- **thread:** very slow factory using full featured, standard threads.
They are slow but very standard. Some debuggers or profilers only work with this factory.
- - **java:** Java applications are virtualized onto java threads (that
- are regular system threads registered to the JVM)
- **ucontext:** fast factory using System V contexts (Linux and FreeBSD only)
- **boost:** This uses the `context
implementation <http://www.boost.org/doc/libs/1_59_0/libs/context/doc/html/index.html>`_