OS 10.8.5 on iMac grinding to a halt - fairly usual CPU but up to 7GB Page outs after starting Safari (I think)

Question

Hi
I'm running OS 10.8.5 on an iMac (early 2009, 4GB memory), which has been running fine up until today:
Applications started becoming very unresponsive, and eventually beach balled until it became unusable. Restarts didnt cure it.
Having looked in the Activity Monitor from suggestions in discussions here, the System Memory 'Page Outs' seemed high, at 7GB
For a while, if I didnt run Safari, it seemed to stay at 0
Once I've run Safari, the Page Outs keep climbing, even when Safari is closed
The Page Outs are raising at between 1MB/sec and 7MB/sec, which i think seems high based on reading people's comments
(CPU usage seemed fairly normal, except for an occaisional 'Safari Web Content' jump to 20% once in a while)
Im using Safari 6.0.5, and aside from Java being enabled (I've tried turning it off, same effect), theres nothing unusual about what I'm doing in Safari
Any ideas about what could be causing this / what to do?
Many thanks in advance

Answer 1

There is excessive swapping of data between physical memory and virtual memory. That can happen for two reasons:
You have a long-running process with a memory leak (i.e., a bug), or
You don't have enough memory installed for your usage pattern.
Tracking down a memory leak can be difficult, and it may come down to a process of elimination.
When you notice the swap activity, open the Activity Monitor application and select All Processes from the menu in the toolbar, if not already selected. Click the heading of the Real Mem column in the process table twice to sort the table with the highest value at the top. If you don't see that column, select
View ▹ Columns ▹ Real Memory
from the menu bar.
If one process (excluding "kernel_task") is using much more memory than all the others, that could be an indication of a leak. A better indication would be a process that continually grabs more and more real memory over time without ever releasing it. Here is an example of how it's done.
The process named "Safari Web Content" renders web pages for Safari and other applications. It uses a lot of memory and may leak if certain Safari extensions or third-party web plugins are installed. Consider it a prime suspect.
Another process often implicated in memory leaks is "inkjet8," which is a component of the HP printing softwre. If it's present, force-quit the process in Activity Monitor to solve the problem temporarily. If you don't use an HP printer, remove the software. Otherwise, if the problem is recurrent, update the software (which may not help) or contact HP support.
If you don't have an obvious memory leak, your options are to install more memory (if possible) or to run fewer programs simultaneously.
The next suggestion is only for users familiar with the shell. For a more precise, but potentially misleading, test, run the following command: 
sudo leaks -nocontext -nostacks process | grep total
where process is the name of a process you suspect of leaking memory. Almost every process will leak some memory; the question is how much, and especially how much the leak increases with time. I can’t be more specific. See the leaks(1) man page and the Apple developer documentation for details.