mirror of https://github.com/mkerrisk/man-pages
1008 lines
34 KiB
Groff
1008 lines
34 KiB
Groff
.\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
|
|
.\" and Copyright (C) 2014 Peter Zijlstra <peterz@infradead.org>
|
|
.\" and Copyright (C) 2014 Juri Lelli <juri.lelli@gmail.com>
|
|
.\" Various pieces from the old sched_setscheduler(2) page
|
|
.\" Copyright (C) Tom Bjorkholm, Markus Kuhn & David A. Wheeler 1996-1999
|
|
.\" and Copyright (C) 2007 Carsten Emde <Carsten.Emde@osadl.org>
|
|
.\" and Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>
|
|
.\"
|
|
.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
|
|
.\" This is free documentation; you can redistribute it and/or
|
|
.\" modify it under the terms of the GNU General Public License as
|
|
.\" published by the Free Software Foundation; either version 2 of
|
|
.\" the License, or (at your option) any later version.
|
|
.\"
|
|
.\" The GNU General Public License's references to "object code"
|
|
.\" and "executables" are to be interpreted as the output of any
|
|
.\" document formatting or typesetting system, including
|
|
.\" intermediate and printed output.
|
|
.\"
|
|
.\" This manual is distributed in the hope that it will be useful,
|
|
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
.\" GNU General Public License for more details.
|
|
.\"
|
|
.\" You should have received a copy of the GNU General Public
|
|
.\" License along with this manual; if not, see
|
|
.\" <http://www.gnu.org/licenses/>.
|
|
.\" %%%LICENSE_END
|
|
.\"
|
|
.\" Worth looking at: http://rt.wiki.kernel.org/index.php
|
|
.\"
|
|
.TH SCHED 7 2021-03-22 "Linux" "Linux Programmer's Manual"
|
|
.SH NAME
|
|
sched \- overview of CPU scheduling
|
|
.SH DESCRIPTION
|
|
Since Linux 2.6.23, the default scheduler is CFS,
|
|
the "Completely Fair Scheduler".
|
|
The CFS scheduler replaced the earlier "O(1)" scheduler.
|
|
.\"
|
|
.SS API summary
|
|
Linux provides the following system calls for controlling
|
|
the CPU scheduling behavior, policy, and priority of processes
|
|
(or, more precisely, threads).
|
|
.TP
|
|
.BR nice (2)
|
|
Set a new nice value for the calling thread,
|
|
and return the new nice value.
|
|
.TP
|
|
.BR getpriority (2)
|
|
Return the nice value of a thread, a process group,
|
|
or the set of threads owned by a specified user.
|
|
.TP
|
|
.BR setpriority (2)
|
|
Set the nice value of a thread, a process group,
|
|
or the set of threads owned by a specified user.
|
|
.TP
|
|
.BR sched_setscheduler (2)
|
|
Set the scheduling policy and parameters of a specified thread.
|
|
.TP
|
|
.BR sched_getscheduler (2)
|
|
Return the scheduling policy of a specified thread.
|
|
.TP
|
|
.BR sched_setparam (2)
|
|
Set the scheduling parameters of a specified thread.
|
|
.TP
|
|
.BR sched_getparam (2)
|
|
Fetch the scheduling parameters of a specified thread.
|
|
.TP
|
|
.BR sched_get_priority_max (2)
|
|
Return the maximum priority available in a specified scheduling policy.
|
|
.TP
|
|
.BR sched_get_priority_min (2)
|
|
Return the minimum priority available in a specified scheduling policy.
|
|
.TP
|
|
.BR sched_rr_get_interval (2)
|
|
Fetch the quantum used for threads that are scheduled under
|
|
the "round-robin" scheduling policy.
|
|
.TP
|
|
.BR sched_yield (2)
|
|
Cause the caller to relinquish the CPU,
|
|
so that some other thread be executed.
|
|
.TP
|
|
.BR sched_setaffinity (2)
|
|
(Linux-specific)
|
|
Set the CPU affinity of a specified thread.
|
|
.TP
|
|
.BR sched_getaffinity (2)
|
|
(Linux-specific)
|
|
Get the CPU affinity of a specified thread.
|
|
.TP
|
|
.BR sched_setattr (2)
|
|
Set the scheduling policy and parameters of a specified thread.
|
|
This (Linux-specific) system call provides a superset of the functionality of
|
|
.BR sched_setscheduler (2)
|
|
and
|
|
.BR sched_setparam (2).
|
|
.TP
|
|
.BR sched_getattr (2)
|
|
Fetch the scheduling policy and parameters of a specified thread.
|
|
This (Linux-specific) system call provides a superset of the functionality of
|
|
.BR sched_getscheduler (2)
|
|
and
|
|
.BR sched_getparam (2).
|
|
.\"
|
|
.SS Scheduling policies
|
|
The scheduler is the kernel component that decides which runnable thread
|
|
will be executed by the CPU next.
|
|
Each thread has an associated scheduling policy and a \fIstatic\fP
|
|
scheduling priority,
|
|
.IR sched_priority .
|
|
The scheduler makes its decisions based on knowledge of the scheduling
|
|
policy and static priority of all threads on the system.
|
|
.PP
|
|
For threads scheduled under one of the normal scheduling policies
|
|
(\fBSCHED_OTHER\fP, \fBSCHED_IDLE\fP, \fBSCHED_BATCH\fP),
|
|
\fIsched_priority\fP is not used in scheduling
|
|
decisions (it must be specified as 0).
|
|
.PP
|
|
Processes scheduled under one of the real-time policies
|
|
(\fBSCHED_FIFO\fP, \fBSCHED_RR\fP) have a
|
|
\fIsched_priority\fP value in the range 1 (low) to 99 (high).
|
|
(As the numbers imply, real-time threads always have higher priority
|
|
than normal threads.)
|
|
Note well: POSIX.1 requires an implementation to support only a
|
|
minimum 32 distinct priority levels for the real-time policies,
|
|
and some systems supply just this minimum.
|
|
Portable programs should use
|
|
.BR sched_get_priority_min (2)
|
|
and
|
|
.BR sched_get_priority_max (2)
|
|
to find the range of priorities supported for a particular policy.
|
|
.PP
|
|
Conceptually, the scheduler maintains a list of runnable
|
|
threads for each possible \fIsched_priority\fP value.
|
|
In order to determine which thread runs next, the scheduler looks for
|
|
the nonempty list with the highest static priority and selects the
|
|
thread at the head of this list.
|
|
.PP
|
|
A thread's scheduling policy determines
|
|
where it will be inserted into the list of threads
|
|
with equal static priority and how it will move inside this list.
|
|
.PP
|
|
All scheduling is preemptive: if a thread with a higher static
|
|
priority becomes ready to run, the currently running thread
|
|
will be preempted and
|
|
returned to the wait list for its static priority level.
|
|
The scheduling policy determines the
|
|
ordering only within the list of runnable threads with equal static
|
|
priority.
|
|
.SS SCHED_FIFO: First in-first out scheduling
|
|
\fBSCHED_FIFO\fP can be used only with static priorities higher than
|
|
0, which means that when a \fBSCHED_FIFO\fP thread becomes runnable,
|
|
it will always immediately preempt any currently running
|
|
\fBSCHED_OTHER\fP, \fBSCHED_BATCH\fP, or \fBSCHED_IDLE\fP thread.
|
|
\fBSCHED_FIFO\fP is a simple scheduling
|
|
algorithm without time slicing.
|
|
For threads scheduled under the
|
|
\fBSCHED_FIFO\fP policy, the following rules apply:
|
|
.IP 1) 3
|
|
A running \fBSCHED_FIFO\fP thread that has been preempted by another thread of
|
|
higher priority will stay at the head of the list for its priority and
|
|
will resume execution as soon as all threads of higher priority are
|
|
blocked again.
|
|
.IP 2)
|
|
When a blocked \fBSCHED_FIFO\fP thread becomes runnable, it
|
|
will be inserted at the end of the list for its priority.
|
|
.IP 3)
|
|
If a call to
|
|
.BR sched_setscheduler (2),
|
|
.BR sched_setparam (2),
|
|
.BR sched_setattr (2),
|
|
.BR pthread_setschedparam (3),
|
|
or
|
|
.BR pthread_setschedprio (3)
|
|
changes the priority of the running or runnable
|
|
.B SCHED_FIFO
|
|
thread identified by
|
|
.I pid
|
|
the effect on the thread's position in the list depends on
|
|
the direction of the change to threads priority:
|
|
.RS
|
|
.IP \(bu 3
|
|
If the thread's priority is raised,
|
|
it is placed at the end of the list for its new priority.
|
|
As a consequence,
|
|
it may preempt a currently running thread with the same priority.
|
|
.IP \(bu
|
|
If the thread's priority is unchanged,
|
|
its position in the run list is unchanged.
|
|
.IP \(bu
|
|
If the thread's priority is lowered,
|
|
it is placed at the front of the list for its new priority.
|
|
.RE
|
|
.IP
|
|
According to POSIX.1-2008,
|
|
changes to a thread's priority (or policy) using any mechanism other than
|
|
.BR pthread_setschedprio (3)
|
|
should result in the thread being placed at the end of
|
|
the list for its priority.
|
|
.\" In 2.2.x and 2.4.x, the thread is placed at the front of the queue
|
|
.\" In 2.0.x, the Right Thing happened: the thread went to the back -- MTK
|
|
.IP 4)
|
|
A thread calling
|
|
.BR sched_yield (2)
|
|
will be put at the end of the list.
|
|
.PP
|
|
No other events will move a thread
|
|
scheduled under the \fBSCHED_FIFO\fP policy in the wait list of
|
|
runnable threads with equal static priority.
|
|
.PP
|
|
A \fBSCHED_FIFO\fP
|
|
thread runs until either it is blocked by an I/O request, it is
|
|
preempted by a higher priority thread, or it calls
|
|
.BR sched_yield (2).
|
|
.SS SCHED_RR: Round-robin scheduling
|
|
\fBSCHED_RR\fP is a simple enhancement of \fBSCHED_FIFO\fP.
|
|
Everything
|
|
described above for \fBSCHED_FIFO\fP also applies to \fBSCHED_RR\fP,
|
|
except that each thread is allowed to run only for a maximum time
|
|
quantum.
|
|
If a \fBSCHED_RR\fP thread has been running for a time
|
|
period equal to or longer than the time quantum, it will be put at the
|
|
end of the list for its priority.
|
|
A \fBSCHED_RR\fP thread that has
|
|
been preempted by a higher priority thread and subsequently resumes
|
|
execution as a running thread will complete the unexpired portion of
|
|
its round-robin time quantum.
|
|
The length of the time quantum can be
|
|
retrieved using
|
|
.BR sched_rr_get_interval (2).
|
|
.\" On Linux 2.4, the length of the RR interval is influenced
|
|
.\" by the process nice value -- MTK
|
|
.\"
|
|
.SS SCHED_DEADLINE: Sporadic task model deadline scheduling
|
|
Since version 3.14, Linux provides a deadline scheduling policy
|
|
.RB ( SCHED_DEADLINE ).
|
|
This policy is currently implemented using
|
|
GEDF (Global Earliest Deadline First)
|
|
in conjunction with CBS (Constant Bandwidth Server).
|
|
To set and fetch this policy and associated attributes,
|
|
one must use the Linux-specific
|
|
.BR sched_setattr (2)
|
|
and
|
|
.BR sched_getattr (2)
|
|
system calls.
|
|
.PP
|
|
A sporadic task is one that has a sequence of jobs, where each
|
|
job is activated at most once per period.
|
|
Each job also has a
|
|
.IR "relative deadline" ,
|
|
before which it should finish execution, and a
|
|
.IR "computation time" ,
|
|
which is the CPU time necessary for executing the job.
|
|
The moment when a task wakes up
|
|
because a new job has to be executed is called the
|
|
.IR "arrival time"
|
|
(also referred to as the request time or release time).
|
|
The
|
|
.IR "start time"
|
|
is the time at which a task starts its execution.
|
|
The
|
|
.I "absolute deadline"
|
|
is thus obtained by adding the relative deadline to the arrival time.
|
|
.PP
|
|
The following diagram clarifies these terms:
|
|
.PP
|
|
.in +4n
|
|
.EX
|
|
arrival/wakeup absolute deadline
|
|
| start time |
|
|
| | |
|
|
v v v
|
|
-----x--------xooooooooooooooooo--------x--------x---
|
|
|<- comp. time ->|
|
|
|<------- relative deadline ------>|
|
|
|<-------------- period ------------------->|
|
|
.EE
|
|
.in
|
|
.PP
|
|
When setting a
|
|
.B SCHED_DEADLINE
|
|
policy for a thread using
|
|
.BR sched_setattr (2),
|
|
one can specify three parameters:
|
|
.IR Runtime ,
|
|
.IR Deadline ,
|
|
and
|
|
.IR Period .
|
|
These parameters do not necessarily correspond to the aforementioned terms:
|
|
usual practice is to set Runtime to something bigger than the average
|
|
computation time (or worst-case execution time for hard real-time tasks),
|
|
Deadline to the relative deadline, and Period to the period of the task.
|
|
Thus, for
|
|
.BR SCHED_DEADLINE
|
|
scheduling, we have:
|
|
.PP
|
|
.in +4n
|
|
.EX
|
|
arrival/wakeup absolute deadline
|
|
| start time |
|
|
| | |
|
|
v v v
|
|
-----x--------xooooooooooooooooo--------x--------x---
|
|
|<-- Runtime ------->|
|
|
|<----------- Deadline ----------->|
|
|
|<-------------- Period ------------------->|
|
|
.EE
|
|
.in
|
|
.PP
|
|
The three deadline-scheduling parameters correspond to the
|
|
.IR sched_runtime ,
|
|
.IR sched_deadline ,
|
|
and
|
|
.IR sched_period
|
|
fields of the
|
|
.I sched_attr
|
|
structure; see
|
|
.BR sched_setattr (2).
|
|
These fields express values in nanoseconds.
|
|
.\" FIXME It looks as though specifying sched_period as 0 means
|
|
.\" "make sched_period the same as sched_deadline".
|
|
.\" This needs to be documented.
|
|
If
|
|
.IR sched_period
|
|
is specified as 0, then it is made the same as
|
|
.IR sched_deadline .
|
|
.PP
|
|
The kernel requires that:
|
|
.PP
|
|
sched_runtime <= sched_deadline <= sched_period
|
|
.PP
|
|
.\" See __checkparam_dl in kernel/sched/core.c
|
|
In addition, under the current implementation,
|
|
all of the parameter values must be at least 1024
|
|
(i.e., just over one microsecond,
|
|
which is the resolution of the implementation), and less than 2^63.
|
|
If any of these checks fails,
|
|
.BR sched_setattr (2)
|
|
fails with the error
|
|
.BR EINVAL .
|
|
.PP
|
|
The CBS guarantees non-interference between tasks, by throttling
|
|
threads that attempt to over-run their specified Runtime.
|
|
.PP
|
|
To ensure deadline scheduling guarantees,
|
|
the kernel must prevent situations where the set of
|
|
.B SCHED_DEADLINE
|
|
threads is not feasible (schedulable) within the given constraints.
|
|
The kernel thus performs an admittance test when setting or changing
|
|
.B SCHED_DEADLINE
|
|
policy and attributes.
|
|
This admission test calculates whether the change is feasible;
|
|
if it is not,
|
|
.BR sched_setattr (2)
|
|
fails with the error
|
|
.BR EBUSY .
|
|
.PP
|
|
For example, it is required (but not necessarily sufficient) for
|
|
the total utilization to be less than or equal to the total number of
|
|
CPUs available, where, since each thread can maximally run for
|
|
Runtime per Period, that thread's utilization is its
|
|
Runtime divided by its Period.
|
|
.PP
|
|
In order to fulfill the guarantees that are made when
|
|
a thread is admitted to the
|
|
.BR SCHED_DEADLINE
|
|
policy,
|
|
.BR SCHED_DEADLINE
|
|
threads are the highest priority (user controllable) threads in the
|
|
system; if any
|
|
.BR SCHED_DEADLINE
|
|
thread is runnable,
|
|
it will preempt any thread scheduled under one of the other policies.
|
|
.PP
|
|
A call to
|
|
.BR fork (2)
|
|
by a thread scheduled under the
|
|
.B SCHED_DEADLINE
|
|
policy fails with the error
|
|
.BR EAGAIN ,
|
|
unless the thread has its reset-on-fork flag set (see below).
|
|
.PP
|
|
A
|
|
.B SCHED_DEADLINE
|
|
thread that calls
|
|
.BR sched_yield (2)
|
|
will yield the current job and wait for a new period to begin.
|
|
.\"
|
|
.\" FIXME Calling sched_getparam() on a SCHED_DEADLINE thread
|
|
.\" fails with EINVAL, but sched_getscheduler() succeeds.
|
|
.\" Is that intended? (Why?)
|
|
.\"
|
|
.SS SCHED_OTHER: Default Linux time-sharing scheduling
|
|
\fBSCHED_OTHER\fP can be used at only static priority 0
|
|
(i.e., threads under real-time policies always have priority over
|
|
.B SCHED_OTHER
|
|
processes).
|
|
\fBSCHED_OTHER\fP is the standard Linux time-sharing scheduler that is
|
|
intended for all threads that do not require the special
|
|
real-time mechanisms.
|
|
.PP
|
|
The thread to run is chosen from the static
|
|
priority 0 list based on a \fIdynamic\fP priority that is determined only
|
|
inside this list.
|
|
The dynamic priority is based on the nice value (see below)
|
|
and is increased for each time quantum the thread is ready to run,
|
|
but denied to run by the scheduler.
|
|
This ensures fair progress among all \fBSCHED_OTHER\fP threads.
|
|
.PP
|
|
In the Linux kernel source code, the
|
|
.B SCHED_OTHER
|
|
policy is actually named
|
|
.BR SCHED_NORMAL .
|
|
.\"
|
|
.SS The nice value
|
|
The nice value is an attribute
|
|
that can be used to influence the CPU scheduler to
|
|
favor or disfavor a process in scheduling decisions.
|
|
It affects the scheduling of
|
|
.BR SCHED_OTHER
|
|
and
|
|
.BR SCHED_BATCH
|
|
(see below) processes.
|
|
The nice value can be modified using
|
|
.BR nice (2),
|
|
.BR setpriority (2),
|
|
or
|
|
.BR sched_setattr (2).
|
|
.PP
|
|
According to POSIX.1, the nice value is a per-process attribute;
|
|
that is, the threads in a process should share a nice value.
|
|
However, on Linux, the nice value is a per-thread attribute:
|
|
different threads in the same process may have different nice values.
|
|
.PP
|
|
The range of the nice value
|
|
varies across UNIX systems.
|
|
On modern Linux, the range is \-20 (high priority) to +19 (low priority).
|
|
On some other systems, the range is \-20..20.
|
|
Very early Linux kernels (Before Linux 2.0) had the range \-infinity..15.
|
|
.\" Linux before 1.3.36 had \-infinity..15.
|
|
.\" Since kernel 1.3.43, Linux has the range \-20..19.
|
|
.PP
|
|
The degree to which the nice value affects the relative scheduling of
|
|
.BR SCHED_OTHER
|
|
processes likewise varies across UNIX systems and
|
|
across Linux kernel versions.
|
|
.PP
|
|
With the advent of the CFS scheduler in kernel 2.6.23,
|
|
Linux adopted an algorithm that causes
|
|
relative differences in nice values to have a much stronger effect.
|
|
In the current implementation, each unit of difference in the
|
|
nice values of two processes results in a factor of 1.25
|
|
in the degree to which the scheduler favors the higher priority process.
|
|
This causes very low nice values (+19) to truly provide little CPU
|
|
to a process whenever there is any other
|
|
higher priority load on the system,
|
|
and makes high nice values (\-20) deliver most of the CPU to applications
|
|
that require it (e.g., some audio applications).
|
|
.PP
|
|
On Linux, the
|
|
.BR RLIMIT_NICE
|
|
resource limit can be used to define a limit to which
|
|
an unprivileged process's nice value can be raised; see
|
|
.BR setrlimit (2)
|
|
for details.
|
|
.PP
|
|
For further details on the nice value, see the subsections on
|
|
the autogroup feature and group scheduling, below.
|
|
.\"
|
|
.SS SCHED_BATCH: Scheduling batch processes
|
|
(Since Linux 2.6.16.)
|
|
\fBSCHED_BATCH\fP can be used only at static priority 0.
|
|
This policy is similar to \fBSCHED_OTHER\fP in that it schedules
|
|
the thread according to its dynamic priority
|
|
(based on the nice value).
|
|
The difference is that this policy
|
|
will cause the scheduler to always assume
|
|
that the thread is CPU-intensive.
|
|
Consequently, the scheduler will apply a small scheduling
|
|
penalty with respect to wakeup behavior,
|
|
so that this thread is mildly disfavored in scheduling decisions.
|
|
.PP
|
|
.\" The following paragraph is drawn largely from the text that
|
|
.\" accompanied Ingo Molnar's patch for the implementation of
|
|
.\" SCHED_BATCH.
|
|
.\" commit b0a9499c3dd50d333e2aedb7e894873c58da3785
|
|
This policy is useful for workloads that are noninteractive,
|
|
but do not want to lower their nice value,
|
|
and for workloads that want a deterministic scheduling policy without
|
|
interactivity causing extra preemptions (between the workload's tasks).
|
|
.\"
|
|
.SS SCHED_IDLE: Scheduling very low priority jobs
|
|
(Since Linux 2.6.23.)
|
|
\fBSCHED_IDLE\fP can be used only at static priority 0;
|
|
the process nice value has no influence for this policy.
|
|
.PP
|
|
This policy is intended for running jobs at extremely low
|
|
priority (lower even than a +19 nice value with the
|
|
.B SCHED_OTHER
|
|
or
|
|
.B SCHED_BATCH
|
|
policies).
|
|
.\"
|
|
.SS Resetting scheduling policy for child processes
|
|
Each thread has a reset-on-fork scheduling flag.
|
|
When this flag is set, children created by
|
|
.BR fork (2)
|
|
do not inherit privileged scheduling policies.
|
|
The reset-on-fork flag can be set by either:
|
|
.IP * 3
|
|
ORing the
|
|
.B SCHED_RESET_ON_FORK
|
|
flag into the
|
|
.I policy
|
|
argument when calling
|
|
.BR sched_setscheduler (2)
|
|
(since Linux 2.6.32);
|
|
or
|
|
.IP *
|
|
specifying the
|
|
.B SCHED_FLAG_RESET_ON_FORK
|
|
flag in
|
|
.IR attr.sched_flags
|
|
when calling
|
|
.BR sched_setattr (2).
|
|
.PP
|
|
Note that the constants used with these two APIs have different names.
|
|
The state of the reset-on-fork flag can analogously be retrieved using
|
|
.BR sched_getscheduler (2)
|
|
and
|
|
.BR sched_getattr (2).
|
|
.PP
|
|
The reset-on-fork feature is intended for media-playback applications,
|
|
and can be used to prevent applications evading the
|
|
.BR RLIMIT_RTTIME
|
|
resource limit (see
|
|
.BR getrlimit (2))
|
|
by creating multiple child processes.
|
|
.PP
|
|
More precisely, if the reset-on-fork flag is set,
|
|
the following rules apply for subsequently created children:
|
|
.IP * 3
|
|
If the calling thread has a scheduling policy of
|
|
.B SCHED_FIFO
|
|
or
|
|
.BR SCHED_RR ,
|
|
the policy is reset to
|
|
.BR SCHED_OTHER
|
|
in child processes.
|
|
.IP *
|
|
If the calling process has a negative nice value,
|
|
the nice value is reset to zero in child processes.
|
|
.PP
|
|
After the reset-on-fork flag has been enabled,
|
|
it can be reset only if the thread has the
|
|
.BR CAP_SYS_NICE
|
|
capability.
|
|
This flag is disabled in child processes created by
|
|
.BR fork (2).
|
|
.\"
|
|
.SS Privileges and resource limits
|
|
In Linux kernels before 2.6.12, only privileged
|
|
.RB ( CAP_SYS_NICE )
|
|
threads can set a nonzero static priority (i.e., set a real-time
|
|
scheduling policy).
|
|
The only change that an unprivileged thread can make is to set the
|
|
.B SCHED_OTHER
|
|
policy, and this can be done only if the effective user ID of the caller
|
|
matches the real or effective user ID of the target thread
|
|
(i.e., the thread specified by
|
|
.IR pid )
|
|
whose policy is being changed.
|
|
.PP
|
|
A thread must be privileged
|
|
.RB ( CAP_SYS_NICE )
|
|
in order to set or modify a
|
|
.BR SCHED_DEADLINE
|
|
policy.
|
|
.PP
|
|
Since Linux 2.6.12, the
|
|
.B RLIMIT_RTPRIO
|
|
resource limit defines a ceiling on an unprivileged thread's
|
|
static priority for the
|
|
.B SCHED_RR
|
|
and
|
|
.B SCHED_FIFO
|
|
policies.
|
|
The rules for changing scheduling policy and priority are as follows:
|
|
.IP * 3
|
|
If an unprivileged thread has a nonzero
|
|
.B RLIMIT_RTPRIO
|
|
soft limit, then it can change its scheduling policy and priority,
|
|
subject to the restriction that the priority cannot be set to a
|
|
value higher than the maximum of its current priority and its
|
|
.B RLIMIT_RTPRIO
|
|
soft limit.
|
|
.IP *
|
|
If the
|
|
.B RLIMIT_RTPRIO
|
|
soft limit is 0, then the only permitted changes are to lower the priority,
|
|
or to switch to a non-real-time policy.
|
|
.IP *
|
|
Subject to the same rules,
|
|
another unprivileged thread can also make these changes,
|
|
as long as the effective user ID of the thread making the change
|
|
matches the real or effective user ID of the target thread.
|
|
.IP *
|
|
Special rules apply for the
|
|
.BR SCHED_IDLE
|
|
policy.
|
|
In Linux kernels before 2.6.39,
|
|
an unprivileged thread operating under this policy cannot
|
|
change its policy, regardless of the value of its
|
|
.BR RLIMIT_RTPRIO
|
|
resource limit.
|
|
In Linux kernels since 2.6.39,
|
|
.\" commit c02aa73b1d18e43cfd79c2f193b225e84ca497c8
|
|
an unprivileged thread can switch to either the
|
|
.BR SCHED_BATCH
|
|
or the
|
|
.BR SCHED_OTHER
|
|
policy so long as its nice value falls within the range permitted by its
|
|
.BR RLIMIT_NICE
|
|
resource limit (see
|
|
.BR getrlimit (2)).
|
|
.PP
|
|
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
|
|
.BR getrlimit (2)
|
|
for further information on
|
|
.BR RLIMIT_RTPRIO .
|
|
.SS Limiting the CPU usage of real-time and deadline processes
|
|
A nonblocking infinite loop in a thread scheduled under the
|
|
.BR SCHED_FIFO ,
|
|
.BR SCHED_RR ,
|
|
or
|
|
.BR SCHED_DEADLINE
|
|
policy can potentially block all other threads from accessing
|
|
the CPU forever.
|
|
Prior to Linux 2.6.25, the only way of preventing a runaway real-time
|
|
process from freezing the system was to run (at the console)
|
|
a shell scheduled under a higher static priority than the tested application.
|
|
This allows an emergency kill of tested
|
|
real-time applications that do not block or terminate as expected.
|
|
.PP
|
|
Since Linux 2.6.25, there are other techniques for dealing with runaway
|
|
real-time and deadline processes.
|
|
One of these is to use the
|
|
.BR RLIMIT_RTTIME
|
|
resource limit to set a ceiling on the CPU time that
|
|
a real-time process may consume.
|
|
See
|
|
.BR getrlimit (2)
|
|
for details.
|
|
.PP
|
|
Since version 2.6.25, Linux also provides two
|
|
.I /proc
|
|
files that can be used to reserve a certain amount of CPU time
|
|
to be used by non-real-time processes.
|
|
Reserving CPU time in this fashion allows some CPU time to be
|
|
allocated to (say) a root shell that can be used to kill a runaway process.
|
|
Both of these files specify time values in microseconds:
|
|
.TP
|
|
.IR /proc/sys/kernel/sched_rt_period_us
|
|
This file specifies a scheduling period that is equivalent to
|
|
100% CPU bandwidth.
|
|
The value in this file can range from 1 to
|
|
.BR INT_MAX ,
|
|
giving an operating range of 1 microsecond to around 35 minutes.
|
|
The default value in this file is 1,000,000 (1 second).
|
|
.TP
|
|
.IR /proc/sys/kernel/sched_rt_runtime_us
|
|
The value in this file specifies how much of the "period" time
|
|
can be used by all real-time and deadline scheduled processes
|
|
on the system.
|
|
The value in this file can range from \-1 to
|
|
.BR INT_MAX \-1.
|
|
Specifying \-1 makes the run time the same as the period;
|
|
that is, no CPU time is set aside for non-real-time processes
|
|
(which was the Linux behavior before kernel 2.6.25).
|
|
The default value in this file is 950,000 (0.95 seconds),
|
|
meaning that 5% of the CPU time is reserved for processes that
|
|
don't run under a real-time or deadline scheduling policy.
|
|
.SS Response time
|
|
A blocked high priority thread waiting for 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
|
|
.\" .BR request_irq (9).
|
|
.SS Miscellaneous
|
|
Child processes inherit the scheduling policy and parameters across a
|
|
.BR fork (2).
|
|
The scheduling policy and parameters are preserved across
|
|
.BR execve (2).
|
|
.PP
|
|
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).
|
|
.\"
|
|
.SS The autogroup feature
|
|
.\" commit 5091faa449ee0b7d73bc296a93bca9540fc51d0a
|
|
Since Linux 2.6.38,
|
|
the kernel provides a feature known as autogrouping to improve interactive
|
|
desktop performance in the face of multiprocess, CPU-intensive
|
|
workloads such as building the Linux kernel with large numbers of
|
|
parallel build processes (i.e., the
|
|
.BR make (1)
|
|
.BR \-j
|
|
flag).
|
|
.PP
|
|
This feature operates in conjunction with the
|
|
CFS scheduler and requires a kernel that is configured with
|
|
.BR CONFIG_SCHED_AUTOGROUP .
|
|
On a running system, this feature is enabled or disabled via the file
|
|
.IR /proc/sys/kernel/sched_autogroup_enabled ;
|
|
a value of 0 disables the feature, while a value of 1 enables it.
|
|
The default value in this file is 1, unless the kernel was booted with the
|
|
.IR noautogroup
|
|
parameter.
|
|
.PP
|
|
A new autogroup is created when a new session is created via
|
|
.BR setsid (2);
|
|
this happens, for example, when a new terminal window is started.
|
|
A new process created by
|
|
.BR fork (2)
|
|
inherits its parent's autogroup membership.
|
|
Thus, all of the processes in a session are members of the same autogroup.
|
|
An autogroup is automatically destroyed when the last process
|
|
in the group terminates.
|
|
.PP
|
|
When autogrouping is enabled, all of the members of an autogroup
|
|
are placed in the same kernel scheduler "task group".
|
|
The CFS scheduler employs an algorithm that equalizes the
|
|
distribution of CPU cycles across task groups.
|
|
The benefits of this for interactive desktop performance
|
|
can be described via the following example.
|
|
.PP
|
|
Suppose that there are two autogroups competing for the same CPU
|
|
(i.e., presume either a single CPU system or the use of
|
|
.BR taskset (1)
|
|
to confine all the processes to the same CPU on an SMP system).
|
|
The first group contains ten CPU-bound processes from
|
|
a kernel build started with
|
|
.IR "make\ \-j10" .
|
|
The other contains a single CPU-bound process: a video player.
|
|
The effect of autogrouping is that the two groups will
|
|
each receive half of the CPU cycles.
|
|
That is, the video player will receive 50% of the CPU cycles,
|
|
rather than just 9% of the cycles,
|
|
which would likely lead to degraded video playback.
|
|
The situation on an SMP system is more complex,
|
|
.\" Mike Galbraith, 25 Nov 2016:
|
|
.\" I'd say something more wishy-washy here, like cycles are
|
|
.\" distributed fairly across groups and leave it at that, as your
|
|
.\" detailed example is incorrect due to SMP fairness (which I don't
|
|
.\" like much because [very unlikely] worst case scenario
|
|
.\" renders a box sized group incapable of utilizing more that
|
|
.\" a single CPU total). For example, if a group of NR_CPUS
|
|
.\" size competes with a singleton, load balancing will try to give
|
|
.\" the singleton a full CPU of its very own. If groups intersect for
|
|
.\" whatever reason on say my quad lappy, distribution is 80/20 in
|
|
.\" favor of the singleton.
|
|
but the general effect is the same:
|
|
the scheduler distributes CPU cycles across task groups such that
|
|
an autogroup that contains a large number of CPU-bound processes
|
|
does not end up hogging CPU cycles at the expense of the other
|
|
jobs on the system.
|
|
.PP
|
|
A process's autogroup (task group) membership can be viewed via the file
|
|
.IR /proc/[pid]/autogroup :
|
|
.PP
|
|
.in +4n
|
|
.EX
|
|
$ \fBcat /proc/1/autogroup\fP
|
|
/autogroup\-1 nice 0
|
|
.EE
|
|
.in
|
|
.PP
|
|
This file can also be used to modify the CPU bandwidth allocated
|
|
to an autogroup.
|
|
This is done by writing a number in the "nice" range to the file
|
|
to set the autogroup's nice value.
|
|
The allowed range is from +19 (low priority) to \-20 (high priority).
|
|
(Writing values outside of this range causes
|
|
.BR write (2)
|
|
to fail with the error
|
|
.BR EINVAL .)
|
|
.\" FIXME .
|
|
.\" Because of a bug introduced in Linux 4.7
|
|
.\" (commit 2159197d66770ec01f75c93fb11dc66df81fd45b made changes
|
|
.\" that exposed the fact that autogroup didn't call scale_load()),
|
|
.\" it happened that *all* values in this range caused a task group
|
|
.\" to be further disfavored by the scheduler, with \-20 resulting
|
|
.\" in the scheduler mildly disfavoring the task group and +19 greatly
|
|
.\" disfavoring it.
|
|
.\"
|
|
.\" A patch was posted on 23 Nov 2016
|
|
.\" ("sched/autogroup: Fix 64bit kernel nice adjustment";
|
|
.\" check later to see in which kernel version it lands.
|
|
.PP
|
|
The autogroup nice setting has the same meaning as the process nice value,
|
|
but applies to distribution of CPU cycles to the autogroup as a whole,
|
|
based on the relative nice values of other autogroups.
|
|
For a process inside an autogroup, the CPU cycles that it receives
|
|
will be a product of the autogroup's nice value
|
|
(compared to other autogroups)
|
|
and the process's nice value
|
|
(compared to other processes in the same autogroup.
|
|
.PP
|
|
The use of the
|
|
.BR cgroups (7)
|
|
CPU controller to place processes in cgroups other than the
|
|
root CPU cgroup overrides the effect of autogrouping.
|
|
.PP
|
|
The autogroup feature groups only processes scheduled under
|
|
non-real-time policies
|
|
.RB ( SCHED_OTHER ,
|
|
.BR SCHED_BATCH ,
|
|
and
|
|
.BR SCHED_IDLE ).
|
|
It does not group processes scheduled under real-time and
|
|
deadline policies.
|
|
Those processes are scheduled according to the rules described earlier.
|
|
.\"
|
|
.SS The nice value and group scheduling
|
|
When scheduling non-real-time processes (i.e., those scheduled under the
|
|
.BR SCHED_OTHER ,
|
|
.BR SCHED_BATCH ,
|
|
and
|
|
.BR SCHED_IDLE
|
|
policies), the CFS scheduler employs a technique known as "group scheduling",
|
|
if the kernel was configured with the
|
|
.BR CONFIG_FAIR_GROUP_SCHED
|
|
option (which is typical).
|
|
.PP
|
|
Under group scheduling, threads are scheduled in "task groups".
|
|
Task groups have a hierarchical relationship,
|
|
rooted under the initial task group on the system,
|
|
known as the "root task group".
|
|
Task groups are formed in the following circumstances:
|
|
.IP * 3
|
|
All of the threads in a CPU cgroup form a task group.
|
|
The parent of this task group is the task group of the
|
|
corresponding parent cgroup.
|
|
.IP *
|
|
If autogrouping is enabled,
|
|
then all of the threads that are (implicitly) placed in an autogroup
|
|
(i.e., the same session, as created by
|
|
.BR setsid (2))
|
|
form a task group.
|
|
Each new autogroup is thus a separate task group.
|
|
The root task group is the parent of all such autogroups.
|
|
.IP *
|
|
If autogrouping is enabled, then the root task group consists of
|
|
all processes in the root CPU cgroup that were not
|
|
otherwise implicitly placed into a new autogroup.
|
|
.IP *
|
|
If autogrouping is disabled, then the root task group consists of
|
|
all processes in the root CPU cgroup.
|
|
.IP *
|
|
If group scheduling was disabled (i.e., the kernel was configured without
|
|
.BR CONFIG_FAIR_GROUP_SCHED ),
|
|
then all of the processes on the system are notionally placed
|
|
in a single task group.
|
|
.PP
|
|
Under group scheduling,
|
|
a thread's nice value has an effect for scheduling decisions
|
|
.IR "only relative to other threads in the same task group" .
|
|
This has some surprising consequences in terms of the traditional semantics
|
|
of the nice value on UNIX systems.
|
|
In particular, if autogrouping
|
|
is enabled (which is the default in various distributions), then employing
|
|
.BR setpriority (2)
|
|
or
|
|
.BR nice (1)
|
|
on a process has an effect only for scheduling relative
|
|
to other processes executed in the same session
|
|
(typically: the same terminal window).
|
|
.PP
|
|
Conversely, for two processes that are (for example)
|
|
the sole CPU-bound processes in different sessions
|
|
(e.g., different terminal windows,
|
|
each of whose jobs are tied to different autogroups),
|
|
.IR "modifying the nice value of the process in one of the sessions"
|
|
.IR "has no effect"
|
|
in terms of the scheduler's decisions relative to the
|
|
process in the other session.
|
|
.\" More succinctly: the nice(1) command is in many cases a no-op since
|
|
.\" Linux 2.6.38.
|
|
.\"
|
|
A possibly useful workaround here is to use a command such as
|
|
the following to modify the autogroup nice value for
|
|
.I all
|
|
of the processes in a terminal session:
|
|
.PP
|
|
.in +4n
|
|
.EX
|
|
$ \fBecho 10 > /proc/self/autogroup\fP
|
|
.EE
|
|
.in
|
|
.SS Real-time features in the mainline Linux kernel
|
|
.\" FIXME . Probably this text will need some minor tweaking
|
|
.\" ask Carsten Emde about this.
|
|
Since kernel version 2.6.18, Linux is gradually
|
|
becoming equipped with real-time capabilities,
|
|
most of which are derived from the former
|
|
.I realtime\-preempt
|
|
patch set.
|
|
Until the patches have been completely merged into the
|
|
mainline kernel,
|
|
they must be installed to achieve the best real-time performance.
|
|
These patches are named:
|
|
.PP
|
|
.in +4n
|
|
.EX
|
|
patch\-\fIkernelversion\fP\-rt\fIpatchversion\fP
|
|
.EE
|
|
.in
|
|
.PP
|
|
and can be downloaded from
|
|
.UR http://www.kernel.org\:/pub\:/linux\:/kernel\:/projects\:/rt/
|
|
.UE .
|
|
.PP
|
|
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.
|
|
.PP
|
|
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 are then used to run a thread
|
|
with true real-time priority and a minimum worst-case scheduling latency.
|
|
.SH NOTES
|
|
The
|
|
.BR cgroups (7)
|
|
CPU controller can be used to limit the CPU consumption of
|
|
groups of processes.
|
|
.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.
|
|
.SH SEE ALSO
|
|
.ad l
|
|
.nh
|
|
.BR chcpu (1),
|
|
.BR chrt (1),
|
|
.BR lscpu (1),
|
|
.BR ps (1),
|
|
.BR taskset (1),
|
|
.BR top (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_getscheduler (2),
|
|
.BR sched_rr_get_interval (2),
|
|
.BR sched_setaffinity (2),
|
|
.BR sched_setparam (2),
|
|
.BR sched_setscheduler (2),
|
|
.BR sched_yield (2),
|
|
.BR setpriority (2),
|
|
.BR pthread_getaffinity_np (3),
|
|
.BR pthread_getschedparam (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 ,
|
|
.IR Documentation/scheduler/sched\-design\-CFS.txt ,
|
|
and
|
|
.IR Documentation/scheduler/sched\-nice\-design.txt
|