Swap size for large memory machines

[Carl S. Gutekunst]

In article <61633 at uunet.UU.NET> rick at uunet.UU.NET (Rick Adams) writes:
>The amount of swap space that you need is totally dependant on your job
>load.... The only rule that makes any sense at all is main memory + swap
>space must be greater than the sum of the memory use of all the processes
>on your system at peak load. (This is obvious if you think about it.)

Almost. Swap space and main memory do *not* add together on a demand-paged
virtual memory system; swap space alone must be greater than the sum of all
processes running on the system.

The general rule back in ye olden days was that you allocated enough swap
space to cover all your concurrently running processes, then bought as much
RAM as you could afford. This meant a lot of paging (or swapping, back in
those days). If the system thrashed too much, you tightened your belt and
bought more RAM.

These days in commercial installations, the goal seems to be to have enough 
RAM so that you don't page, and/or so response time is minimized. A trans-
action processing system with 100 users that does one database query per user
every 15 seconds would probably want at least 100MB of swap space, but might
see no difference between 16MB RAM and 32MB RAM.

My rule of thumb: 1MB of swap space per login user. 1MB of RAM per *active*
user, plus 4MB for the kernel. Remember that users with windowing terminals
(AT&T 630 or X Server) count as several users. And make adjustments for mongo
sized applications that eat a horrific amount of memory, like FrameMaker.

And as the folks at Silent Radio pointed out, please do not fiddle with the
default disk partitions. It is easy to do, but a big headache for the service
techs. Far better is to just use different or multiple partitions. When using
multiple swap partitions, putting then on different drives is a big win.

<csg>