If you’ve just installed a machine with 16Gb of RAM and want to install an Oracle instance, how much memory should you allocate to the SGA, the PGA target, and the operating system.
There’s an old ‘rule of thumb’ that I see from time to time that says something like: 50% to the SGA, 30% to the PGA, and 20% to the O/S.
That seems to me to be a naive (though possibly fairly safe) suggestion for many OLTP systems. Why do you need so much memory for the operating system – are you failing to make best use of your memory if you set aside so much.
Take the time to read, and listen to, this presentation from Christo Kutrovsky from Pythian and you’ll find out what that 20% is needed for – and how you can avoid “wasting” it if you configure your system properly.
Take note, especially, of the sections on “Page Tables” and “Huge Pages”. (Many of the examples reference Linux, but the principles do apply to other flavours of Unix).