My cron_schedule table is full of "missed" and "running" rows, what does that mean?

Rows stuck in running usually mean a previous cron:run crashed mid-job and never released the slot, so new runs overlap and skip. Rows piling up as missed mean cron fell behind its schedule_lifetime window. We clear the stale rows safely, fix whatever job was hanging, and tune the cron group config (schedule_generate_every, schedule_lifetime, use_separate_process) so the queue stops clogging.