What does 'maximum number of FD events ... received' mean?

Question:

In my named logs, I'm frequently seeing maximum number of FD events (64) received - is this cause for concern?

Answer:

maximum number of FD events means that when checking to see if any sockets are ready to be read from, there were more than 64 of them. As commented in the code, this isn't an error, but it is unexpectedly high. If the message is logged frequently and persists for a long period of time, then it may be that recompiling with a higher value of ISC_SOCKET_MAXEVENTS will make this message disappear. But if there is another underlying problem that isn't diagnosed and addressed, you may still reach the maximum number of events, no matter how high the limit is set.

On larger machines that have high query rates, increasing this setting can be done when you build the named binary by setting ISC_SOCKET_MAXEVENTS when invoking configure. For example:

STD_CDEFINES='-DISC_SOCKET_MAXEVENTS=256' ./configure

Please also see:

What to do with a misbehaving BIND server

Background info about 'maximum number of FD events' log messages

--with-tuning=large - about using this build-time option