Guitarix at ultra-low latency - CPU affinity

[Rippert]

I've been using taskset to move the guitarix realtime threads to isolated cpus, but since changing presets can spawn new realtime threads, for the convolvers I assume, that are children of the main, non-realtime, thread, It is not a reliable method.

Can you tell me where the primary realtime thread and the convolver realtime threads, are spawned in the code? I will try to use a cpp method to set the affinity and make a new command line option for it.

[brummer10]

Hi The primary realtime thread is owned by jackd, not guitarix. The convolver threads been spawned by zita-convolver, /trunk/src/zita-convolver/zita-convolver.cc line 615, called from guitarix in gx_convolver.cpp line 216.

[Rippert]

What I observe is that there is one realtime thread owned by Jack, and one (using the -D CLI switch, two without) owned by guitarix (also more if running any convolvers). Here is a process listing of threads while running guitarix (I moved the non-realtime guitarix parent thread to CPU0 manually with taskset after guitarix started up):

$ ps -eL -o pid,tid,psr,rtprio,pcpu,comm,cmd|egrep "guitar|jackd|%CPU"
    PID     TID PSR RTPRIO %CPU COMMAND         CMD
    629     629   0      -  0.0 jackd           jackd --silent -P 75 -d alsa -p 64 -r 96000 -X seq
    629     632   0      -  0.0 jackd           jackd --silent -P 75 -d alsa -p 64 -r 96000 -X seq
    629     633   1      -  0.0 jackd           jackd --silent -P 75 -d alsa -p 64 -r 96000 -X seq
    629     639   0      -  0.0 jackd           jackd --silent -P 75 -d alsa -p 64 -r 96000 -X seq
    629     640   1     75 11.1 jackd           jackd --silent -P 75 -d alsa -p 64 -r 96000 -X seq
    629     641   1      -  0.0 jackd           jackd --silent -P 75 -d alsa -p 64 -r 96000 -X seq
    650     650   0      - 97.4 guitarix        guitarix -N -D -C -p 7000
    650     652   2      -  0.0 guitarix        guitarix -N -D -C -p 7000
    650     654   2      -  0.0 guitarix        guitarix -N -D -C -p 7000
    650     655   2      -  0.0 guitarix        guitarix -N -D -C -p 7000
    650     656   2      -  0.0 guitarix        guitarix -N -D -C -p 7000
    650     657   2      -  0.0 guitarix        guitarix -N -D -C -p 7000
    650     658   2      -  0.0 guitarix        guitarix -N -D -C -p 7000
    650     659   2     70 15.3 guitarix        guitarix -N -D -C -p 7000
    650     660   2      -  0.0 gmain           guitarix -N -D -C -p 7000

I can set the affinity of guitarix when it is first called on the commandline, and all other threads will share that affinity, not the affinity of jackd. I have to manually change the affinity of the realtime thread from the one active non-realtime, parent thread (or vice-versa) after startup to separate them. After that , all convolver threads will share the affinity of the parent thread, and must be moved manually if desired. I'll test setting the affinity of the convolver threads at the point you mention in the code and see if it works.

Anyway, it shouldn't matter who the owner is in terms of cpu affinity. I should just need to use pthread_setaffinity_np at the point where the thread is created, which should be in your code somewhere. At least that is how I understand it at this point.

[brummer10]

650 659 2 70 15.3 guitarix guitarix -N -D -C -p 7000

This should be the convolver thread, it use jack priority - 5. The other one you seen may be the pitch tracker, which use jack priority - 6 so in your setup it should have a priority of 69. (gx_pitch_tracker.cpp line 154) (gx_internal_ui_plugins.cpp line 135 set the priority for the pitch tracker)