Exokernel

Exokernel is a term every system researcher has heard of at some point in life. However, according to the PDOS group at MIT, there aren’t any exokernel-based operating systems in active use today. It’s interesting to discover what ideas exokernels brought to the OS high-level design and some potential drawbacks of such design choice.

Perhaps the most important thing to keep in mind is that exokernel operating system architecture pushes management of physical resources to the application level, contrary to what most monolithic kernel would do: providing hardware resource management through some form of abstraction, usually hiding hardware-related details.

Limitations of Traditional Approaches

Monolithic kernels usually enforce centralized resource management via a set of abstractions. In microkernel-based system, they are usually provided through some form of trusted user-level servers. There are several drawbacks:

• Too general. Over generalizing can limit application diversity and have performance implications (domain/application-specific approach usually have performance improvements, in the cost of, well, being more “specific”.). For example, in UNIX, two applications exhibiting rather different memory access patterns are subject to the general-purpose OS scheduler and page replacement policy. Letting applications define such policies can open doors for performance improvements since applications have better knowledge of their...