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Realtime Linux

Linux can grant RealTime (RT) rights to users, allowing RT applications to run as normal users. For this purpose, a customized kernel must be compiled and a corresponding module must be created. The following instructions are based on a Debian etch and are taken from the file /usr/share/doc/realtime-lsm/README.Debian after installation and adapted.

  • aptitude install realtime-lsm
  • aptitude install realtime-lsm-source
  • aptitude install linux-source*
  • aptitude install module-assistent
  • Unpack the kernel sources in the director /usr/src
  • Copy the kernel configuration: cp /boot/config* /usr/src/linux-source*/.config
  • Change of the option CONFIG_SECURITY_CAPABILITIES=y to CONFIG_SECURITY_CAPABILITIES=m in .config
  • Compile Kernel: make-kpkg –initrd –revision 1 –append-to-version -1-lsm kernel_image
  • Install Kernel: dpkg -i /usr/src/*-1-lsm*.deb
  • Edit /etc/grub/menu.lst
  • m-a prepare
  • m-a build realtime-lsm
  • Edit /etc/default/realtime:
    • ENABLE=yes
    • PARAMETERS=„gid=1000 any=0 mlock=1“ (gid is the GROUP-ID of the realtime goup)
    • jack hard priority 60
    • jack hard nice -19
    • jack hard rtprio 89
  • Reboot the new Kernel (*-1-lsm*)

Additional packages

The package schedutils is recommended for installation. It contains:

  • chrt
  • ionice
  • taskset

Manpages:

CHRT(1)                       Linux User’s Manual                      CHRT(1)



NAME
       chrt - manipulate real-time attributes of a process

SYNOPSIS
       chrt [options] [prio] [pid | command [arg]...]

DESCRIPTION
       chrt(1)  sets  or  retrieves  the real-time scheduling attributes of an
       existing PID or runs COMMAND with the given  attributes.   Both  policy
       (one  of  SCHED_FIFO, SCHED_RR, or SCHED_OTHER) and priority can be set
       and retrieved.

OPTIONS
       -p, --pid
              operate on an existing PID and do not launch a new task



       -f, --fifo
              set scheduling policy to SCHED_FIFO

       -m, --max
              show minimum and maximum valid priorities, then exit

       -o, --other
              set policy scheduling policy to SCHED_OTHER

       -r, --rr
              set scheduling policy to SCHED_RR (the default)

       -h, --help
              display usage information and exit

       -v, --version
              output version information and exit

USAGE
       The default behavior is to run a new command::
              chrt [prio] -- [command] [arguments]

       You can also retrieve the real-time attributes  of  an  existing
       task:
              chrt -p [pid]

       Or set them:
              chrt -p [prio] [pid]

PERMISSIONS
       A user  must  possess  CAP_SYS_NICE  to  change  the  scheduling
       attributes  of  a process.  Any user can retrieve the scheduling
       information.

AUTHOR
       Written by Robert M. Love.

REPORTING BUGS
       Report bugs to <rml@tech9.net>.

COPYRIGHT
       Copyright © 2004 Robert M. Love
       This is free software; see the source  for  copying  conditions.
       There  is  NO  warranty; not even for MERCHANTABILITY or FITNESS
       FOR A PARTICULAR PURPOSE.

SEE ALSO
       taskset(1), nice(1), renice(1)

       See  sched_setscheduler(2)  for  a  description  of  the   Linux
       scheduling scheme.



schedutils                         Apr 2003                            CHRT(1)
ionice(1)                                                            ionice(1)



NAME
       ionice - get/set program io scheduling class and priority

SYNOPSIS
       ionice [-c] [-n] [-p] [COMMAND [ARG...]]


DESCRIPTION
       This  program  sets the io scheduling class and priority for a program.
       As of this writing, Linux supports 3 scheduling classes:

       Idle.  A program running with idle io priority will only get disk  time
       when no other program has asked for disk io for a defined grace period.
       The impact of idle io processes on normal  system  activity  should  be
       zero. This scheduling class does not take a priority argument.

       Best effort.  This is the default scheduling class for any process that
       hasn’t asked for a specific io priority. Programs inherit the CPU  nice
       setting  for  io  priorities. This class takes a priority argument from
       0-7, with lower number being higher priority. Programs running  at  the
       same best effort priority are served in a round-robin fashion.

       Real  time.  The RT scheduling class is given first access to the disk,
       regardless of what else is going on in the system. Thus  the  RT  class
       needs  to  be used with some care, as it can starve other processes. As
       with the best effort class, 8 priority levels are defined denoting  how
       big  a  time slice a given process will receive on each scheduling win‐
       dow.

       If no arguments or just -p is given, ionice will query the  current  io
       scheduling class and priority for that process.


OPTIONS
       -c     The  scheduling class. 1 for real time, 2 for best-effort, 3 for
              idle.

       -n     The scheduling class data. This defines the class data,  if  the
              class accepts an argument. For real time and best-effort, 0-7 is
              valid data.

       -p     Pass in a process pid to change an already running  process.  If
              this  argument  is not given, ionice will run the listed program
              with the given parameters.


EXAMPLES
       # ionice -c3 -p89

       Sets process with PID 89 as an idle io process.

       # ionice -c2 -n0 bash

       Runs ’bash’ as a best-effort program with highest priority.

       # ionice -p89

       Returns the class and priority of the process with PID 89.


NOTES
       Linux supports io scheduling priorities and classes since  2.6.13  with
       the CFQ io scheduler.


AUTHORS
       Jens Axboe <axboe@suse.de>



ionice                            August 2005                        ionice(1)
Formatiere taskset(1) neu, bitte warten...
TASKSET(1)                    Linux User’s Manual                   TASKSET(1)



NAME
       taskset - retrieve or set a processes’s CPU affinity

SYNOPSIS
       taskset [options] [mask | list ] [pid | command [arg]...]

DESCRIPTION
       taskset  is  used to set or retrieve the CPU affinity of a running pro‐
       cess given its PID or to launch a new COMMAND with a given  CPU  affin‐
       ity.   CPU affinity is a scheduler property that "bonds" a process to a
       given set of CPUs on the system.  The Linux scheduler  will  honor  the
       given  CPU  affinity  and  the  process will not run on any other CPUs.
       Note that the Linux scheduler also supports natural CPU  affinity:  the
       scheduler attempts to keep processes on the same CPU as long as practi‐
       cal for performance reasons.  Therefore, forcing a specific CPU  affin‐
       ity is useful only in certain applications.

       The CPU affinity is represented as a bitmask, with the lowest order bit
       corresponding to the first logical CPU and the highest order bit corre‐
       sponding  to  the  last logical CPU.  Not all CPUs may exist on a given
       system but a mask may specify more CPUs than are present.  A  retrieved
       mask  will  reflect only the bits that correspond to CPUs physically on
       the system.  If an invalid mask is given (i.e., one that corresponds to
       no  valid  CPUs on the current system) an error is returned.  The masks
       are typically given in hexadecimal.  For example,

       0x00000001
              is processor #0

       0x00000003
              is processors #0 and #1

       0xFFFFFFFF
              is all processors (#0 through #31)

       When taskset returns, it is guaranteed that the given program has  been
       scheduled to a legal CPU.

OPTIONS
       -p, --pid
              operate on an existing PID and not launch a new task

       -c, --cpu-list
              specifiy  a  numerical  list of processors instead of a bitmask.
              The list may contain multiple items,  separated  by  comma,  and
              ranges.  For example, 0,5,7,9-11.

       -h, --help
              display usage information and exit

       -V, --version
              output version information and exit

USAGE
       The  default  behavior  is  to  run a new command with a given affinity
       mask:
              taskset [mask] -- [command] [arguments]

       You can also retrieve the CPU affinity of an existing task:
              taskset -p [pid]

       Or set it:
              taskset -p [mask] [pid]

PERMISSIONS
       A  user  must possess CAP_SYS_NICE to change the CPU affinity of a pro‐
       cess.  Any user can retrieve the affinity mask.

AUTHOR
       Written by Robert M. Love.

REPORTING BUGS
       Report bugs to <rml@tech9.net>.

COPYRIGHT
       Copyright © 2004 Robert M. Love
       This is free software; see the source for copying conditions.  There is
       NO  warranty;  not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
       PURPOSE.

SEE ALSO
       chrt(1), nice(1), renice(1), sched_setaffinity(2), sched_getaffinity(2)

       See  sched_setscheduler(2)  for  a  description of the Linux scheduling
       scheme.



schedutils                         Apr 2003                         TASKSET(1)
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