====== jackfreqd ======

Inspired by discussion with Fernando Lopez-Lezcano on [[http://lists.linuxaudio.org/pipermail/linux-audio-user/2010-December/074539.html|LAU]] and [[http://permalink.gmane.org/gmane.comp.audio.jackit/22839|jack-dev]]. I became motivated to create a tool that changes the CPU frequency according to DSP-load in order to address an issue on multi-core systems where system-load may still be too low for a default CPU governor to react while DSP load is already at the limit, causing x-runs.

''jackfreqd'' is heavily based on [[http://www.deater.net/john/powernowd.html|powernowd]]. Instead of taking CPU load as parameter for deciding on the CPU frequency ''jackfreqd'' uses [[http://jackaudio.org|JACK]] DSP-load and ''jackfreqd'' only supports the powernowd's //aggressive// mode ((Aggressive mode changes frequency by a sawtooth function. The system immediately jumps to the highest frequency whenever usage exceeds a given threshold (//high-water-mark//), and decreases by "step" Hz as usage drops below a //low-water-mark//)). Optionally ''jackfreqd'' can also take CPU load into account which comes in handy when the JACK-daemon is temporarily unavailable or if frequency-scaling should also be done for on non-audio processes.

In order to gain access to modify CPU-speed and do so quickly, ''jackfreqd'' needs to be run as //root//. ''jackfreqd'' includes support to assume a given user-id which runs the JACK-daemon, and also to auto-detect a running JACK instances and (re) connect to it.

===== Source =====

In Feb 2020 Oleg Samarin has taken over maintenance. The repo was moved to github, please update your URL:

  * <del>[[http://rg42.org/gitweb/?p=jackfreqd.git|Repository, Changelog]]</del>
  * https://github.com/oleg68/jackfreqd

''jackfreqd'' can be distributed in terms of the [[http://www.gnu.org/licenses/gpl2.html|GNU General Public License]].

===== Usage =====

Consult the man page ''man jackfreqd'' or run ''jackfreqd -h'' for details.

Note that, while CPU frequency scaling is usually not a problem with JACK, Front-Side-Bus frequency scaling and PCI power-saving often causes x-runs with JACK. The latter can usually be disabled in the BIOS.

===== Discussion with Fernando =====
>>> //Fernando://
>>> To reproduce the issue you need to be quite "tuned" (in the wrong way),
>>> you need an application that uses two cores at about 50-60% and then you
>>> change the connections so that they have to execute in series in jackd.
>>> Before the connection change everything is fine as they execute in
>>> parallel. After the connection no cores trigger the cpuspeed scaling but
>>> jackd misses the deadline anyway... In my case it was even worse, when I
>>> set the speed to "high" it would almost work - doing other things would
>>> trigger xruns and then to cap it off stopping cpuspeed would push things
>>> over the edge and I would get no xruns. Bad luck in a way...
>>> 
>> //Robin://
>> yes, there is a short delay when the CPU switches speeds, but it should
>> be irrelevant. I intent 'jackfreqd' to do the same as you do: switch to
>> "high CPU performance" when starting serious audio work and go back to
>> low-power only at the end of the session.
>>
>> It uses very small High- and Low-watermarks: as soon as the DSP load
>> goes above 50% : CPU freq switches to 100%.
>> If and only if DSP load drops below 10% the CPU-speed is reduced by
>> one-step again.
>>
>> There can be a x-run at the start of the session: If you manage to go
>> from <50% to >100% DSP load in one step.. (since 0.0.2 jackfreq checks
>> DSP load periodically and on each JACK graph change. The algorithm can
>> be optimized: lower the periodic update period after each graph-change
>> for a few cycles..).  I still need to play around to find good defaults
>> for the high/low watermarks, etc.. For the time being I'm aiming at a
>> proof-of-concept.
>>
> //Fernando://
>Sounds like a good solution and strategy and it would be very useful to have... 

{{tag>audio development floss jack}}