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Emulab FAQ: Using the Testbed: Why can't I swap in my N node experiment, when there are more than N nodes free?

Emulab FAQ: Using the Testbed: Why can't I swap in my N node experiment, when there are more than N nodes free?

Even if the "Free PC" count shows that there are more than N nodes "free", you may still not be able to swapin an N node experiment. The common reasons are:

  • You did not take into account traffic shaping nodes. If you have shaped links in your experiment, additional "shaping nodes" are required to perform the shaping. In the current testbed, a minimum of one shaping node is needed for every two shaped links. For shaped LANs, one shaping node is needed for every two members of the LAN. Worse case, it can require one shaping node per shaped link (per LAN member).
  • Not all nodes can run all OSes. Some images are intended for, or restricted to, specific nodes or node types. For example, experiments requring wireless networks can only run on nodes with wireless interfaces! Ditto for Gigabit Ethernet. Also, some older images do not have drivers for newer hardware and are thus restricted to running on older machines.
  • The physical testbed topology may limit which nodes can be used for an experiment. In particular, the inter-switch bandwidth between particular switches may limit the nodes that can be used. For example, LANs of 40 or more nodes may have to be "clustered" on a single switch. If you absolutely, positively have to have that LAN of 100+ nodes and "best effort" bandwidth is good enough, then you can trick assign into allowing it by specifying each connection to the LAN to have a small bandwidth (e.g., 10Kbps) in the make-lan command and then adding:
      	tb-set-noshaping $lan 1
    
    The low bandwidth value will ensure that assign can map the LAN given our current inter-switch bandwidth, and the "no shaping" flag tells Emulab to not actually enforce shaping (i.e., to not allocate shaping nodes).
  • The physical to virtual mapping tool is not perfect. Assign uses a heuristic algorithm and may not find a solution (mapping) when one exists. This typically occurs when there are very few nodes free in the testbed and you ask for most of them.