[Jack-Devel] Some observations re Jack and systemd

Hi everyone, after scratching my head for a long time to make systemd behave when running jack as a service, I thought I'd share my findings: Example service file: [Unit] Description=JACK Audio Connection Kit After=sound.target After=ntp.service After=time-sync.target Before=jackd.target Requires=jackd.target [Service] EnvironmentFile=-/your/config/file/containing/JACKD_OPTIONS LimitRTPRIO=85 LimitMEMLOCK=700000000 User=nettings Environment="DBUS_SESSION_BUS_ADDRESS=unix:path=/run/dbus/system_bus_socket" ExecStartPre=/bin/sleep 10 ExecStart=/usr/bin/jackd $JACKD_OPTIONS RestartSec=5 Restart=on-failure [Install] WantedBy=multi-user.target So it's easy to start a service as a user, but that user's environment is not automatically pulled in. That means even though user nettings does have the necessary permissions in /etc/security/limits.conf, you still need to spell them out for systemd. Another thing to note is that at least on current raspbian, the time-sync.target is broken. Its promise is to only be triggered after ntp has worked its magic and the system time is monotonous. However, I've seen time jumps which combined with cheap USB hardware or the builtin Raspi sound will hang jack. Hence the ugly sleep. Final goodness: systemd might try to clean up after you. So when you have a jackd service started in your name, all is well, until you have logged in somehow and then closed the last shell, at which point the janitor from hell will happily clear /dev/shm and with it all your jack semaphores, leading to instant dumpster fire and silence. This magic can be avoided by setting RemoveIPC=no in /etc/systemd/logind.conf After clearing these hurdles, I've been quite happy with how systemd handles jack clients. The usecase here is a number of clients running in an embedded appliance, and systemd takes care of restoring them when I cause them to crash, which is good. Not that they crash, but it's good to have defense in depth. Figuring out how to order services is hard because you cannot assume a service is really ready when systemd has fired it off. So I've used a ExecStartPre script that blocks until a downstream jack port has appeared in those cases where I want to start clients in a particular order (matters most for the one job that needs to connect all the ports, so I let it wait on a test port for each client it needs to connect). Old-time SysV user here, and granted, systemd has given me many a WTF moment, but it does its job. Unfortunately, it does too many other jobs as well, but hey... -- Jörn Nettingsmeier Tuinbouwstraat 180, 1097 ZB Amsterdam, Nederland Tel. +49 177 7937487 Meister für Veranstaltungstechnik (Bühne/Studio), Tonmeister VDT http://stackingdwarves.net