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The priocntl command displays or sets scheduling parameters of the specified process(es). It can also be used to display the current configuration information for the system’s process scheduler or execute a command with specified scheduling parameters.
Processes fall into distinct classes with a separate scheduling policy applied to each class. The process classes currently supported are the real-time class, time-sharing class, and the interactive class. The characteristics of these classes and the class-specific options they accept are described below in the USAGE section under the headings Real-Time Class, Time-Sharing Class, and Inter-Active Class. With appropriate permissions, the priocntl command can change the class and other scheduling parameters associated with a running process.
In the default configuration, a runnable real-time process runs before any other process. Therefore, inappropriate use of real-time processes can have a dramatic negative impact on system performance.
If an idlist is present it must appear last on the command line and the elements of the list must be separated by white space. If no idlist is present an idtype argument of pid, ppid, pgid, sid, class, uid, or gid specifies the process ID , parent process ID , process group ID , session ID , class, user ID , or group ID , respectively, of the priocntl command itself.
The command
priocntl -d [-i idtype] [idlist]
displays the class and class-specific scheduling parameters of the process(es) specified by idtype and idlist.
The command
priocntl -s [-c class] [class-specific options] [-i idtype] [idlist]
sets the class and class-specific parameters of the specified processes to the values given on the command line. The -c class option specifies the class to be set. (The valid class arguments are RT for real-time TS for time-sharing or IA for inter-active.)
The class-specific parameters to be set are specified by the class-specific options as explained under the appropriate heading below. If the -c class option is omitted, idtype and idlist must specify a set of processes which are all in the same class, otherwise an error results. If no class-specific options are specified the process’s class-specific parameters are set to the default values for the class specified by -c class (or to the default parameter values for the process’s current class if the -c class option is also omitted).
In order to change the scheduling parameters of a process using priocntl the real or effective user ID (respectively, groupID) of the user invoking priocntl must match the real or effective user ID (respectively, groupID) of the receiving process or the effective user ID of the user must be super-user. These are the minimum permission requirements enforced for all classes. An individual class may impose additional permissions requirements when setting processes to that class or when setting class-specific scheduling parameters.
When idtype and idlist specify a set of processes, priocntl acts on the processes in the set in an implementation-specific order. If priocntl encounters an error for one or more of the target processes, it may or may not continue through the set of processes, depending on the nature of the error.
If the error is related to permissions, priocntl prints an error message and then continue through the process set, resetting the parameters for all target processes for which the user has appropriate permissions. If priocntl encounters an error other than permissions, it does not continue through the process set but prints an error message and exits immediately.
A special sys scheduling class exists for the purpose of scheduling the execution of certain special system processes (such as the swapper process). It is not possible to change the class of any process to sys. In addition, any processes in the sys class that are included in the set of processes specified by idtype and idlist are disregarded by priocntl. For example, if idtype were uid, an idlist consisting of a zero would specify all processes with a UID of 0, except processes in the sys class and (if changing the parameters using the -s option) the init process.
The init process (process ID 1) is a special case. In order for the priocntl command to change the class or other scheduling parameters of the init process, idtype must be pid and idlist must be consist of only a 1. The init process may be assigned to any class configured on the system, but the time-sharing class is almost always the appropriate choice. (Other choices may be highly undesirable; see the for more information.)
The command
priocntl -e [-c class] [class-specific options] command [argument...]
executes the specified command with the class and scheduling parameters specified on the command line (arguments are the arguments to the command). If the -c class option is omitted the command is run in the user’s current class.
- -i pid
- idlist is a list of process ID s. The priocntl command applies to the specified processes.
- -i ppid
- idlist is a list of parent process ID s. The priocntl command applies to all processes whose parent process ID is in the list.
- -i pgid
- idlist is a list of process group ID s. The priocntl command applies to all processes in the specified process groups.
- -i sid
- idlist is a list of session ID s. The priocntl command applies to all processes in the specified sessions.
- -i class
- idlist consists of a single class name (RT for real-time or TS for time-sharing or IA for inter-active). The priocntl command applies to all processes in the specified class.
- -i uid
- idlist is a list of user ID s. The priocntl command applies to all processes with an effective user ID equal to an ID from the list.
- -i gid
- idlist is a list of group ID s. The priocntl command applies to all processes with an effective group ID equal to an ID from the list.
- -i all
- The priocntl command applies to all existing processes. No idlist should be specified (if one is it is ignored). The permission restrictions described below still apply.
If the -i idtype option is omitted when using the -d or -s options the default idtype of pid is assumed.
The valid class-specific options for setting real-time parameters are:
The valid class-specific options for setting time-sharing parameters are:
The valid class-specific options for setting inter-active parameters are:
The real-time class has a range of real-time priority (rtpri) values that may be assigned to processes within the class. Real-time priorities range from 0 to x, where the value of x is configurable and can be displayed for a specific installation that has already configured a real-time scheduler, by using the command
priocntl -l
The real-time scheduling policy is a fixed priority policy. The scheduling priority of a real-time process never changes except as the result of an explicit request by the user/application to change the rtpri value of the process.
For processes in the real-time class, the rtpri value is, for all practical purposes, equivalent to the scheduling priority of the process. The rtpri value completely determines the scheduling priority of a real-time process relative to other processes within its class. Numerically higher rtpri values represent higher priorities. Since the real-time class controls the highest range of scheduling priorities in the system it is guaranteed that the runnable real-time process with the highest rtpri value is always selected to run before any other process in the system.
In addition to providing control over priority, priocntl provides for control over the length of the time quantum allotted to processes in the real-time class. The time quantum value specifies the maximum amount of time a process may run assuming that it does not complete or enter a resource or event wait state (sleep). Note that if another process becomes runnable at a higher priority, the currently running process may be preempted before receiving its full time quantum.
The command
priocntl -d [-i idtype] [idlist]
displays the real-time priority and time quantum (in millisecond resolution) for each real-time process in the set specified by idtype and idlist.
Any combination of the -p and
-t options may be used with priocntl -s or priocntl -e for the real-time class.
If an option is omitted and the process is currently real-time, the associated
parameter is unaffected. If an option is omitted when changing the class
of a process to real-time from some other class, the associated parameter
is set to a default value. The default value for rtpri is 0 and the default
for time quantum is dependent on the value of rtpri and on the system configuration;
see rt_dptbl(4)
.
When using the -t tqntm option you may optionally specify a resolution using the -r res option. (If no resolution is specified, millisecond resolution is assumed.) If res is specified it must be a positive integer between 1 and 1,000,000,000 inclusive and the resolution used is the reciprocal of res in seconds. For example, specifying -t 10 -r 100 would set the resolution to hundredths of a second and the resulting time quantum length would be 10/100 seconds (one tenth of a second). Although very fine (nanosecond) resolution may be specified, the time quantum length is rounded up by the system to the next integral multiple of the system clock’s resolution. Requests for time quantums of zero or quantums greater than the (typically very large) implementation-specific maximum quantum result in an error.
In order to change the class of a process to real-time (from any other class) the user invoking priocntl must have super-user privilege. In order to change the rtpri value or time quantum of a real-time process the user invoking priocntl must either be super-user, or must currently be in the real-time class (shell running as a real-time process) with a real or effective user ID matching the real or effective user ID of the target process.
The real-time priority and time quantum are inherited across the fork(2) and exec(2) system calls.
The time-sharing scheduling policy provides for a fair and effective allocation of the CPU resource among processes with varying CPU consumption characteristics. The objectives of the time-sharing policy are to provide good response time to interactive processes and good throughput to CPU-bound jobs while providing a degree of user/application control over scheduling.
The time-sharing class has a range of time-sharing user priority (tsupri) values that may be assigned to processes within the class. User priorities range from -x to +x, where the value of x is configurable. The range for a specific installation can be displayed by using the command
priocntl -l
The purpose of the user priority is to provide some degree of user/application control over the scheduling of processes in the time-sharing class. Raising or lowering the tsupri value of a process in the time-sharing class raises or lowers the scheduling priority of the process. It is not guaranteed, however, that a time-sharing process with a higher tsupri value will run before one with a lower tsupri value. This is because the tsupri value is just one factor used to determine the scheduling priority of a time-sharing process. The system may dynamically adjust the internal scheduling priority of a time-sharing process based on other factors such as recent CPU usage.
In addition to the system-wide limits on user priority (displayed with priocntl -l), there is a per process user priority limit (tsuprilim), which specifies the maximum tsupri value that may be set for a given process.
The command
priocntl -d [-i idtype] [idlist]
displays the user priority and user priority limit for each time-sharing process in the set specified by idtype and idlist.
Any time-sharing process may lower its own tsuprilim (or that of another process with the same user ID ). Only a time-sharing process with super-user privilege may raise a tsuprilim. When changing the class of a process to time-sharing from some other class, super-user privilege is required in order to set the initial tsuprilim to a value greater than zero.
Any time-sharing process may set its own tsupri (or that of another process with the same user ID ) to any value less than or equal to the process’s tsuprilim. Attempts to set the tsupri above the tsuprilim (and/or set the tsuprilim below the tsupri) result in the tsupri being set equal to the tsuprilim.
Any combination of the -m and -p options may be used with priocntl -s or priocntl -e for the time-sharing class. If an option is omitted and the process is currently time-sharing the associated parameter is normally unaffected. The exception is when the -p option is omitted and -m is used to set a tsuprilim below the current tsupri. In this case the tsupri is set equal to the tsuprilim which is being set. If an option is omitted when changing the class of a process to time-sharing from some other class, the associated parameter is set to a default value. The default value for tsuprilim is 0 and the default for tsupri is to set it equal to the tsuprilim value which is being set.
The time-sharing user priority and user priority limit are inherited across the fork(2) and exec(2) system calls.
The inter-active scheduling policy provides for a fair and effective allocation of the CPU resource among processes with varying CPU consumption characteristics while providing good responsiveness for user interaction. The objectives of the inter-active policy are to provide good response time to interactive processes and good throughput to CPU-bound jobs. Only the super user has access to the inter-active class, the user has no control over scheduling policys.
Real-Time Class examples follow:
example% priocntl -s -c RT -t 1 -r 10 -i idtype idlist
The above example sets the class of any non-real-time processes selected by idtype and idlist to real-time and sets their real-time priority to the default value of 0. The real-time priorities of any processes currently in the real-time class are unaffected. The time quantums of all of the specified processes are set to 1/10 seconds.
example% priocntl -e -c RT -p 15 -t 20 command
This example executes command in the real-time class with a real-time priority of 15 and a time quantum of 20 milliseconds.
Time-Sharing Class examples follow:
example% priocntl -s -c TS -i idtype idlist
The above example sets the class of any non-time-sharing processes selected by idtype and idlist to time-sharing and sets both their user priority limit and user priority to 0. Processes already in the time-sharing class are unaffected.
This example executes command with the arguments arguments in the time-sharing class with a user priority limit of 0 and a user priority of -15.
example% priocntl -e -c TS -m 0 -p -15 command [arguments]
priocntl prints the following error messages: