mirror of https://github.com/mkerrisk/man-pages
sched.7: New page providing an overview of the scheduling APIs
Most of this content derives from sched_setscheduler(2). In preparation for adding a sched_setattr(2) page, it makes sense to isolate out this general content to a separate page that is referred to by the other scheduling pages. Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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.\" 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
|
||||
.\" 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 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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|
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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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Since Linux 2.6.32, the
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.B SCHED_RESET_ON_FORK
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flag can be ORed in
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.I policy
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when calling
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.BR sched_setscheduler ().
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As a result of including this flag, children created by
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.BR fork (2)
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do not inherit privileged scheduling policies.
|
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This 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
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.BR SCHED_RESET_ON_FORK
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flag is specified,
|
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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
|
||||
.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
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.BR SCHED_RESET_ON_FORK
|
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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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The
|
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.B SCHED_RESET_ON_FORK
|
||||
flag is visible in the policy value returned by
|
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.BR sched_getscheduler ()
|
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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
|
||||
(i.e., the thread specified by
|
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.IR pid )
|
||||
whose policy is being changed.
|
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|
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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
|
||||
policies.
|
||||
The rules for changing scheduling policy and priority are as follows:
|
||||
.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.
|
||||
.IP *
|
||||
Special rules apply for the
|
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.BR SCHED_IDLE .
|
||||
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
|
||||
policy so long as its nice value falls within the range permitted by its
|
||||
.BR RLIMIT_NICE
|
||||
resource limit (see
|
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.BR getrlimit (2)).
|
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.PP
|
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Privileged
|
||||
.RB ( CAP_SYS_NICE )
|
||||
threads ignore the
|
||||
.B RLIMIT_RTPRIO
|
||||
limit; as with older kernels,
|
||||
they can make arbitrary changes to scheduling policy and priority.
|
||||
See
|
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.BR getrlimit (2)
|
||||
for further information on
|
||||
.BR RLIMIT_RTPRIO .
|
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.SS Response time
|
||||
A blocked high priority thread waiting for the I/O has a certain
|
||||
response time before it is scheduled again.
|
||||
The device driver writer
|
||||
can greatly reduce this response time by using a "slow interrupt"
|
||||
interrupt handler.
|
||||
.\" as described in
|
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.\" .BR request_irq (9).
|
||||
.SS Miscellaneous
|
||||
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).
|
||||
|
||||
Memory locking is usually needed for real-time processes to avoid
|
||||
paging delays; this can be done with
|
||||
.BR mlock (2)
|
||||
or
|
||||
.BR mlockall (2).
|
||||
|
||||
Since a nonblocking infinite loop in a thread scheduled under
|
||||
\fBSCHED_FIFO\fP or \fBSCHED_RR\fP will block all threads with lower
|
||||
priority forever, a software developer should always keep available on
|
||||
the console a shell scheduled under a higher static priority than the
|
||||
tested application.
|
||||
This will allow an emergency kill of tested
|
||||
real-time applications that do not block or terminate as expected.
|
||||
See also the description of the
|
||||
.BR RLIMIT_RTTIME
|
||||
resource limit in
|
||||
.BR getrlimit (2).
|
||||
|
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POSIX systems on which
|
||||
.BR sched_setscheduler ()
|
||||
and
|
||||
.BR sched_getscheduler ()
|
||||
are available define
|
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.B _POSIX_PRIORITY_SCHEDULING
|
||||
in \fI<unistd.h>\fP.
|
||||
.SH RETURN VALUE
|
||||
On success,
|
||||
.BR sched_setscheduler ()
|
||||
returns zero.
|
||||
On success,
|
||||
.BR sched_getscheduler ()
|
||||
returns the policy for the thread (a nonnegative integer).
|
||||
On error, \-1 is returned, and
|
||||
.I errno
|
||||
is set appropriately.
|
||||
.SH ERRORS
|
||||
.TP
|
||||
.B EINVAL
|
||||
The scheduling \fIpolicy\fP is not one of the recognized policies,
|
||||
\fIparam\fP is NULL,
|
||||
or \fIparam\fP does not make sense for the \fIpolicy\fP.
|
||||
.TP
|
||||
.B EPERM
|
||||
The calling thread does not have appropriate privileges.
|
||||
.TP
|
||||
.B ESRCH
|
||||
The thread whose ID is \fIpid\fP could not be found.
|
||||
.SH CONFORMING TO
|
||||
POSIX.1-2001 (but see BUGS below).
|
||||
The \fBSCHED_BATCH\fP and \fBSCHED_IDLE\fP policies are Linux-specific.
|
||||
.SH NOTES
|
||||
POSIX.1 does not detail the permissions that an unprivileged
|
||||
thread requires in order to call
|
||||
.BR sched_setscheduler (),
|
||||
and details vary across systems.
|
||||
For example, the Solaris 7 manual page says that
|
||||
the real or effective user ID of the caller must
|
||||
match the real user ID or the save set-user-ID of the target.
|
||||
.PP
|
||||
The scheduling policy and parameters are in fact per-thread
|
||||
attributes on Linux.
|
||||
The value returned from a call to
|
||||
.BR gettid (2)
|
||||
can be passed in the argument
|
||||
.IR pid .
|
||||
Specifying
|
||||
.I pid
|
||||
as 0 will operate on the attribute for the calling thread,
|
||||
and passing the value returned from a call to
|
||||
.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
|
||||
.BR pthread_setschedparam (3),
|
||||
.BR pthread_getschedparam (3),
|
||||
and
|
||||
.BR pthread_setschedprio (3),
|
||||
instead of the
|
||||
.BR sched_* (2)
|
||||
system calls.)
|
||||
.PP
|
||||
Originally, Standard Linux was intended as a general-purpose operating
|
||||
system being able to handle background processes, interactive
|
||||
applications, and less demanding real-time applications (applications that
|
||||
need to usually meet timing deadlines).
|
||||
Although the Linux kernel 2.6
|
||||
allowed for kernel preemption and the newly introduced O(1) scheduler
|
||||
ensures that the time needed to schedule is fixed and deterministic
|
||||
irrespective of the number of active tasks, true real-time computing
|
||||
was not possible up to kernel version 2.6.17.
|
||||
.SS Real-time features in the mainline Linux kernel
|
||||
.\" FIXME . Probably this text will need some minor tweaking
|
||||
.\" by about the time of 2.6.30; ask Carsten Emde about this then.
|
||||
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,
|
||||
Steven Rostedt, and others.
|
||||
Until the patches have been completely merged into the
|
||||
mainline kernel
|
||||
(this is expected to be around kernel version 2.6.30),
|
||||
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_getaffinity (2),
|
||||
.BR sched_getparam (2),
|
||||
.BR sched_rr_get_interval (2),
|
||||
.BR sched_setaffinity (2),
|
||||
.BR sched_setparam (2),
|
||||
.BR sched_yield (2),
|
||||
.BR setpriority (2),
|
||||
.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 file
|
||||
.I Documentation/scheduler/sched-rt-group.txt
|
Loading…
Reference in New Issue