mirror of https://github.com/mkerrisk/man-pages
552 lines
18 KiB
Groff
552 lines
18 KiB
Groff
.\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Various pieces from the old sched_setscheduler(2) page
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.\" Copyright (C) Tom Bjorkholm, Markus Kuhn & David A. Wheeler 1996-1999
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.\" and Copyright (C) 2007 Carsten Emde <Carsten.Emde@osadl.org>
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.\" and Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.\"
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.\" Worth looking at: http://rt.wiki.kernel.org/index.php
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.\"
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.TH SCHED 7 2014-04-28 "Linux" "Linux Programmer's Manual"
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.SH NAME
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sched \- overview of scheduling APIs
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.SH DESCRIPTION
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.SS API summary
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The Linux scheduling APIs are as follows:
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.TP
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.BR sched_setscheduler (2)
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Set the scheduling policy and parameters of a specified thread.
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.TP
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.BR sched_getscheduler (2)
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Return the scheduling policy of a specified thread.
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.TP
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.BR sched_setparam (2)
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Set the scheduling parameters of a specified thread.
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.TP
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.BR sched_getparam (2)
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Fetch the scheduling parameters of a specified thread.
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.TP
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.BR sched_get_priority_max (2)
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Return the minimum priority available in a specified scheduling policy.
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.TP
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.BR sched_get_priority_min (2)
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Return the maximum priority available in a specified scheduling policy.
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.TP
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.BR sched_rr_get_interval (2)
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Fetch the quantum used for threads that are scheduled under
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the "round-robin" scheduling policy.
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.TP
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.BR sched_yield (2)
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Cause the caller to relinquish the CPU,
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so that some other thread be executed.
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.TP
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.BR sched_setaffinity (2)
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(Linux-specific)
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Set the CPU affinity of a specified thread.
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.TP
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.BR sched_getaffinity (2)
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(Linux-specific)
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Set the CPU affinity of a specified thread.
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.TP
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.BR sched_setattr (2)
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(Linux-specific)
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A generalized API for setting the scheduling policy and parameters
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of a specified thread.
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.TP
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.BR sched_getattr (2)
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(Linux-specific)
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A generalized API for fetching the scheduling policy and parameters
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of a specified thread.
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.\"
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.SS Scheduling policies
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The scheduler is the kernel component that decides which runnable thread
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will be executed by the CPU next.
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Each thread has an associated scheduling policy and a \fIstatic\fP
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scheduling priority, \fIsched_priority\fP; these are the settings
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that are modified by
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.BR sched_setscheduler ().
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The scheduler makes it decisions based on knowledge of the scheduling
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policy and static priority of all threads on the system.
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For threads scheduled under one of the normal scheduling policies
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(\fBSCHED_OTHER\fP, \fBSCHED_IDLE\fP, \fBSCHED_BATCH\fP),
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\fIsched_priority\fP is not used in scheduling
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decisions (it must be specified as 0).
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Processes scheduled under one of the real-time policies
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(\fBSCHED_FIFO\fP, \fBSCHED_RR\fP) have a
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\fIsched_priority\fP value in the range 1 (low) to 99 (high).
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(As the numbers imply, real-time threads always have higher priority
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than normal threads.)
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Note well: POSIX.1-2001 requires an implementation to support only a
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minimum 32 distinct priority levels for the real-time policies,
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and some systems supply just this minimum.
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Portable programs should use
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.BR sched_get_priority_min (2)
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and
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.BR sched_get_priority_max (2)
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to find the range of priorities supported for a particular policy.
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Conceptually, the scheduler maintains a list of runnable
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threads for each possible \fIsched_priority\fP value.
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In order to determine which thread runs next, the scheduler looks for
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the nonempty list with the highest static priority and selects the
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thread at the head of this list.
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A thread's scheduling policy determines
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where it will be inserted into the list of threads
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with equal static priority and how it will move inside this list.
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All scheduling is preemptive: if a thread with a higher static
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priority becomes ready to run, the currently running thread
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will be preempted and
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returned to the wait list for its static priority level.
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The scheduling policy determines the
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ordering only within the list of runnable threads with equal static
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priority.
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.SS SCHED_FIFO: First in-first out scheduling
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\fBSCHED_FIFO\fP can be used only with static priorities higher than
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0, which means that when a \fBSCHED_FIFO\fP threads becomes runnable,
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it will always immediately preempt any currently running
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\fBSCHED_OTHER\fP, \fBSCHED_BATCH\fP, or \fBSCHED_IDLE\fP thread.
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\fBSCHED_FIFO\fP is a simple scheduling
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algorithm without time slicing.
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For threads scheduled under the
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\fBSCHED_FIFO\fP policy, the following rules apply:
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.IP * 3
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A \fBSCHED_FIFO\fP thread that has been preempted by another thread of
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higher priority will stay at the head of the list for its priority and
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will resume execution as soon as all threads of higher priority are
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blocked again.
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.IP *
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When a \fBSCHED_FIFO\fP thread becomes runnable, it
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will be inserted at the end of the list for its priority.
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.IP *
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A call to
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.BR sched_setscheduler ()
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or
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.BR sched_setparam (2)
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will put the
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\fBSCHED_FIFO\fP (or \fBSCHED_RR\fP) thread identified by
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\fIpid\fP at the start of the list if it was runnable.
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As a consequence, it may preempt the currently running thread if
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it has the same priority.
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(POSIX.1-2001 specifies that the thread should go to the end
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of the list.)
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.\" In 2.2.x and 2.4.x, the thread is placed at the front of the queue
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.\" In 2.0.x, the Right Thing happened: the thread went to the back -- MTK
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.IP *
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A thread calling
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.BR sched_yield (2)
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will be put at the end of the list.
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.PP
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No other events will move a thread
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scheduled under the \fBSCHED_FIFO\fP policy in the wait list of
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runnable threads with equal static priority.
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A \fBSCHED_FIFO\fP
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thread runs until either it is blocked by an I/O request, it is
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preempted by a higher priority thread, or it calls
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.BR sched_yield (2).
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.SS SCHED_RR: Round-robin scheduling
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\fBSCHED_RR\fP is a simple enhancement of \fBSCHED_FIFO\fP.
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Everything
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described above for \fBSCHED_FIFO\fP also applies to \fBSCHED_RR\fP,
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except that each thread is allowed to run only for a maximum time
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quantum.
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If a \fBSCHED_RR\fP thread has been running for a time
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period equal to or longer than the time quantum, it will be put at the
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end of the list for its priority.
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A \fBSCHED_RR\fP thread that has
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been preempted by a higher priority thread and subsequently resumes
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execution as a running thread will complete the unexpired portion of
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its round-robin time quantum.
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The length of the time quantum can be
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retrieved using
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.BR sched_rr_get_interval (2).
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.\" On Linux 2.4, the length of the RR interval is influenced
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.\" by the process nice value -- MTK
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.\"
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.SS SCHED_OTHER: Default Linux time-sharing scheduling
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\fBSCHED_OTHER\fP can be used at only static priority 0.
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\fBSCHED_OTHER\fP is the standard Linux time-sharing scheduler that is
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intended for all threads that do not require the special
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real-time mechanisms.
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The thread to run is chosen from the static
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priority 0 list based on a \fIdynamic\fP priority that is determined only
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inside this list.
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The dynamic priority is based on the nice value (set by
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.BR nice (2)
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or
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.BR setpriority (2))
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and increased for each time quantum the thread is ready to run,
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but denied to run by the scheduler.
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This ensures fair progress among all \fBSCHED_OTHER\fP threads.
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.\"
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.SS SCHED_BATCH: Scheduling batch processes
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(Since Linux 2.6.16.)
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\fBSCHED_BATCH\fP can be used only at static priority 0.
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This policy is similar to \fBSCHED_OTHER\fP in that it schedules
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the thread according to its dynamic priority
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(based on the nice value).
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The difference is that this policy
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will cause the scheduler to always assume
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that the thread is CPU-intensive.
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Consequently, the scheduler will apply a small scheduling
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penalty with respect to wakeup behaviour,
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so that this thread is mildly disfavored in scheduling decisions.
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.\" The following paragraph is drawn largely from the text that
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.\" accompanied Ingo Molnar's patch for the implementation of
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.\" SCHED_BATCH.
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.\" commit b0a9499c3dd50d333e2aedb7e894873c58da3785
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This policy is useful for workloads that are noninteractive,
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but do not want to lower their nice value,
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and for workloads that want a deterministic scheduling policy without
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interactivity causing extra preemptions (between the workload's tasks).
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.\"
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.SS SCHED_IDLE: Scheduling very low priority jobs
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(Since Linux 2.6.23.)
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\fBSCHED_IDLE\fP can be used only at static priority 0;
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the process nice value has no influence for this policy.
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This policy is intended for running jobs at extremely low
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priority (lower even than a +19 nice value with the
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.B SCHED_OTHER
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or
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.B SCHED_BATCH
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policies).
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.\"
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.SS Resetting scheduling policy for child processes
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Each thread has a reset-on-fork scheduling flag.
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When this flag is set, children created by
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.BR fork (2)
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do not inherit privileged scheduling policies.
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The reset-on-fork flag can be set by either:
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.IP * 3
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ORing the
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.B SCHED_RESET_ON_FORK
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flag into the
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.I policy
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argument when calling
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.BR sched_setscheduler (2)
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(since Linux 2.6.32);
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or
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.IP *
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specifying the
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.B SCHED_FLAG_RESET_ON_FORK
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flag in
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.IR attr.sched_flags
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when calling
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.BR sched_setattr (2).
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.PP
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Note that the constants used with these two APIs have different names.
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The state of the reset-on-fork flag can analogously be retrieved using
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.BR sched_getscheduler (2)
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and
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.BR sched_getattr (2).
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The reset-on-fork feature is intended for media-playback applications,
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and can be used to prevent applications evading the
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.BR RLIMIT_RTTIME
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resource limit (see
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.BR getrlimit (2))
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by creating multiple child processes.
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More precisely, if the reset-on-fork flag is set,
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the following rules apply for subsequently created children:
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.IP * 3
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If the calling thread has a scheduling policy of
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.B SCHED_FIFO
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or
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.BR SCHED_RR ,
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the policy is reset to
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.BR SCHED_OTHER
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in child processes.
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.IP *
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If the calling process has a negative nice value,
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the nice value is reset to zero in child processes.
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.PP
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After the reset-on-fork flag has been enabled,
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it can be reset only if the thread has the
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.BR CAP_SYS_NICE
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capability.
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This flag is disabled in child processes created by
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.BR fork (2).
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.\"
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.SS Privileges and resource limits
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In Linux kernels before 2.6.12, only privileged
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.RB ( CAP_SYS_NICE )
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threads can set a nonzero static priority (i.e., set a real-time
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scheduling policy).
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The only change that an unprivileged thread can make is to set the
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.B SCHED_OTHER
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policy, and this can be done only if the effective user ID of the caller of
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.BR sched_setscheduler ()
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matches the real or effective user ID of the target thread
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(i.e., the thread specified by
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.IR pid )
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whose policy is being changed.
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Since Linux 2.6.12, the
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.B RLIMIT_RTPRIO
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resource limit defines a ceiling on an unprivileged thread's
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static priority for the
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.B SCHED_RR
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and
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.B SCHED_FIFO
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policies.
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The rules for changing scheduling policy and priority are as follows:
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.IP * 3
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If an unprivileged thread has a nonzero
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.B RLIMIT_RTPRIO
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soft limit, then it can change its scheduling policy and priority,
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subject to the restriction that the priority cannot be set to a
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value higher than the maximum of its current priority and its
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.B RLIMIT_RTPRIO
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soft limit.
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.IP *
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If the
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.B RLIMIT_RTPRIO
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soft limit is 0, then the only permitted changes are to lower the priority,
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or to switch to a non-real-time policy.
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.IP *
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Subject to the same rules,
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another unprivileged thread can also make these changes,
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as long as the effective user ID of the thread making the change
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matches the real or effective user ID of the target thread.
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.IP *
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Special rules apply for the
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.BR SCHED_IDLE .
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In Linux kernels before 2.6.39,
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an unprivileged thread operating under this policy cannot
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change its policy, regardless of the value of its
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.BR RLIMIT_RTPRIO
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resource limit.
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In Linux kernels since 2.6.39,
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.\" commit c02aa73b1d18e43cfd79c2f193b225e84ca497c8
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an unprivileged thread can switch to either the
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.BR SCHED_BATCH
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or the
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.BR SCHED_NORMAL
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policy so long as its nice value falls within the range permitted by its
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.BR RLIMIT_NICE
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resource limit (see
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.BR getrlimit (2)).
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.PP
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Privileged
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.RB ( CAP_SYS_NICE )
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threads ignore the
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.B RLIMIT_RTPRIO
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limit; as with older kernels,
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they can make arbitrary changes to scheduling policy and priority.
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See
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.BR getrlimit (2)
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for further information on
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.BR RLIMIT_RTPRIO .
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.SS Response time
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A blocked high priority thread waiting for the I/O has a certain
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response time before it is scheduled again.
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The device driver writer
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can greatly reduce this response time by using a "slow interrupt"
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interrupt handler.
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.\" as described in
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.\" .BR request_irq (9).
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.SS Miscellaneous
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Child processes inherit the scheduling policy and parameters across a
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.BR fork (2).
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The scheduling policy and parameters are preserved across
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.BR execve (2).
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Memory locking is usually needed for real-time processes to avoid
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paging delays; this can be done with
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.BR mlock (2)
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or
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.BR mlockall (2).
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Since a nonblocking infinite loop in a thread scheduled under
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\fBSCHED_FIFO\fP or \fBSCHED_RR\fP will block all threads with lower
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priority forever, a software developer should always keep available on
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the console a shell scheduled under a higher static priority than the
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tested application.
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This will allow an emergency kill of tested
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real-time applications that do not block or terminate as expected.
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See also the description of the
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.BR RLIMIT_RTTIME
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resource limit in
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.BR getrlimit (2).
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POSIX systems on which
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.BR sched_setscheduler ()
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and
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.BR sched_getscheduler ()
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are available define
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.B _POSIX_PRIORITY_SCHEDULING
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in \fI<unistd.h>\fP.
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.SH RETURN VALUE
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On success,
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.BR sched_setscheduler ()
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returns zero.
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On success,
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.BR sched_getscheduler ()
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returns the policy for the thread (a nonnegative integer).
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On error, \-1 is returned, and
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.I errno
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is set appropriately.
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.SH ERRORS
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.TP
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.B EINVAL
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The scheduling \fIpolicy\fP is not one of the recognized policies,
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\fIparam\fP is NULL,
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or \fIparam\fP does not make sense for the \fIpolicy\fP.
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.TP
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.B EPERM
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The calling thread does not have appropriate privileges.
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.TP
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.B ESRCH
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The thread whose ID is \fIpid\fP could not be found.
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.SH CONFORMING TO
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POSIX.1-2001 (but see BUGS below).
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The \fBSCHED_BATCH\fP and \fBSCHED_IDLE\fP policies are Linux-specific.
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.SH NOTES
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|
POSIX.1 does not detail the permissions that an unprivileged
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thread requires in order to call
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.BR sched_setscheduler (),
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and details vary across systems.
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|
For example, the Solaris 7 manual page says that
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the real or effective user ID of the caller must
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match the real user ID or the save set-user-ID of the target.
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.PP
|
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The scheduling policy and parameters are in fact per-thread
|
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attributes on Linux.
|
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The value returned from a call to
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.BR gettid (2)
|
|
can be passed in the argument
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.IR pid .
|
|
Specifying
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.I pid
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|
as 0 will operate on the attribute for the calling thread,
|
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and passing the value returned from a call to
|
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.BR getpid (2)
|
|
will operate on the attribute for the main thread of the thread group.
|
|
(If you are using the POSIX threads API, then use
|
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.BR pthread_setschedparam (3),
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.BR pthread_getschedparam (3),
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and
|
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.BR pthread_setschedprio (3),
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instead of the
|
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.BR sched_* (2)
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system calls.)
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.PP
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|
Originally, Standard Linux was intended as a general-purpose operating
|
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system being able to handle background processes, interactive
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applications, and less demanding real-time applications (applications that
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need to usually meet timing deadlines).
|
|
Although the Linux kernel 2.6
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|
allowed for kernel preemption and the newly introduced O(1) scheduler
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ensures that the time needed to schedule is fixed and deterministic
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irrespective of the number of active tasks, true real-time computing
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was not possible up to kernel version 2.6.17.
|
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.SS Real-time features in the mainline Linux kernel
|
|
.\" FIXME . Probably this text will need some minor tweaking
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|
.\" by about the time of 2.6.30; ask Carsten Emde about this then.
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|
From kernel version 2.6.18 onward, however, Linux is gradually
|
|
becoming equipped with real-time capabilities,
|
|
most of which are derived from the former
|
|
.I realtime-preempt
|
|
patches developed by Ingo Molnar, Thomas Gleixner,
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Steven Rostedt, and others.
|
|
Until the patches have been completely merged into the
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mainline kernel
|
|
(this is expected to be around kernel version 2.6.30),
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|
they must be installed to achieve the best real-time performance.
|
|
These patches are named:
|
|
.in +4n
|
|
.nf
|
|
|
|
patch-\fIkernelversion\fP-rt\fIpatchversion\fP
|
|
.fi
|
|
.in
|
|
.PP
|
|
and can be downloaded from
|
|
.UR http://www.kernel.org\:/pub\:/linux\:/kernel\:/projects\:/rt/
|
|
.UE .
|
|
|
|
Without the patches and prior to their full inclusion into the mainline
|
|
kernel, the kernel configuration offers only the three preemption classes
|
|
.BR CONFIG_PREEMPT_NONE ,
|
|
.BR CONFIG_PREEMPT_VOLUNTARY ,
|
|
and
|
|
.B CONFIG_PREEMPT_DESKTOP
|
|
which respectively provide no, some, and considerable
|
|
reduction of the worst-case scheduling latency.
|
|
|
|
With the patches applied or after their full inclusion into the mainline
|
|
kernel, the additional configuration item
|
|
.B CONFIG_PREEMPT_RT
|
|
becomes available.
|
|
If this is selected, Linux is transformed into a regular
|
|
real-time operating system.
|
|
The FIFO and RR scheduling policies that can be selected using
|
|
.BR sched_setscheduler ()
|
|
are then used to run a thread
|
|
with true real-time priority and a minimum worst-case scheduling latency.
|
|
.SH BUGS
|
|
POSIX says that on success,
|
|
.SH SEE ALSO
|
|
.ad l
|
|
.nh
|
|
.BR chrt (1),
|
|
.BR getpriority (2),
|
|
.BR mlock (2),
|
|
.BR mlockall (2),
|
|
.BR munlock (2),
|
|
.BR munlockall (2),
|
|
.BR nice (2),
|
|
.BR sched_get_priority_max (2),
|
|
.BR sched_get_priority_min (2),
|
|
.BR sched_getscheduler (2),
|
|
.BR sched_getaffinity (2),
|
|
.BR sched_getparam (2),
|
|
.BR sched_rr_get_interval (2),
|
|
.BR sched_setaffinity (2),
|
|
.BR sched_setscheduler (2),
|
|
.BR sched_setparam (2),
|
|
.BR sched_yield (2),
|
|
.BR setpriority (2),
|
|
.BR pthread_getaffinity_np (3),
|
|
.BR pthread_setaffinity_np (3),
|
|
.BR sched_getcpu (3),
|
|
.BR capabilities (7),
|
|
.BR cpuset (7)
|
|
.ad
|
|
.PP
|
|
.I Programming for the real world \- POSIX.4
|
|
by Bill O. Gallmeister, O'Reilly & Associates, Inc., ISBN 1-56592-074-0.
|
|
.PP
|
|
The Linux kernel source files
|
|
.IR Documentation/scheduler/sched-deadline.txt ,
|
|
.IR Documentation/scheduler/sched-rt-group.txt ,
|
|
and
|
|
.IR Documentation/scheduler/sched-nice-designtxt
|