mirror of https://github.com/mkerrisk/man-pages
Add pointer to discussion of RLIMIT_RTTIME in getrlimit.2.
Rewrote and restructured various parts of the page for greater clarity.
This commit is contained in:
Michael Kerrisk
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.\"
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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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.\" 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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@ -37,13 +38,13 @@
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.\" 2007-07-10, Carsten Emde <Carsten.Emde@osadl.org>
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.\" Add text on real-time features that are currently being
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.\" added to the mainline kernel.
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.\" FIXME 2.6..25-rc2 has RLIMIT_RTTIME, which sould probably get
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.\" documented on this page.
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.\" 2008-05-07, mtk; Rewrote and restructured various parts of the page to
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.\" improve readability.
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.\"
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.TH SCHED_SETSCHEDULER 2 2008-03-07 "Linux" "Linux Programmer's Manual"
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.TH SCHED_SETSCHEDULER 2 2008-06-20 "Linux" "Linux Programmer's Manual"
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.SH NAME
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sched_setscheduler, sched_getscheduler \-
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set and get scheduling algorithm/parameters
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set and get scheduling policy/parameters
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.SH SYNOPSIS
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.nf
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.B #include <sched.h>
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@ -63,74 +64,87 @@ set and get scheduling algorithm/parameters
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.SH DESCRIPTION
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.BR sched_setscheduler ()
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sets both the scheduling policy and the associated parameters for the
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process identified by \fIpid\fP.
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process whose ID is specified in \fIpid\fP.
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If \fIpid\fP equals zero, the
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scheduler of the calling process will be set.
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scheduling policy and parameters of the calling process will be set.
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The interpretation of
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the parameter \fIparam\fP depends on the selected policy.
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Currently, the
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following scheduling policies are supported under Linux:
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.BR SCHED_FIFO ,
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.BR SCHED_RR ,
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.BR SCHED_OTHER ,
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Currently, Linux supports the following "normal" scheduling policies:
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.TP 14
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.BR SCHED_OTHER
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the standard round-robin time-sharing policy;
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.\" In the 2.6 kernel sources, SCHED_OTHER is actually called
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.\" SCHED_NORMAL.
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.BR SCHED_BATCH ,
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and
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.BR SCHED_IDLE ;
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their respective semantics are described below.
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.TP
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.BR SCHED_BATCH
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for "batch" style execution of processes; and
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.TP
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.BR SCHED_IDLE
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for running
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.I very
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low priority background jobs.
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.PP
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The following "real-time" policies are also supported,
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for special time-critical applications that need precise control over
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the way in which runnable processes are selected for execution:
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.TP 14
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.BR SCHED_FIFO
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a first-in, first-out policy; and
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.TP
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.BR SCHED_RR
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a round-robin policy.
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.PP
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The semantics of each of these policies are detailed below.
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.BR sched_getscheduler ()
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queries the scheduling policy currently applied to the process
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identified by \fIpid\fP.
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If \fIpid\fP equals zero, the policy of the
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calling process will be retrieved.
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.\"
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.SS Scheduling Policies
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The scheduler is the kernel part that decides which runnable process
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The scheduler is the kernel component that decides which runnable process
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will be executed by the CPU next.
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The Linux scheduler offers three
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different scheduling policies, one for normal processes and two for
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real-time applications.
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A static priority value \fIsched_priority\fP
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is assigned to each process and this value can be changed only via
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system calls.
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Conceptually, the scheduler maintains a list of runnable
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processes for each possible \fIsched_priority\fP value, and
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\fIsched_priority\fP can have a value in the range 0 to 99.
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In order
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to determine the process that runs next, the Linux scheduler looks for
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the non-empty list with the highest static priority and takes the
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process at the head of this list.
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The scheduling policy determines for
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each process, where it will be inserted into the list of processes
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with equal static priority and how it will move inside this list.
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Each process 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 processes on the system.
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\fBSCHED_OTHER\fP is the default universal time-sharing scheduler
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policy used by most processes.
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\fBSCHED_BATCH\fP is intended for "batch" style execution of processes.
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\fBSCHED_IDLE\fP is intended for running \fIvery\fP
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low priority background jobs.
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\fBSCHED_FIFO\fP and \fBSCHED_RR\fP are
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intended for special time-critical applications that need precise
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control over the way in which runnable processes are selected for
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execution.
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For processes 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 with \fBSCHED_OTHER\fP, \fBSCHED_BATCH\fP, or
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\fBSCHED_IDLE\fP
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must be assigned the static priority 0.
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Processes scheduled under \fBSCHED_FIFO\fP or
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\fBSCHED_RR\fP can have a static priority in the range 1 to 99.
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The system calls
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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 processes always have higher priority
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than normal processes.)
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Note well: POSIX.1-2001 only requires an implementation to support a
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minimum 32 distinct priority levels for the real-time policies,
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and some 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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can be used to find out the valid
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priority range for a scheduling policy in a portable way on all
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POSIX.1-2001 conforming systems.
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to find the range of priorities supported for a particular policy.
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All scheduling is preemptive: If a process with a higher static
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priority gets ready to run, the calling process will be preempted and
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returned into its wait list.
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Conceptually, the scheduler maintains a list of runnable
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processes for each possible \fIsched_priority\fP value.
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In order to determine which process runs next, the scheduler looks for
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the non-empty list with the highest static priority and selects the
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process at the head of this list.
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A process's scheduling policy determines
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where it will be inserted into the list of processes
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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 process with a higher static
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priority becomes ready to run, the currently running process
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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 only determines the
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ordering within the list of runnable processes with equal static
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priority.
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@ -142,13 +156,16 @@ it will always immediately preempt any currently running
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\fBSCHED_FIFO\fP is a simple scheduling
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algorithm without time slicing.
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For processes scheduled under the
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\fBSCHED_FIFO\fP policy, the following rules are applied: A
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\fBSCHED_FIFO\fP process that has been preempted by another process of
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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 process that has been preempted by another process 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 processes of higher priority are
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blocked again.
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.IP *
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When a \fBSCHED_FIFO\fP process 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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@ -162,13 +179,15 @@ it has the same priority.
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of the list.)
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.\" In 2.2.x and 2.4.x, the process is placed at the front of the queue
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.\" In 2.0.x, the Right Thing happened: the process went to the back -- MTK
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.IP *
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A process calling
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.BR sched_yield (2)
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will be
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put at the end of the list.
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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 process
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scheduled under the \fBSCHED_FIFO\fP policy in the wait list of
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runnable processes with equal static priority.
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A \fBSCHED_FIFO\fP
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process runs until either it is blocked by an I/O request, it is
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preempted by a higher priority process, or it calls
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@ -195,21 +214,19 @@ retrieved using
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.SS SCHED_OTHER: Default Linux time-sharing scheduling
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\fBSCHED_OTHER\fP can only be used at 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 processes that do not require special static priority
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intended for all processes that do not require the special
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real-time mechanisms.
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The process to run is chosen from the static
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priority 0 list based on a dynamic priority that is determined only
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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
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by
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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
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each time quantum the process is ready to run, but denied to run by
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the scheduler.
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This ensures fair progress among all \fBSCHED_OTHER\fP
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processes.
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and increased for each time quantum the process 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 processes.
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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 only be used at static priority 0.
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@ -222,6 +239,7 @@ that the process 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 process 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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@ -234,6 +252,7 @@ interactivity causing extra preemptions (between the workload's tasks).
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(Since Linux 2.6.23.)
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\fBSCHED_IDLE\fP can only be used 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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@ -244,7 +263,8 @@ policies).
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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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processes can set a non-zero static priority.
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processes can set a non-zero static priority (i.e., set a real-time
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scheduling policy).
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The only change that an unprivileged process can make is to set the
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.B SCHED_OTHER
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policy, and this can only be done if the effective user ID of the caller of
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@ -257,7 +277,7 @@ 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 process's
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priority for the
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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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@ -293,24 +313,28 @@ 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 algorithm and parameters across a
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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 algorithm and parameters are preserved across
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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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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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As a non-blocking end-less loop in a process scheduled under
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Since a non-blocking infinite loop in a process scheduled under
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\fBSCHED_FIFO\fP or \fBSCHED_RR\fP will block all processes 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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