Ensayo

Exokernel: An Operating System Architecture for
Application-Level Resource Management
Dawson R. Engler, M. Frans Kaashoek, and James O’Toole Jr.
M.I.T. Laboratory for Computer Science
Cambridge, MA 02139, U.S.A
engler, kaashoek, james @lcs.mit.edu

 

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Traditional operating systems limit the performance, flexibility, and
functionality of applications by fixingthe interface and implementation of operating system abstractions such as interprocess communication and virtual memory. The exokernel operating system
architecture addresses this problem by providing application-level
management of physical resources. In the exokernel architecture, a
small kernel securely exports all hardware resources through a lowlevel interface to untrusted library operatingsystems. Library operating systems use this interface to implement system objects and
policies. This separation of resource protection from management
allows application-specific customization of traditional operating
system abstractions by extending, specializing, or even replacing
libraries.
We have implemented a prototype exokernel operating system.
Measurements show that most primitivekernel operations (such
as exception handling and protected control transfer) are ten to 100
times faster than in Ultrix, a mature monolithic UNIX operating system. In addition, we demonstrate that an exokernel allows applications to control machine resources in ways not possible in traditional
operating systems. For instance, virtual memory and interprocess
communication abstractions areimplemented entirely within an
application-level library. Measurements show that application-level
virtual memory and interprocess communication primitives are five
to 40 times faster than Ultrix’s kernel primitives. Compared to
state-of-the-art implementations from the literature, the prototype
exokernel system is at least five times faster on operations such as
exception dispatching andinterprocess communication.

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Operating systems define the interface between applications and
physical resources. Unfortunately, this interface can significantly
limit the performance and implementation freedom of applications.
Traditionally, operating systems hide information about machine
resources behind high-level abstractions such as processes, files,
address spacesand interprocess communication. These abstractions define a virtual machine on which applications execute; their
implementation cannot be replaced or modified by untrusted applications. Hardcoding the implementations of these abstractions is
This research was supported in part by the Advanced Research Projects Agency under
contract N00014-94-1-0985 and by a NSF National Young Investigator Award.(

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inappropriate for three main reasons: it denies applications the advantages ofdomain-specific optimizations, it discourages changes
to the implementations of existing abstractions, and it restricts the
flexibility of application builders, since new abstractions can only
be added by awkward emulation on top of existing ones (if they can
be added at all).
We believe these problems can be solved through applicationlevel (i.e., untrusted) resource management. To this end, we...