reading the file, the level numbers in
the comments should agree with this ordering. If for some reason they do
not, dispadmin is unaffected.

# Fixed Priority Dispatcher Configuration File RES=1000

# (fx_quantum) LEVEL
200 # 0
200 # 1
200 # 2
200 # 3
200 # 4
200 # 5
200 # 6
200 # 7
. . .
. . .
. . .
20 # 58
20 # 59
20 # 60

Example 2 fx_dptbl.c File Used for Building the New fx_dptbl

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

/* BEGIN fx_dptbl.c */

#include <sys/proc.h>
#include <sys/priocntl.h>
#include <sys/class.h>
#include <sys/disp.h>
#include <sys/fx.h>
#include <sys/fxpriocntl.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, "Fixed priority dispatch table"

static struct modlinkage modlinkage = {
MODREV_1, &modlmisc, 0

return (mod_install(&modlinkage));

struct modinfo *modinfop;
return (mod_info(&modlinkage, modinfop));

#define FXGPUP0 0 /* Global priority for FX user priority 0 */
fxdpent_t config_fx_dptbl[] = {

/* glbpri qntm */

FXGPUP0+0, 20,
FXGPUP0+1, 20,
FXGPUP0+2, 20,
FXGPUP0+3, 20,
FXGPUP0+4, 20,
FXGPUP0+5, 20,
FXGPUP0+6, 20,
FXGPUP0+7, 20,
FXGPUP0+8, 20,
FXGPUP0+9, 20,
FXGPUP0+10, 16,
FXGPUP0+11, 16,
FXGPUP0+12, 16,
FXGPUP0+13, 16,
FXGPUP0+14, 16,
FXGPUP0+15, 16,
FXGPUP0+16, 16,
FXGPUP0+17, 16,
FXGPUP0+18, 16,
FXGPUP0+19, 16,
FXGPUP0+20, 12,
FXGPUP0+21, 12,
FXGPUP0+22, 12,
FXGPUP0+23, 12,
FXGPUP0+24, 12,
FXGPUP0+25, 12,
FXGPUP0+26, 12,
FXGPUP0+27, 12,
FXGPUP0+28, 12,
FXGPUP0+29, 12,
FXGPUP0+30, 8,
FXGPUP0+31, 8,
FXGPUP0+32, 8,
FXGPUP0+33, 8,
FXGPUP0+34, 8,
FXGPUP0+35, 8,
FXGPUP0+36, 8,
FXGPUP0+37, 8,
FXGPUP0+38, 8,
FXGPUP0+39, 8,
FXGPUP0+40, 4,
FXGPUP0+41, 4,
FXGPUP0+42, 4,
FXGPUP0+43, 4,
FXGPUP0+44, 4,
FXGPUP0+45, 4,
FXGPUP0+46, 4,
FXGPUP0+47, 4,
FXGPUP0+48, 4,
FXGPUP0+49, 4,
FXGPUP0+50, 4,
FXGPUP0+51, 4,
FXGPUP0+52, 4,
FXGPUP0+53, 4,
FXGPUP0+54, 4,
FXGPUP0+55, 4,
FXGPUP0+56, 4,
FXGPUP0+57, 4,
FXGPUP0+58, 4,
FXGPUP0+59, 2,
FXGPUP0+60 2,

pri_t config_fx_maxumdpri =
sizeof (config_fx_dptbl) / sizeof (fxdpent_t) - 1;

* Return the address of config_fx_dptbl
fxdpent_t *
return (config_fx_dptbl);

* Return the address of fx_maxumdpri
* the config_fx_dptbl table.
return (config_fx_maxumdpri);


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

System Administration Guide, Volume 1, System Interface Guide


In order to improve performance under heavy system load, both the nfsd
daemon and the lockd daemon utilize the maximum priority in the FX class.
Unusual fx_dptbl configurations may have significant negative impact on
the performance of the nfsd and lockd daemons.

October 15, 2002 FX_DPTBL(5)