fp_dptbl(4)

fp_dptbl -- fixed priority 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 that 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 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 (fp_dptbl). 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 header file.

fp_globpri: 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 configuration. The fp_globpri values cannot be changed with dispadmin(1M).
fp_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 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.

Dispadmin configuration

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

   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 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 level. 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 level.

Example

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 on input. dispadmin assumes that the lines in the file are ordered by consecutive, increasing priority level (from 0 to the maximum configured fixed 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 # (fp_quantum) LEVEL 100 # 0 100 # 1 100 # 2 100 # 3 100 # 4 100 # 5 90 # 6 90 # 7 . . . . . . . . . 10 # 58 10 # 59

Files

/usr/include/sys/fp.h

References

dispadmin(1M), priocntl(1), priocntl(2)