It is possible to run multiple instances of the Hurd in parallel, on a single instance of Mach. Other than performance crosstalk, they are essentially isolated. Practically, as many devices do not allow multiple non-cooperating users, e.g., hard drive and network this is not currently possible. It can be overcome, however, by virtualizing these problematic devices. When extra hardware is not available, it is possible to use a sub-hurd. A sub-Hurd is like a neighbor Hurd, however, makes use of some resources provided by another Hurd. For instance, backing store and the console. Sub-hurds are extremely useful for debugging core servers as it is possible to attach to them with gdb from the parent. This avoids deadlock, e.g., when the instance of gdb stops the server but requires its use. (Note: it is possible to use [[NonInvasiveDebugging]], but this is less flexible.) A [tutorial](http://www.gnu.org/software/hurd/howto/subhurd.html) on setting up sub-hurds is available.