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Is there any obvious reason that it's bad to have two timers running simultaneously, one using timerfd (started via timerfd_settime) and one using a POSIX timer (started via timer_settime)? Here's the context: Emacs runs timers. Until recently it used timerfd if available, and otherwise a POSIX timer. After a recent change, it starts both kinds of timers. This causes a severe slowdown on Cygwin (https://debbugs.gnu.org/cgi/bugreport.cgi?bug=51734#32). The easiest thing is just to disable that change on Cygwin, but I'd feel better understanding why this is a problem. Ken