fixed priority dispatcher parameter table
The process scheduler (or dispatcher) is the portion of the kernel that
controls allocation of the
The scheduler supports the notion of scheduling classes where each
class defines a scheduling policy, used to schedule processes within
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 that are available to processes within the
(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 fixed priority class maintains an in-core table, with an entry for each
priority level, giving the properties of that level.
This table is called the fixed priority dispatcher parameter table
The fp_dptbl consists of an array of parameter structures
(struct fp_dpent), one for each of the n priority levels.
The properties of a given priority level i are specified by
the ith parameter structure in this array (fp_dptbli).
A parameter structure consists of the following members.
These are also described in the /usr/include/sys/fp.h
The global scheduling priority associated with this priority level.
The mapping between fixed priority levels and global
scheduling priorities is determined at boot time by the system
The fp_globpri values cannot be changed
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 command or the
priocntl system call.
An administrator can affect the behavior of the fixed priority
portion of the scheduler by reconfiguring the fp_dptbl.
There are two methods available for doing this.
The fp_quantum values in the fp_dptbl can be examined and
modified on a running system using the
Invoking dispadmin for the fixed priority class allows the
administrator to retrieve the current fp_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
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
The system clock's resolution is hardware-dependent;
this resolution can be calculated from the value of HZ,
which is defined in the file /usr/include/sys/param.h.
HZ gives the number of clock ticks per second of the system clock.
For example, an HZ of 100 specifies 100 clock ticks per second,
or one tick every 10 milliseconds (that is, this system clock has a resolution
of 10 milliseconds).
If the -t and -r options
are used to specify a time quantum of 34 milliseconds, it is rounded up
to 4 ticks (40 milliseconds) on a machine with an HZ of 100.
The remaining lines in the file are used to specify the fp_quantum
values for each of the fixed 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 on.
There must be exactly one line for each configured fixed priority
Each fp_quantum entry must be either a positive integer
specifying the desired time quantum (in the resolution given by res),
or the symbol RT_TQINF indicating an infinite time quantum for that
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 fixed priority class, and the mapping
between these fixed priorities and the corresponding global
scheduling priorities is determined by the configuration specified in
the fp master file.
The level numbers are strictly for the convenience of the administrator reading
the file and, as with any comment, they are ignored by dispadmin
dispadmin assumes that the lines in the file are ordered
by consecutive, increasing priority level (from 0 to the maximum configured
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
# TIME QUANTUM PRIORITY
# (fp_quantum) LEVEL
100 # 0
100 # 1
100 # 2
100 # 3
100 # 4
100 # 5
90 # 6
90 # 7
. . .
. . .
. . .
10 # 58
10 # 59
© 2005 The SCO Group, Inc. All rights reserved.
SCO OpenServer Release 6.0.0 - 02 June 2005