ts_dptbl(4)

ts_dptbl -- time-sharing 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).

Processes in the time-sharing class which are running in user mode (or in kernel mode before going to sleep) are scheduled according to the parameters in a time-sharing dispatcher parameter table (ts_dptbl). (Time-sharing processes running in kernel mode after sleeping are run within a special range of priorities reserved for such processes and are not affected by the parameters in the ts_dptbl until they return to user mode.) The ts_dptbl consists of an array of parameter structures (struct ts_dpent), one for each of the n priority levels used by time-sharing processes in user mode. The properties of a given priority level i are specified by the ith parameter structure in this array (ts_dptbli).

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

ts_globpri: The global scheduling priority associated with this priority level. The mapping between time-sharing priority levels and global scheduling priorities is determined at boot time by the system configuration. ts_globpri is the only member of the ts_dptbl which cannot be changed with dispadmin(1M).
ts_quantum: The length of the time quantum allocated to processes at this level in ticks (HZ).
ts_tqexp: Priority level of the new queue on which to place a process running at the current level if it exceeds its time quantum. Normally this field links to a lower priority time-sharing level that has a larger quantum.
ts_slpret: Priority level of the new queue on which to place a process, that was previously in user mode at this level, when it returns to user mode after sleeping. Normally this field links to a higher priority level that has a smaller quantum.
ts_maxwait: A per process counter, ts_dispwait is initialized to zero each time a time-sharing process is placed back on the dispatcher queue after its time quantum has expired or when it is awakened (ts_dispwait is not reset to zero when a process is preempted by a higher priority process). This counter is incremented once per second for each process on the dispatcher queue. If a process's ts_dispwait value exceeds the ts_maxwait value for its level, the process's priority is changed to that indicated by ts_lwait. The purpose of this field is to prevent starvation.
ts_lwait: Move a process to this new priority level if ts_dispwait is greater than ts_maxwait.

An administrator can affect the behavior of the time-sharing portion of the scheduler by reconfiguring the ts_dptbl. There are two methods available for doing this.

dispadmin configuration

With the exception of ts_globpri all of the members of the ts_dptbl can be examined and modified on a running system using the dispadmin(1M) command. Invoking dispadmin for the time-sharing class allows the administrator to retrieve the current ts_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 ts_quantum 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 parameter values for each of the time-sharing priority levels. The first line specifies the parameters for time-sharing level 0, the second line specifies the parameters for time-sharing level 1, and so on. There must be exactly one line for each configured time-sharing priority level.

Files

/usr/include/sys/ts.h

References

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

Notices

dispadmin does some limited sanity checking on the values supplied in the configuration file. The sanity checking is intended to ensure that the new ts_dptbl values do not cause the system to panic. The sanity checking does not attempt to analyze the effect that the new values will have on the performance of the system. Unusual ts_dptbl configurations may have a dramatic negative impact on the performance of the system.

No sanity checking is done on the ts_dptbl values specified in the ts master file. Specifying an inconsistent or nonsensical ts_dptbl configuration through the ts master file could cause serious performance problems and/or cause the system to panic.

Examples

The following excerpt from a dispadmin configuration file illustrates the 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 time-sharing class, and the mapping between these time-sharing priorities and the corresponding global scheduling priorities is determined by the configuration specified in the ts 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 time-sharing priority). The level numbers in the comments should normally agree with this ordering; if for some reason they don't, however, dispadmin is unaffected.

# Time-Sharing Dispatcher Configuration File RES=1000

# ts_quantum ts_tqexp ts_slpret ts_maxwait ts_lwait PRIORITY LEVEL 500 0 10 5 10 # 0 500 0 11 5 11 # 1 500 1 12 5 12 # 2 500 1 13 5 13 # 3 500 2 14 5 14 # 4 500 2 15 5 15 # 5 450 3 16 5 16 # 6 450 3 17 5 17 # 7 . . . . . . . . . . . . . . . . . . . . . 50 48 59 5 59 # 58 50 49 59 5 59 # 59