priocntl(2)

NAME

priocntl − process scheduler control

SYNOPSIS

#include <sys/types.h>
#include <sys/priocntl.h>
#include <sys/rtpriocntl.h>
#include <sys/tspriocntl.h>

long priocntl(idtype_t idtype, id_t id, int cmd, /∗ arg ∗/ ...);

DESCRIPTION

priocntl() provides for control over the scheduling of active processes.

Processes fall into distinct classes with a separate scheduling policy applied to each class. The two classes currently supported are the real-time class and the time-sharing class. The characteristics of these classes are described under the corresponding headings below. The class attribute of a process is inherited across the fork and exec(2)functions. priocntl() can be used to dynamically change the class and other scheduling parameters associated with a running process or set of processes given the appropriate permissions as explained below.

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.

priocntl() provides a interface for specifying a process or set of processes to which the function is to apply. The priocntlset() function provides the same functions as priocntl(), but allows a more general interface for specifying the set of processes to which the function is to apply.

For priocntl(), the idtype and id arguments are used together to specify the set of processes. The interpretation of id depends on the value of idtype. The possible values for idtype and corresponding interpretations of id are as follows:

P_PIDid is a process ID specifying a single process to which the priocntl() function is to apply.

P_PPIDid is a parent process ID. The priocntl() function applies to all processes with the specified parent process ID.

P_PGIDid is a process group ID. The priocntl() function applies to all processes in the specified process group.

P_SIDid is a session ID. The priocntl() function applies to all processes in the specified session.

P_CIDid is a class ID (returned by priocntl() PC_GETCID as explained below). The priocntl() function applies to all processes in the specified class.

P_UIDid is a user ID. The priocntl() function applies to all processes with this effective user ID.

P_GIDid is a group ID. The priocntl() function applies to all processes with this effective group ID.

P_ALLThe priocntl() function applies to all existing processes. The value of id is ignored. The permission restrictions described below still apply.

An id value of P_MYID can be used in conjunction with the idtype value to specify the calling process’s process ID, parent process ID, process group ID, session ID, class ID, user ID, or group ID.

In order to change the scheduling parameters of a process (using the PC_SETPARMS command as explained below) the real or effective user ID of the process calling priocntl() must match the real or effective user ID of the receiving process or the effective user ID of the calling process 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 and/or when setting class-specific scheduling parameters.

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 a specified set of processes are disregarded by priocntl(). For example, an idtype of P_UID and an id value of zero would specify all processes with a user ID of zero except processes in the sys class and (if changing the parameters using PC_SETPARMS) the init process.

The init process is a special case. In order for a priocntl() call to change the class or other scheduling parameters of the init process (process ID 1), it must be the only process specified by idtype and id. 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 SunOS 5.1 Routine System Administration Guide for more information.)

The data type and value of arg are specific to the type of command specified by cmd.

A structure with the following members is used by the PC_GETCID and PC_GETCLINFO commands.

id_tpc_cid;/∗ Class id ∗/
charpc_clname[PC_CLNMSZ]; /∗ Class name ∗/
longpc_clinfo[PC_CLINFOSZ]; /∗ Class information ∗/

pc_cid is a class ID returned by priocntl() PC_GETCID. pc_clname is a buffer of size PC_CLNMSZ (defined in <sys/priocntl.h>) used to hold the class name (RT for real-time or TS for time-sharing).

pc_clinfo is a buffer of size PC_CLINFOSZ (defined in <sys/priocntl.h>) used to return data describing the attributes of a specific class. The format of this data is class-specific and is described under the appropriate heading (REAL-TIME CLASS or TIME-SHARING CLASS) below.

A structure with the following elements is used by the PC_SETPARMS and PC_GETPARMS commands.

id_tpc_cid;/∗ Process class ∗/
longpc_clparms[PC_CLPARMSZ]; /∗ Class-specific params ∗/

pc_cid is a class ID (returned by priocntl() PC_GETCID). The special class ID PC_CLNULL can also be assigned to pc_cid when using the PC_GETPARMS command as explained below.

The pc_clparms buffer holds class-specific scheduling parameters. The format of this parameter data for a particular class is described under the appropriate heading below. PC_CLPARMSZ is the length of the pc_clparms buffer and is defined in <sys/priocntl.h>.

Commands

Available priocntl() commands are:

PC_GETCID
Get class ID and class attributes for a specific class given class name. The idtype and id arguments are ignored. If arg is non-null, it points to a structure of type pcinfo_t. The pc_clname buffer contains the name of the class whose attributes you are getting.

On success, the class ID is returned in pc_cid, the class attributes are returned in the pc_clinfo buffer, and the priocntl() call returns the total number of classes configured in the system (including the sys class). If the class specified by pc_clname is invalid or is not currently configured the priocntl() call returns −1 with errno set to EINVAL. The format of the attribute data returned for a given class is defined in the <sys/rtpriocntl.h> or <sys/tspriocntl.h> header file and described under the appropriate heading below.

If arg is a NULL pointer, no attribute data is returned but the priocntl() call still returns the number of configured classes.

PC_GETCLINFO
Get class name and class attributes for a specific class given class ID. The idtype and id arguments are ignored. If arg is non-null, it points to a structure of type pcinfo_t. pc_cid is the class ID of the class whose attributes you are getting.

On success, the class name is returned in the pc_clname buffer, the class attributes are returned in the pc_clinfo buffer, and the priocntl() call returns the total number of classes configured in the system (including the sys class). The format of the attribute data returned for a given class is defined in the <sys/rtpriocntl.h> or <sys/tspriocntl.h> header file and described under the appropriate heading below.

If arg is a NULL pointer, no attribute data is returned but the priocntl() call still returns the number of configured classes.

PC_SETPARMS
Set the class and class-specific scheduling parameters of the specified process(es). arg points to a structure of type pcparms_t. pc_cid specifies the class you are setting and the pc_clparms buffer contains the class-specific parameters you are setting. The format of the class-specific parameter data is defined in the <sys/rtpriocntl.h> or <sys/tspriocntl.h> header and described under the appropriate class heading below.

When setting parameters for 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 (EPERM), priocntl() continues through the process set, resetting the parameters for all target processes for which the calling process has appropriate permissions. priocntl() then returns −1 with errno set to EPERM to indicate that the operation failed for one or more of the target processes. If priocntl() encounters an error other than permissions, it does not continue through the set of target processes but returns the error immediately.

PC_GETPARMS
Get the class and/or class-specific scheduling parameters of a process. arg points the a structure of type pcparms_t.

If pc_cid specifies a configured class and a single process belonging to that class is specified by the idtype and id values or the procset structure, then the scheduling parameters of that process are returned in the pc_clparms buffer. If the process specified does not exist or does not belong to the specified class, the priocntl() call returns −1 with errno set to ESRCH.

If pc_cid specifies a configured class and a set of processes is specified, the scheduling parameters of one of the specified processes belonging to the specified class are returned in the pc_clparms buffer and the priocntl() call returns the process ID of the selected process. The criteria for selecting a process to return in this case is class dependent. If none of the specified processes exist or none of them belong to the specified class the priocntl() call returns −1 with errno set to ESRCH.

If pc_cid is PC_CLNULL and a single process is specified the class of the specified process is returned in pc_cid and its scheduling parameters are returned in the pc_clparms buffer.

PC_ADMIN
This command provides functionality needed for the implementation of the dispadmin(1M) command. It is not intended for general use by other applications.

REAL-TIME CLASS

The real-time class provides a fixed priority preemptive scheduling policy for those processes requiring fast and deterministic response and absolute user/application control of scheduling priorities. If the real-time class is configured in the system it should have exclusive control of the highest range of scheduling priorities on the system. This ensures that a runnable real-time process is given CPU service before any process belonging to any other class.

The real-time class has a range of real-time priority (rt_pri) 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 determined for a specific installation by using the priocntl() PC_GETCID or PC_GETCLINFO command.

The real-time scheduling policy is a fixed priority policy. The scheduling priority of a real-time process is never changed except as the result of an explicit request by the user/application to change the rt_pri value of the process.

For processes in the real-time class, the rt_pri value is, for all practical purposes, equivalent to the scheduling priority of the process. The rt_pri value completely determines the scheduling priority of a real-time process relative to other processes within its class. Numerically higher rt_pri 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 rt_pri 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 system’s process scheduler keeps the runnable real-time processes on a set of scheduling queues. There is a separate queue for each configured real-time priority and all real-time processes with a given rt_pri value are kept together on the appropriate queue. The processes on a given queue are ordered in FIFO order (that is, the process at the front of the queue has been waiting longest for service and receives the CPU first). Real-time processes that wake up after sleeping, processes which change to the real-time class from some other class, processes which have used their full time quantum, and runnable processes whose priority is reset by priocntl() are all placed at the back of the appropriate queue for their priority. A process that is preempted by a higher priority process remains at the front of the queue (with whatever time is remaining in its time quantum) and runs before any other process at this priority. Following a fork(2) function by a real-time process, the parent process continues to run while the child process (which inherits its parent’s rt_pri value) is placed at the back of the queue.

A structure with the following members (defined in <sys/rtpriocntl.h>) defines the format used for the attribute data for the real-time class.

shortrt_maxpri;/∗ Maximum real-time priority ∗/

The priocntl() PC_GETCID and PC_GETCLINFO commands return real-time class attributes in the pc_clinfo buffer in this format.

rt_maxpri specifies the configured maximum rt_pri value for the real-time class (if rt_maxpri is x, the valid real-time priorities range from 0 to x).

A structure with the following members (defined in <sys/rtpriocntl.h>) defines the format used to specify the real-time class-specific scheduling parameters of a process.

shortrt_pri;/∗ Real-Time priority ∗/
ulongrt_tqsecs;/∗ Seconds in time quantum ∗/
longrt_tqnsecs;/∗ Additional nanoseconds in quantum ∗/

When using the priocntl() PC_SETPARMS or PC_GETPARMS commands, if pc_cid specifies the real-time class, the data in the pc_clparms buffer is in this format.

The above commands can be used to set the real-time priority to the specified value or get the current rt_pri value. Setting the rt_pri value of a process that is currently running or runnable (not sleeping) causes the process to be placed at the back of the scheduling queue for the specified priority. The process is placed at the back of the appropriate queue regardless of whether the priority being set is different from the previous rt_pri value of the process. Note that a running process can voluntarily release the CPU and go to the back of the scheduling queue at the same priority by resetting its rt_pri value to its current real-time priority value. In order to change the time quantum of a process without setting the priority or affecting the process’s position on the queue, the rt_pri field should be set to the special value RT_NOCHANGE (defined in <sys/rtpriocntl.h>). Specifying RT_NOCHANGE when changing the class of a process to real-time from some other class results in the real-time priority being set to zero.

For the priocntl() PC_GETPARMS command, if pc_cid specifies the real-time class and more than one real-time process is specified, the scheduling parameters of the real-time process with the highest rt_pri value among the specified processes are returned and the process ID of this process is returned by the priocntl() call. If there is more than one process sharing the highest priority, the one returned is implementation-dependent.

The rt_tqsecs and rt_tqnsecs fields are used for getting or setting the time quantum associated with a process or group of processes. rt_tqsecs is the number of seconds in the time quantum and rt_tqnsecs is the number of additional nanoseconds in the quantum. For example setting rt_tqsecs to 2 and rt_tqnsecs to 500,000,000 (decimal) would result in a time quantum of two and one-half seconds. Specifying a value of 1,000,000,000 or greater in the rt_tqnsecs field results in an error return with errno set to EINVAL. Although the resolution of the tq_nsecs field is very fine, the specified time quantum length is rounded up by the system to the next integral multiple of the system clock’s resolution. The maximum time quantum that can be specified is implementation-specific and equal to LONG_MAX ticks (defined in <limits.h>). Requesting a quantum greater than this maximum results in an error return with errno set to ERANGE (although infinite quantums may be requested using a special value as explained below). Requesting a time quantum of zero (setting both rt_tqsecs and rt_tqnsecs to 0) results in an error return with errno set to EINVAL.

The rt_tqnsecs field can also be set to one of the following special values (defined in <sys/rtpriocntl.h>), in which case the value of rt_tqsecs is ignored.

RT_TQINFSet an infinite time quantum.

RT_TQDEFSet the time quantum to the default for this priority (see rt_dptbl(4)).

RT_NOCHANGEDon’t set the time quantum. This value is useful when you wish to change the real-time priority of a process without affecting the time quantum. Specifying this value when changing the class of a process to real-time from some other class is equivalent to specifying RT_TQDEF.

In order to change the class of a process to real-time (from any other class) the process invoking priocntl() must have super-user privileges. In order to change the priority or time quantum setting of a real-time process the process invoking priocntl() must have super-user privileges or must itself be a real-time process whose real or effective user ID matches the real of effective user ID of the target process.

The real-time priority and time quantum are inherited across the fork(2) and exec(2) functions.

TIME-SHARING CLASS

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 (see ts_upri below) values that may be assigned to processes within the class. A ts_upri value of zero is defined as the default base priority for the time-sharing class. User priorities range from −x to +x where the value of x is configurable and can be determined for a specific installation by using the priocntl() PC_GETCID or PC_GETCLINFO command.

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 ts_upri 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 process with a higher ts_upri value will run before one with a lower ts_upri value. This is because the ts_upri 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 (returned by the PC_GETCID and PC_GETCLINFO commands) there is a per process user priority limit (see ts_uprilim below), which specifies the maximum ts_upri value that may be set for a given process; by default, ts_uprilim is zero.

A structure with the following members (defined in <sys/tspriocntl.h>) defines the format used for the attribute data for the time-sharing class.

shortts_maxupri;/∗ Limits of user priority range ∗/

The priocntl() PC_GETCID and PC_GETCLINFO commands return time-sharing class attributes in the pc_clinfo buffer in this format.

ts_maxupri specifies the configured maximum user priority value for the time-sharing class. If ts_maxupri is x, the valid range for both user priorities and user priority limits is from −x to +x.

A structure with the following members (defined in <sys/tspriocntl.h>) defines the format used to specify the time-sharing class-specific scheduling parameters of a process.

shortts_uprilim;/∗ Time-Sharing user priority limit ∗/
shortts_upri;/∗ Time-Sharing user priority ∗/

When using the priocntl() PC_SETPARMS or PC_GETPARMS commands, if pc_cid specifies the time-sharing class, the data in the pc_clparms buffer is in this format.

For the priocntl() PC_GETPARMS command, if pc_cid specifies the time-sharing class and more than one time-sharing process is specified, the scheduling parameters of the time-sharing process with the highest ts_upri value among the specified processes is returned and the process ID of this process is returned by the priocntl() call. If there is more than one process sharing the highest user priority, the one returned is implementation-dependent.

Any time-sharing process may lower its own ts_uprilim (or that of another process with the same user ID). Only a time-sharing process with super-user privileges may raise a ts_uprilim. When changing the class of a process to time-sharing from some other class, super-user privileges are required in order to set the initial ts_uprilim to a value greater than zero. Attempts by a non-super-user process to raise a ts_uprilim or set an initial ts_uprilim greater than zero fail with a return value of −1 and errno set to EPERM.

Any time-sharing process may set its own ts_upri (or that of another process with the same user ID) to any value less than or equal to the process’s ts_uprilim. Attempts to set the ts_upri above the ts_uprilim (and/or set the ts_uprilim below the ts_upri) result in the ts_upri being set equal to the ts_uprilim.

Either of the ts_uprilim or ts_upri fields may be set to the special value TS_NOCHANGE (defined in <sys/tspriocntl.h>) in order to set one of the values without affecting the other. Specifying TS_NOCHANGE for the ts_upri when the ts_uprilim is being set to a value below the current ts_upri causes the ts_upri to be set equal to the ts_uprilim being set. Specifying TS_NOCHANGE for a parameter when changing the class of a process to time-sharing (from some other class) causes the parameter to be set to a default value. The default value for the ts_uprilim is 0 and the default for the ts_upri is to set it equal to the ts_uprilim which is being set.

The time-sharing user priority and user priority limit are inherited across the fork and exec functions.

RETURN VALUES

Unless otherwise noted above, priocntl() returns a value of 0 on success. priocntl() returns −1 on failure and sets errno to indicate the error.

ERRORS

priocntl() fails if one or more of the following are true :

EAGAIN An attempt to change the class of a process failed because of insufficient resources other than memory (for example, class-specific kernel data structures).

EFAULT One of the arguments points to an illegal address.

EINVAL The argument cmd was invalid, an invalid or unconfigured class was specified, or one of the parameters specified was invalid.

ENOMEM An attempt to change the class of a process failed because of insufficient memory.

EPERM The effective user of the calling process is not super-user.

ERANGE The requested time quantum is out of range.

ESRCH None of the specified processes exist.

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