Realtime group scheduling doesn't know JACK
Jan 11, 2011, 14:32 (0 Talkback[s])
(Other stories by Jonathan Corbet)
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"Realtime scheduling for audio applications (or the lack
thereof) has been a recurring theme over a number of years. LWN
last visited this issue in 2009, when the addition of rtkit was put
forward as the (pulseaudio-based) solution for casual audio use.
Serious audio users - those using frameworks like JACK - have
always wanted more direct access to realtime scheduling, though.
That access has, for some years, been provided through resource
limits. Now it seems that a feature merged for the 2.6.25 kernel
is, two years later, beginning to cause grief for some JACK users.
The resulting discussion is an interesting illustration of
technical differences, how long it can take for new features to
filter through to users, and how one should best deal with the
kernel development community.
"The combination of the RLIMIT_RTPRIO and RLIMIT_RTTIME resource
limits allows the system administrator to give specific users the
ability to run tasks with realtime priority for a bounded period of
time. The feature is easily configured in /etc/security/limits.conf
and will prevent casual users from locking up the system with a
runaway realtime process. This feature is limited in its
flexibility, though, and is relatively easy to circumvent, so it
has never been seen as an ideal solution."