How Big Is Too Big?

You may think, ``I will just give all my RAM to the POSTGRESQL shared buffer cache.'' However, if you do that, there will be no room for the kernel or for any programs to run. The proper size for the POSTGRESQL shared buffer cache is the largest useful size that does not adversely affect other activity.

To understand adverse activity, you need to understand how UNIX operating systems manage memory. If there is enough memory to hold all programs and data, little memory management is required. However, if everything doesn't fit in RAM, the kernel starts forcing memory pages to a disk area called swap. It moves pages that have not been used recently. This operation is called a swap pageout. Pageouts are not a problem because they happen during periods of inactivity. What is bad is when these pages have to be brought back in from swap, meaning an old page that was moved out to swap has to be moved back into RAM. This is called a swap pagein. This is bad because while the page is moved from swap, the program is suspended until the pagein completes.

Pagein activity is shown by system analysis tools like vmstat and sar and indicates there is not enough memory available to function efficiently. Do not confuse swap pageins with ordinary pageins, which may include pages read from the filesystem as part of normal system operation. If you can't find swap pageins, many pageouts is a good indicator you are are also doing swap pageins.