Figure 4-15. A vmstat report on a very active four-CPU core database server. The so column represents the number of pages swapped out per second during the reporting interval. As in this sample, we want to this nearly always to be zero.

Performance analysts know that if an Oracle server process (and, God forbid, a background process) has some of its memory pages swapped out, response time for that process will be negatively impacted if the pages are swapped back in. And let's not forget the pages were swapped out because another active process requested memory and the operating system could not immediately provide the memory.

I tend to focus on the currently active process that is asking for pages at this moment and must wait while the operating system swaps out some pages so it can give them to the process. That's why I focus on pages swapped out. But practically speaking, if there is a memory bottleneck, you will see both pages swapped in and swapped out.