Kernel Queues and Events

Kernel Queues ('kqueue') are a stateful method of event notification. Instead of passing events to monitor with each select(2) or poll(2) system call, the program registers events to receive event notifications for. The registered events can include a pointer to user data. The kernel queues triggered events and returns them when polled for. Kernel Queues currently supports event monitors ('filters') for file descriptors, processes, signals, asynchronous I/O, and VNODEs. This interface is available in FreeBSD 5-CURRENT and 4.0-STABLE and will be in the upcoming 4.1-RELEASE.

Jonathan Lemon <jlemon@freebsd.org> of the FreeBSD project implemented Kernel Queues. The man page explains the orignal kqueue API. If you want the latest/greatest, check out: kqueue(2).

Currently, only a few programs make use of kqueue as the code hasn't been in a full FreeBSD release. Once kqueue is widely available, developers will migrate their programs to kqueue. Hopefully, other projects, like the other BSD systems, will integrate kqueue to provide a common events API.

There was a paper at Usenix 1999 that talked about Kernel Queues.

Current kqueue developments:

• John-Mark Gurney <jmg@freebsd.org> converted inetd to use kqueue. The modified inetd reduces the number of table lookups required to process new connections. A short document describes the expected performance improvement from the changes and a link to the patch.
• kqt is an example program that tracks child processes across a fork(2) without race conditions. Jonathan Lemon is investigating a couple bugs with kqueue that prevents this code from working properly. This should be resolved shortly.
• kqre is a function that monitors a file for changes using the VNODE mechanism.
• Doug White <dwhite@freebsd.org> wrote a Python module that interfaces with kqueue. It's available in the FreeBSD ports tree as devel/py-kqueue.
• The l0pht-watch port for FreeBSD has patches to use Kernel Queues to watch /tmp. It now takes up only a small amount of CPU usage instead of all the available CPU like it used to.
• Adrian Chadd <adrian@freebsd.org> is working on making the HTTP proxy/cache squid use Kernel Queues for async I/O.
• Brian Feldman <green@freebsd.org> is working on making ircd Kernel Queue aware to reduce the load on the server.