A low value means that Gimp sends data vey quickly to disk, not making real use of the available RAM, and making the disks work without any real reason. Too high a value, and other applications start to have less system resources, forcing them to use swap space, which also makes the disks work; or maybe some will even get killed or start to malfunction due lack of RAM.

How to choose a number for the Tile Cache size? Here are some ways you could decide what value to use, as well as a few tricks:

Let's suppose you prefer the last option, and want to get a good value to start with. First, you need to get some data about your computer. This data is the amount of RAM installed in your system, the operating system's swap space available, and a general idea about the speed of the disks that store the operating system's swap and the directory used for Gimp's swap. You do not need to do disk tests, nor check the RPM of the disks, the thing is to see which one seems clearly faster or slower, or whether all are similar. You can change Gimp's swap directory in the Folders page of the Preferences dialog.

The next thing to do is to see how much resources you require for other apps you want to run at the same time than Gimp. So start all your tools and do some work with them, except Gimp of course, and check the usage. You can use applications like free or top, depending in what OS and what environment you use. The numbers you want is the memory left, including file cache. Modern Unix keeps a very small area free, in order to be able to keep large file and buffer caches. Linux's free command does the maths for you: check the column that says “free”, and the line “-/+ buffers/cache”. Note down also the free swap

Now time for decisions and a bit of simple math. Basically the concept is to decide if you want to base all Tile Cache in RAM, or RAM plus operating system swap:

As you can see, all is about checking the free resources, and decide if the OS swap is worth using or will cause more problems than help.

There are some reasons you want to adjust this value, though. The basic one is changes in your computer usage pattern, or changing hardware. That could mean your assumptions about how you use your computer, or the speed of it, are no longer valid. That would require a reevaluation of the previous steps, which can drive you to a similar value or a completly new value.

Another reason to change the value is because it seems that Gimp runs too slowly, while changing to other applications is fast: this means that Gimp could use more memory without impairing the other applications. On the other hand, if you get complaints from other applications about not having enough memory, then it may benefit you to not let Gimp hog so much of it.

If you decided to use only RAM and Gimp runs slowly, you could try increasing the value a bit, but never to use also all the free swap. If the case is the contrary, using both RAM and swap, and you have problems about lack of resources, then you should decrease the amount of RAM available to Gimp.

Another tricks is to put the Swap Dir in a very fast disk, or in a different disk than the one where most of your files reside. Spreading the operating system swap over multiple disks is also a good way to speed up things, in general. And of course, maybe you have to buy more RAM or stop using lots of programs at the same time: you can not expect to edit a poster in a computer with 16MB and be fast.

You can also check what memory requirements your images have. The larger the images, and the number of undos, the more resources you need. This is another way to choose a number, but it is only good if you always work with the same kind of images, and thus the real requirements do not vary. It is also helpful to know if you will require more RAM and/or disk space.