Running JACK server in realtime mode.

The JACK server jackd has the capability to run in a real-time mode
which greatly decreases the chance of audio glitches. The real-time mode
is enabled by passing the -R or --realtime option to jackd when starting
the server. It is only possible to run jackd in real-time mode as a
non-root user by modifying your PAM configuration, PAM stands for
Pluggable Authentication Modules and is the primary authentification
mechanism used on Fedora. The primary source of PAM documentation can be
found at the following at http://www.kernel.org/pub/linux/libs/pam/

The specific PAM configuration file that needs to be modified is
/etc/security/limits.conf and it controls the system resource limits. It
is important to understand that modifying the resource limits
configuration files can decrease the security of your system.
Documentation specific to the resource limits PAM module can be found at
http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/pam-6.html#ss6.12

The resource limits that need to be changed to allow jackd to run in
realtime mode are named rtprio and memlock. To increase the limits for a
specific user named fred you would add the following to
/etc/security/limits.conf

fred - rtprio 20
fred - memlock 50000

The value of rtprio can be set in the range 0 - 99 where any value
greater that 0 will allow the user to change the scheduling policy to
"real-time". By default the JACK server requires a minimum rtprio
setting of 20 but jackd will accept a command line parameter -P or
--realtime-priority which will change the minimum required value of
rtprio that is needed, but the default of 20 is nearly always
sufficient.

The appropriate value for memlock is dependent on the amount of memory
present in the system but a minimum value of 50000(50MB) and a maximum
value of half the available memory can be used as a rough guideline. 

To verify that the resource limits have been modified you can use the
bash built-in ulimit command, for example:

$ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
max nice                        (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 8191
max locked memory       (kbytes, -l) 50000
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
max rt priority                 (-r) 20
stack size              (kbytes, -s) 10240
cpu time               (seconds, -t) unlimited
max user processes              (-u) 8191
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited
file locks                      (-x) unlimited

Keep in mind that you may have to re-login before changes to limits.conf
take effect.