From: Song Xue (speedsx_at_hotmail.com)
Date: Tue Jan 20 2004 - 23:38:21 PST
The paper "Application Performance and Flexibility on Exokernel Systems" describes an exokernel system that safely gives untrusted software efficient control over hardware and software resources by separating management from protection. The goal is to allow specialized applications to achieve high performance without sacrificing the performance of unmodified programs.
The authors believe that traditional systems are flawed because they are designed to accommodate every application and therefore must anticipate all possible needs. They result in poor performance and are error prone. Exokernels, on the other hand, safely gives un-trusted software efficient control over hardware and software resources by separating management from protection:: they protect resources but delegate management to applications.
An exokernel must provide a means to safely multiplex disks among multiple library file systems. The authors first describe XN, Xok's extensible, low-level in-kernel stable storage system. It provides access to stable storage at the level of disk blocks, exporting a buffer cache registry as well as free maps and other on-disk structures. The main purpose of XN is to determine the access rights of a given principal to a given disk block as efficiently as possible. XN's novel solution employs UDFs (untrusted deterministic functions), which are metadata translation functions specified to each file type. UDFs are stored on disk in structures called templates.
The authors build Xok/ExOS, based on which all performance analysis is done. Xok safely multiplexes the physical resources on Intel x86-based computers. ExOS is a libOS that supports most of the abstractions found in 4.4BSD. Xok/ExOS is compared to both FreeBSD 2.2.2 and OpenBSD 2.1 on the same hardware. The authors start performance analysis by measuring the base system performance. The conclusion is that even though ExOS implemtns the bulk of the operating system at the application level, common software development operations perform comparably to the reference systems, which demonstrates that the cost of low-level multiplexing is negligible. The paper then demonstrated some of the interesting possibilities in functionality and performance enabled by application-level resource management. The results range from comparable to significantly. As to global performance, the authors believe that an exokernel can out-perform previous systems.
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