libev4 1:4.27-1 (amd64 binary) in ubuntu focal

 libev provides a full-featured and high-performance event loop that is
 loosely modelled after libevent. It includes relative timers, absolute
 timers with customized rescheduling, synchronous signals, process status
 change events, event watchers dealing with the event loop itself, file
 watchers, and even limited support for fork events. It uses a priority
 queue to manage timers and uses arrays as fundamental data structure. It
 has no artificial limitations on the number of watchers waiting for the
 same event.
 .
 libev supports select, poll, epoll, kqueue, and inotify.