Exokernel: An Operating System Architecture for Application-Level Resource Management" by Dawson R. Engler, M. Frans Kaashoek, and James O'Toole Jr. Chris Franklin CS533 - Concepts of Operating Systems

The Problem  Traditional operating systems fix the interface and implementation of OS abstractions. FIXED Hardware Applications InterfaceAbstractions

The Problem  Performance  Flexibility  Functionality FIXED Hardware Applications InterfaceAbstractions

Example FIXED Hardware Apache InterfaceAbstractions SQL Server Traditional OS

Example Hardware Exokernel – Application level resource management SQL Server Library OS Customized for SQLServer InterfaceAbstractions Library OS Chosen from available Apache InterfaceAbstractions Exokernel

The Issues  Performance Denies applications the advantages of domain- specific optimizations  Flexibility Restricts the flexibility of application builders  Functionality Discourages changes to the implementations of existing abstractions

Performance  There is no single way to abstract physical resources or to implement an abstraction that is best for all applications.  OS is forced to make trade-offs  Performance improvements of application- specific policies could be substantial

Flexibility  Fixed high-level abstractions hide information from applications.  Makes it difficult or impossible for applications to implement their own resource management abstractions.

Functionality  Only one available interface between applications and hardware resources.  Because all applications must share one set of abstractions, changes to these abstractions occur rarely, if ever

The Solution  Separate protection from management Allow user level to manage resources Application libraries implement OS abstractions Exokernel exports resources Low level interface Protects, does not manage Expose hardware

End-to-End Argument  “if something has to be done by the user program itself, it is wasteful to do it in a lower level as well.”  Why should the OS do anything that the user program can do itself?  In other words - all an OS should do is securely allocate resources.

The Exokernel  A thin veneer that multiplexes and exports physical resources securely. Simplicity allows efficiency The lower the level of a primitive, the more efficiently it can be implemented, and the more latitude it grants to implementers of higher level abstractions.

The Exokernel  Resource management is restricted to allocation, revocation, sharing ownership tracking

Library operating systems  Use the low level exokernel interface  Higher level abstractions  Special purpose implementations An application can choose the library which best suits its needs, or even build its own.

Example Exokernel

Another Example

Design Challenge How can an Exokernel allow libOSes to freely manage physical resources while protecting them from each other?

Design Challenge How can an Exokernel allow libOSes to freely manage physical resources while protecting them from each other? Track ownership of resources Guard all resource usage Revoke access to resources

Track ownership of resources Secure bindings – libOS can securely bind to machine resources

Secure Bindings  Exokernel allows libOSes to bind resources using secure bindings Multiplex resources securely Protection for mutually distrusted apps Efficient

Secure Bindings  Secure Binding – a protection mechanism that decouples authorization from actual use of a resource Allows the kernel to protect resources without having to understand them

Guard all resource usage Invisible resource revocation -Efficient – application layer not involved -Traditional OS Visible resource revocation -Allows libOS to guide deallocation and track availability of resources. -Exokernel

Revoke access to resources Abort protocol – Allows exokernel to break secure bindings of an uncooperative libOS by force

Conclusion  An Exokernel securely multiplexes available hardware raw hardware among applications  Application level library operating systems implement higher-level traditional OS abstractions  LibOSes can specialize an implementation to suit a particular application

Conclusion  The lower the level of a primitive… …the more efficiently it can be implemented … the more latitude it gives to higher level abstractions  So, separate management from protection and… …implement protection at a low level (exokernel) … implement management at a higher level (libOS)

References Modern Operating Systems – Andrew Tanenbaum

Some Features  It is possible to have different libOSes, for example, one could export a Unix API and another a Windows API

Exokernel vs. Microkernel A micro-kernel provides abstractions to the hardware such as files, sockets, graphics etc. An exokernel provides almost raw access to the hardware.