Make sure that your database system's OS is running as efficiently as possible with these tweaks
If you have a very busy MySQL server (serving, say, 800+ queries/second and hundreds of simultaneous clients), you may begin to run into limitations of the operating system that prevent mysql from operating as efficiently as it could. In extreme cases (on systems with a misbehaving threads library), mysql has been observed to suddenly take up all available CPU cycles, artificially driving the load to 100+ once a critical resource threshold has been reached.
By building a dedicated mysql server, and adjusting some default values in glibc, linuxthreads, and the Linux kernel, it is possible to make a single database machine serve thousands of simultaneous requests. Naturally, you'll need hardware capable of supporting the load, but with these modifications, the OS and MYSQL build will probably not be the limiting performance factor.
WARNING: the following hack makes changes to critical, sensitive areas of your server. Don't go monkeying with your libc and kernel source lightly, and even then only if you have a complete, verified, offline (and preferably off-site) backup of your entire system. This procedure should only be followed on dedicated MySQL server machines, and then only when you are certain that everything else is in order (especially your /etc/my.cnf variables). You have been warned!