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Add text on real-time features of mainline Linux kernel.
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Michael Kerrisk
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.\" Hey Emacs! This file is -*- nroff -*- source.
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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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.\"
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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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@ -33,6 +34,9 @@
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.\" A couple of grammar clean-ups
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.\" Modified 2004-05-27 by Michael Kerrisk <mtk-manpages@gmx.net>
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.\" 2005-03-23, mtk, Added description of SCHED_BATCH.
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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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.\"
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.TH SCHED_SETSCHEDULER 2 2006-03-23 "Linux" "Linux Programmer's Manual"
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.SH NAME
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@ -326,33 +330,59 @@ and details vary across systems.
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For example, the Solaris 7 manual page says that
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the real of effective user ID of the calling process
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match the real user ID or the save set-user-ID of the target process.
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Standard Linux is a general-purpose operating system
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and can handle background processes,
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interactive applications, and soft real-time applications
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(applications that need to usually meet timing deadlines).
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This man page is directed at these kinds of applications.
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.PP
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Standard Linux is
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.I not
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designed to support
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hard real-time applications, that is, applications in which deadlines
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(often much shorter than a second) must be guaranteed or the system
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will fail catastrophically.
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Like all general-purpose operating systems, Linux
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is designed to maximize average case performance
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instead of worst case performance.
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Linux's worst case performance for
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interrupt handling is much poorer than its average case, its various
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kernel locks (such as for SMP) produce long maximum wait times, and
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many of its performance improvement techniques decrease average time by
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increasing worst-case time.
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For most situations, that's what you want, but
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if you truly are developing a hard real-time application,
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consider using hard real-time extensions to Linux such as
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RTLinux (http://www.rtlinux.org) or RTAI (http://www.rtai.org)
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or use a different operating system
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designed specifically for hard real-time applications.
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Originally, Standard Linux was intended as a general-purpose operating
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system being able to handle background processes, interactive
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applications, and less demanding real-time applications (applications that
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need to usually meet timing deadlines).
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Although the Linux kernel 2.6
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allowed for kernel preemption and the newly introduced O(1) scheduler
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ensures that the time needed to schedule is fixed and deterministic
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irrespective of the number of active tasks, true real-time computing
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was not possible up to kernel version 2.6.17.
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.SS Real-time features in the mainline Linux kernel
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.\" FIXME . Probably this text will need tome minor tweaking
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.\" by about the time of 2.6.25; ask Carsten Emde about this then.
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From kernel version 2.6.18 onwards, however, Linux is gradually
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becoming equipped with real-time capabilities,
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most of which are derived from the former
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realtime-preempt patches developed by Ingo Molnar, Thomas Gleixner and
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others.
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Until the patches have been completely merged into the
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mainline kernel
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(this is expected to be around kernel version 2.6.24 or 2.6.25),
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the realtime-preempt patches must be installed to achieve the best
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realtime performance.
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These patches are named:
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.in +0.5i
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.nf
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patch-\fIkernelversion\fP-rt\fIpatchversion\fP
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.fi
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.in
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.PP
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and can be downloaded from
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.IR http://people.redhat.com/mingo/realtime-preempt/ .
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Without the patches and prior to their full inclusion into the mainline
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kernel, the kernel configuration offers only the three preemption classes
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.BR CONFIG_PREEMPT_NONE ,
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.BR CONFIG_PREEMPT_VOLUNTARY ,
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and
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.BR CONFIG_PREEMPT_DESKTOP
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which respectively provide no, some, and considerable
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reduction of the worst-case scheduling latency.
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With the patches applied or after their full inclusion into the mainline
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kernel, the additional configuration item
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.BR CONFIG_PREEMPT_RT
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becomes available.
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If this is selected, Linux is transformed into a regular
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real-time operating system.
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The FIFO and RR scheduling policies that can be selected using
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.BR sched_setscheduler ()
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are then used to run a process
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with true real-time priority and a minimum worst-case scheduling latency.
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.SH "SEE ALSO"
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.BR getpriority (2),
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.BR mlock (2),
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