Comment 2 for bug 1578827

> Most probably, pid_max limit has been reached (can be increased via sysctl)

I somewhat doubt it. A typical OSD spawns ~ 100 threads, thus the default value of
/proc/sys/kernel/pid_max (being 32768) is enough for running ~ 300 OSDs (provided the node has
enough RAM).

So the cause might be a bit simpler: the node needs more RAM to run that many OSDs.
It's hard to find out the exact reason without even basic information about the cluster and nodes.