Originally Posted by bakunin

But even then, "swap allocated" is not the same as "swap used" which in fact comes down to: "pages swapped out" and "pages swapped in". This is why it is useless to look at swap -l's output. This only tells you if you need to increase the amount of available swap space. It will not tell you how much is actually written to/read from the swap. This is only told by the pi/po counter of vmstat (and all the other counter discussed above in detail).

I hope this helps.
bakunin

When a process calls the malloc()/sbrk() commands, only virtual swap is allocated. The operating system allocates the memory from physical disk-based swap first. If disk-based swap is exhausted or unconfigured, the reservation is allocated from physical memory.....
Follow the guidelines below to calculate amount of virtual swap usage:Virtual swap = Physical Memory + Fixed Disk swap

But even then, "swap allocated" is not the same as "swap used" which in fact comes down to: "pages swapped out" and "pages swapped in".

Originally Posted by jlliagre

Three gigabytes of memory were used (i.e. read/written) sometime in the past by some process(es). They have not been accessed for a while so the kernel decided to put the data on disk, to keep the free RAM high.

You need not to starve on RAM for paging to occur.
When the processes owning it will die.

That might be just some optimization done by the kernel.
There might be a memory leak in a process, 10 MB per week is not among the fiercest ones.
There might be a growing file in /tmp or any tmpfs based file system. The storage area of tmpfs is virtual memory (not any process virtual memory but the OS virtual memory, i.e. RAM + SWAP as Bakunin wrote).

Note also the free memory might be actually used by the kernel, which isn't constrained by process virtual memory rules.

Solaris uses free memory as UFS and NFS cache, so this free memory contains actual data, but it is nevertheless reported as free by vmstat and similar commands, because it is immediately available for processes allocations.