Of course, swap space cannot completely make up for a lack of physical RAM. Disk access is much slower than RAM access, by several orders of magnitude. Therefore, swap is useful primarily as a means to run a number of programs simultaneously that would not otherwise fit into physical RAM; if you are switching between these programs rapidly you'll notice a lag as pages are swapped to and from disk.

At any rate, Linux supports swap space in two forms: as a separate disk partition or a file somewhere on your existing Linux filesystems. You can have up to 16 swap areas, with each swap area being a disk file or partition up to 128 MB in size (again, these values can differ on non-Intel systems). You math whizzes out there will realize that this allows up to 2 GB of swap space. (If anyone has actually attempted to use this much swap, the authors would love to hear about it, whether you're a math whiz or not.)

Note that using a swap partition can yield better performance, because the disk blocks are guaranteed to be contiguous. In the case of a swap file, however, the disk blocks may be scattered around the filesystem, which can be a serious performance hit in some cases. Many people use a swap file when they must add additional swap space temporarily--for example, if the system is thrashing because of lack of physical RAM and swap. Swap files are a good way to add swap on demand.

The "shared" column lists the amount of physical memory shared between multiple processes. Here, we see that about 27 MB of pages are being shared, which means that memory is being utilized well. The "buffers" column shows the amount of memory being used by the kernel buffer cache. The buffer cache (described briefly in the previous section) is used to speed up disk operations, by allowing disk reads and writes to be serviced directly from memory. The buffer cache size will increase or decrease as memory usage on the system changes; this memory is reclaimed if it is needed by applications. Therefore, although we see that 126 MB of system memory is in use, not all (but most) of it is being used by application programs. The "cache" column indicates how many memory pages the kernel has cached for faster access later.

Since the memory used for buffers and cache can easily be reclaimed for use by applications, the second line (-/+ buffers/cache) provides an indication of the memory actually used by applications (the "used" column) or available to applications (the "free" column). The sum of the memory used by buffers and cache reported in the first line is subtracted from the total used memory and added to the total free memory to give the two figures on the second line.

In the third line, we see the total amount of swap, 130,748 blocks (about 128 MB). In this case, only very little of the swap is being used; there is plenty of physical RAM available. If additional applications were started, larger parts of the buffer cache memory would be used to host them. Swap space is generally used as a last resort when the system can't reclaim physical memory in other ways.

Note that the amount of swap reported by free is somewhat less than the total size of your swap partitions and files. This is because several blocks of each swap area must be used to store a map of how each page in the swap area is being utilized. This overhead should be rather small; only a few kilobytes per swap area.

If you're considering creating a swap file, the df command gives you information on the amount of space remaining on your various filesystems. This command prints a list of filesystems, showing each one's size and what percentage is currently occupied.