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When in heavy load, unbound opens many outside_network sockets for out going queries to delegation servers, which may result in a big fd(maxfd) value(for thread A 65500, for thread B 65501, for thread C ...). There are situations when thread A has a max fd num 65500 where maxfd is of course 65500, thread B has max fd num 20 for now but maxfd is still 65501. Though linux kernel checks whether maxfd+1 passed by select syscall is really the process' maxfd+1. Linux kernel can not tell maxfd+1 passed by thread B select syscall is much bigger(65501+1 or 65500+1 after trimed by kerne) than it should be (20+1). In this situation, when kernel do_select() for thread B, much work is wasted.
Unbound
Unbound is a validating, recursive, caching DNS resolver. It is designed to be fast and lean and incorporates modern features based on open standards. If you have any feedback, we would love to hear from you. Don’t hesitate to create an issue on Github or post a message on the Unbound mailing list. You can learn more about Unbound by reading our documentation.
Compiling
Make sure you have the C toolchain, OpenSSL and its include files, and libexpat installed. Unbound can be compiled and installed using:
./configure && make && make install
You can use libevent if you want. libevent is useful when using many (10000) outgoing ports. By default max 256 ports are opened at the same time and the builtin alternative is equally capable and a little faster.
Use the --with-libevent=dir configure option to compile Unbound with libevent
support.
Unbound configuration
All of Unbound's configuration options are described in the man pages, which will be installed and are available on the Unbound documentation page.
An example configuration file is located in doc/example.conf.