Comment by sleepydog
10 days ago
I used to work for a huge email sender (constant contact). Our mail servers needed to perform an absurd number of lookups while receiving and validating mail. When I was there, we used dnscache, running locally, on all our mail servers. But even with a local dnscache, the overhead of making DNS requests and handling their responses was high enough that adding nscd made a noticeable improvement in CPU usage.
i guess this shows that looking up getent hostname database cache is faster than looking up local dns cache because the former is simpler in data structure?
I didn't dig into it too deeply at the time, but I think part of it was that you don't need to open and write to a socket, so that's avoiding some system calls (socket(), bind(), sendto(), close()). IIRC we had nscd set up so clients directly read from shared memory that nscd kept updated, rather than getting requests over a socket.
There's also probably some savings around not having to convert between the structures used by gethostbyname and DNS questions&answers.