priocntl(2) SYSTEM CALLS 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(idtypet idtype, idt 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) system calls. 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 system call is to apply. The
priocntlset system call provides the same functions as
priocntl, but allows a more general interface for specifying
the set of processes to which the system call 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 fol-
lows:
PPID id is a process ID specifying a single process to
which the priocntl system call is to apply.
PPPID
id is a parent process ID. The priocntl system call
applies to all processes with the specified parent
process ID.
PPGID
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priocntl(2) SYSTEM CALLS priocntl(2)
id is a process group ID. The priocntl system call
applies to all processes in the specified process
group.
PSID id is a session ID. The priocntl system call applies
to all processes in the specified session.
PCID id is a class ID (returned by priocntl PCGETCID as
explained below). The priocntl system call applies to
all processes in the specified class.
PUID id is a user ID. The priocntl system call applies to
all processes with this effective user ID.
PGID id is a group ID. The priocntl system call applies to
all processes with this effective group ID.
PALL The priocntl system call applies to all existing
processes. The value of id is ignored. The permis-
sion restrictions described below still apply.
An id value of PMYID 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 PCSETPARMS 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 PUID 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 PCSETPARMS) 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
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priocntl(2) SYSTEM CALLS priocntl(2)
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 System
Administrator's Guide for more information.)
The data type and value of arg are specific to the type of
command specified by cmd.
The following structure is used by the PCGETCID and
PCGETCLINFO commands.
typedef struct {
idt pccid; /* Class id */
char pcclname[PCCLNMSZ]; /* Class name */
long pcclinfo[PCCLINFOSZ]; /* Class information */
} pcinfot;
pccid is a class ID returned by priocntl PCGETCID.
pcclname is a buffer of size PCCLNMSZ (defined in
<sys/priocntl.h>) used to hold the class name (RT for real-
time or TS for time-sharing).
pcclinfo is a buffer of size PCCLINFOSZ (defined in
<sys/priocntl.h>) used to return data describing the attri-
butes of a specific class. The format of this data is
class-specific and is described under the appropriate head-
ing (REAL-TIME CLASS or TIME-SHARING CLASS) below.
The following structure is used by the PCSETPARMS and
PCGETPARMS commands.
typedef struct {
idt pccid; /* Process class */
long pcclparms[PCCLPARMSZ]; /* Class-specific params */
} pcparmst;
pccid is a class ID (returned by priocntl PCGETCID). The
special class ID PCCLNULL can also be assigned to pccid
when using the PCGETPARMS command as explained below.
The pcclparms buffer holds class-specific scheduling param-
eters. The format of this parameter data for a particular
class is described under the appropriate heading below.
PCCLPARMSZ is the length of the pcclparms buffer and is
defined in <sys/priocntl.h>.
Commands
Available priocntl commands are:
PCGETCID
Get class ID and class attributes for a specific class
given class name. The idtype and id arguments are
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priocntl(2) SYSTEM CALLS priocntl(2)
ignored. If arg is non-null, it points to a structure of
type pcinfot. The pcclname buffer contains the name of
the class whose attributes you are getting.
On success, the class ID is returned in pccid, the class
attributes are returned in the pcclinfo buffer, and the
priocntl call returns the total number of classes config-
ured in the system (including the sys class). If the
class specified by pcclname 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 config-
ured classes.
PCGETCLINFO
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
pcinfot. pccid is the class ID of the class whose
attributes you are getting.
On success, the class name is returned in the pcclname
buffer, the class attributes are returned in the
pcclinfo 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 config-
ured classes.
PCSETPARMS
Set the class and class-specific scheduling parameters of
the specified process(es). arg points to a structure of
type pcparmst. pccid specifies the class you are set-
ting and the pcclparms 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 file 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
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priocntl(2) SYSTEM CALLS priocntl(2)
or more of the target processes, it may or may not con-
tinue through the set of processes, depending on the
nature of the error. If the error is related to permis-
sions (EPERM), priocntl continues through the process
set, resetting the parameters for all target processes
for which the calling process has appropriate permis-
sions. 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 immedi-
ately.
PCGETPARMS
Get the class and/or class-specific scheduling parameters
of a process. arg points the a structure of type
pcparmst.
If pccid specifies a configured class and a single pro-
cess belonging to that class is specified by the idtype
and id values or the procset structure, then the schedul-
ing parameters of that process are returned in the
pcclparms 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 pccid 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 pcclparms buffer and the
priocntl call returns the process ID of the selected pro-
cess. 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 pccid is PCCLNULL and a single process is specified
the class of the specified process is returned in pccid
and its scheduling parameters are returned in the
pcclparms buffer.
PCADMIN
This command provides functionality needed for the imple-
mentation 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
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priocntl(2) SYSTEM CALLS priocntl(2)
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
(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 determined for a
specific installation by using the priocntl PCGETCID or
PCGETCLINFO 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 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 deter-
mines the scheduling priority of a real-time process rela-
tive 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 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 rtpri 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
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priocntl(2) SYSTEM CALLS priocntl(2)
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) system call by a real-time process, the
parent process continues to run while the child process
(which inherits its parent's rtpri value) is placed at the
back of the queue.
The following structure (defined in <sys/rtpriocntl.h>)
defines the format used for the attribute data for the
real-time class.
typedef struct {
short rtmaxpri; /* Maximum real-time priority */
} rtinfot;
The priocntl PCGETCID and PCGETCLINFO commands return
real-time class attributes in the pcclinfo buffer in this
format.
rtmaxpri specifies the configured maximum rtpri value for
the real-time class (if rtmaxpri is x, the valid real-time
priorities range from 0 to x).
The following structure (defined in <sys/rtpriocntl.h>)
defines the format used to specify the real-time class-
specific scheduling parameters of a process.
typedef struct {
short rtpri; /* Real-Time priority */
ulong rttqsecs; /* Seconds in time quantum */
long rttqnsecs; /* Additional nanoseconds in quantum */
} rtparmst;
When using the priocntl PCSETPARMS or PCGETPARMS commands,
if pccid specifies the real-time class, the data in the
pcclparms 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 rtpri value.
Setting the rtpri value of a process that is currently run-
ning 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 rtpri value of the pro-
cess. 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 rtpri value to its current
real-time priority value. In order to change the time quan-
tum of a process without setting the priority or affecting
the process's position on the queue, the rtpri field should
be set to the special value RTNOCHANGE (defined in
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priocntl(2) SYSTEM CALLS priocntl(2)
<sys/rtpriocntl.h>). Specifying RTNOCHANGE 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 PCGETPARMS command, if pccid specifies
the real-time class and more than one real-time process is
specified, the scheduling parameters of the real-time pro-
cess with the highest rtpri 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 rttqsecs and rttqnsecs fields are used for getting or
setting the time quantum associated with a process or group
of processes. rttqsecs is the number of seconds in the
time quantum and rttqnsecs is the number of additional
nanoseconds in the quantum. For example setting rttqsecs
to 2 and rttqnsecs 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 rttqnsecs field
results in an error return with errno set to EINVAL.
Although the resolution of the tqnsecs field is very fine,
the specified time quantum length is rounded up by the sys-
tem to the next integral multiple of the system clock's
resolution. For example, the finest resolution currently
available on the 3B2 is 10 milliseconds (1 ``tick''). Set-
ting rttqsecs to 0 and rttqnsecs to 34,000,000 would
specify a time quantum of 34 milliseconds, which would be
rounded up to 4 ticks (40 milliseconds) on the 3B2. The
maximum time quantum that can be specified is
implementation-specific and equal to LONGMAX ticks (defined
in <limits.h>). Requesting a quantum greater than this max-
imum 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 rttqsecs and rttqnsecs to 0) results in
an error return with errno set to EINVAL.
The rttqnsecs field can also be set to one of the following
special values (defined in <sys/rtpriocntl.h>), in which
case the value of rttqsecs is ignored.
RTTQINF Set an infinite time quantum.
RTTQDEF Set the time quantum to the default for this
priority [see rtdptbl(4)].
RTNOCHANGE
Don't set the time quantum. This value is use-
ful when you wish to change the real-time
priority of a process without affecting the
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priocntl(2) SYSTEM CALLS priocntl(2)
time quantum. Specifying this value when
changing the class of a process to real-time
from some other class is equivalent to specify-
ing RTTQDEF.
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 pro-
cess.
The real-time priority and time quantum are inherited across
the fork(2) and exec(2) system calls.
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 objec-
tives 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 tsupri below) values that may be assigned to
processes within the class. A tsupri 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 PCGETCID or PCGETCLINFO
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 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 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
(returned by the PCGETCID and PCGETCLINFO commands) there
is a per process user priority limit (see tsuprilim below),
which specifies the maximum tsupri value that may be set
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priocntl(2) SYSTEM CALLS priocntl(2)
for a given process; by default, tsuprilim is zero.
The following structure (defined in <sys/tspriocntl.h>)
defines the format used for the attribute data for the
time-sharing class.
typedef struct {
short tsmaxupri; /* Limits of user priority range */
} tsinfot;
The priocntl PCGETCID and PCGETCLINFO commands return
time-sharing class attributes in the pcclinfo buffer in
this format.
tsmaxupri specifies the configured maximum user priority
value for the time-sharing class. If tsmaxupri is x, the
valid range for both user priorities and user priority lim-
its is from -x to +x.
The following structure (defined in <sys/tspriocntl.h>)
defines the format used to specify the time-sharing class-
specific scheduling parameters of a process.
typedef struct {
short tsuprilim; /* Time-Sharing user priority limit */
short tsupri; /* Time-Sharing user priority */
} tsparmst;
When using the priocntl PCSETPARMS or PCGETPARMS commands,
if pccid specifies the time-sharing class, the data in the
pcclparms buffer is in this format.
For the priocntl PCGETPARMS command, if pccid specifies
the time-sharing class and more than one time-sharing pro-
cess is specified, the scheduling parameters of the time-
sharing process with the highest tsupri 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 tsuprilim (or
that of another process with the same user ID). Only a
time-sharing process with super-user privileges may raise a
tsuprilim. 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 tsuprilim to a value
greater than zero. Attempts by a non-super-user process to
raise a tsuprilim or set an initial tsuprilim greater than
zero fail with a return value of -1 and errno set to EPERM.
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priocntl(2) SYSTEM CALLS priocntl(2)
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.
Either of the tsuprilim or tsupri fields may be set to the
special value TSNOCHANGE (defined in <sys/tspriocntl.h>) in
order to set one of the values without affecting the other.
Specifying TSNOCHANGE for the tsupri when the tsuprilim
is being set to a value below the current tsupri causes the
tsupri to be set equal to the tsuprilim being set. Speci-
fying TSNOCHANGE 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 tsuprilim is 0 and the default for the tsupri is
to set it equal to the tsuprilim which is being set.
The time-sharing user priority and user priority limit are
inherited across the fork and exec system calls.
RETURN VALUE
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 :
EPERM The calling process does not have the required per-
missions as explained above.
EINVAL The argument cmd was invalid, an invalid or uncon-
figured class was specified, or one of the parame-
ters specified was invalid.
ERANGE The requested time quantum is out of range.
ESRCH None of the specified processes exist.
EFAULT All or part of the area pointed to by one of the
data pointers is outside the process's address
space.
ENOMEM An attempt to change the class of a process failed
because of insufficient memory.
EAGAIN An attempt to change the class of a process failed
because of insufficient resources other than memory
(for example, class-specific kernel data struc-
tures).
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priocntl(2) SYSTEM CALLS priocntl(2)
SEE ALSO
fork(2), exec(2), nice(2), priocntlset(2)
priocntl(1) in the User's Reference Manual
dispadmin(1M), rtdptbl(4), tsdptbl(4) in the System
Administrator's Reference Manual
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