mjhjmtu

What is the different between SystemD "Tasks" and nproc values?

We were seeing a problem where a service was crashing because the number of current tasks exceeded the Systemd Default value for TasksMax (512). However we had set LimitNPROC=infinity already so we were surprised that the service was throttled at 512.

We resolved the problem by setting

TasksMax=infinity

What is the different between TasksMax and LimitNPROC and when should you use one over the other?

LimitNProc= is an rlimit (these also referred to as a ulimit). "See setrlimit(2) for details on the resource limit
concept... Note that most process resource limits configured with these
options are per-process, and processes may fork in order to acquire
a new set of resources that are accounted independently of the
original process, and may thus escape limits set." This is documented in man systemd.exec. RLIMIT_NPROC is one of the exceptions to this quote; it limits the number of processes "for the real user ID" (UID).

TasksMax= is documented in the more specific man systemd.resource-control, as one of the options which "relies on the
Linux Control Groups (cgroups) kernel concept for organizing processes". All such limits apply to the cgroup created for the systemd service. cgroups cannot be escaped by simply forking a new process.

TasksMax= makes much more sense to me. I don't know any reason why you would use LimitNProc=.