Swapping is the process of instantiating your experiment, i.e., allocating nodes, configuring links, etc. It also refers to the reverse process, in which nodes are released. These are called "swapping in" and "swapping out" respectively. Swapping is when you (or we, or the Emulab system) temporarily swaps out your experiment, releasing all of the nodes in the experiment. Your experiment is still resident in the Emulab database, and you can see its status in the web interface, but no nodes are allocated.
In general, you should do your work, and then terminate your experiment as soon as you're done with it. If you're not done with it, but are through for a while, you should probably "swap out" your experiment It is especially important to swap out your experiment if you're through with it for the weekend. Emulab usually gets heavy use on the weekends by users who need to make very large experiments, so it is important to leave as many nodes available as possible.
Caution: Be aware that we do not currently save any files that you may have placed on your nodes. When your experiment is swapped back in, you will likely get different nodes, with fresh copies of the disk images. For that reason, you should not swap your experiment out unless you make arrangements to save and restore any state you need.
Once an experiment is swapped out, you can swap it back in via the web interface by going to the Experiment Information page for your experiment, and clicking on the swapin option. See the Project File Storage module for more information
You will sometimes notice that the Experiment Information page does not contain the swap link. That is because experiments cannot be swapped when they are in transition. For example, when the experiment is being swapped in (say, after first being created) the link will disappear until the experiment is fully swapped in, and it is capable of being swapped out. You will need to occasionally reload the page so that the updated state is recognized and the swap link appears.