Review of sharing and protection paper

From: Joanna Muench (joannam_at_spro.net)
Date: Wed Jan 14 2004 - 12:34:30 PST

  • Next message: David Coleman: "Review of Chase et al."

    The advent of the 64-bit processor allowed the development of the
    single-address-space OS described in Chase et al. (1994). The paper
    argues that a single-address-space system is a significant improvement
    on traditional systems, providing greater reliability, easing sharing
    and all those good things. The authors motivate the system by pointing
    out the design tradeoffs between protection, performance and integration
    forced by private-address-space systems. The paper summarizes the
    fundamental concepts to the system, including now-familiar capabilities,
    protection domains and portals, the point of access to a domain. Portals
    are required since sharing between threads takes place via shared
    memory. The discussion of how the prototype was implemented on a Mach
    microkernel mentions the surprising notion of persisting capabilities,
    segments and other resources across restarts.

    The later sections of the paper on application of the system, issues and
    relation to the past helped place this work in context. The paper gives
    a compelling performance example, showing how overlapping domains
    between applications can provide the safety of a decomposed system along
    with the performance of a monolithic system. I didn't have time to fully
    appreciate the issues section - that will take rereading. The final
    section of the paper presents how Opal has worked off of other systems,
    especially capability based architectures, and how improvements in
    processor performance allowed the authors to bring some of those
    concepts to their logical conclusion.

    While this paper was long I felt it gave a full picture of the Opal
    system. It will take a second reading (at least) to fully appreciate
    some of the finer details. I like the fact that the authors provided
    some good, concrete examples to drive home their points, such as the
    explicit example they gave in section 5.2 of the flexibility that UNIX
    tools like awk and grep provide, but at the cost of copying and
    converting the data multiple times. The authors outlined very clearly
    the important design philosophies, such as the idea that storage
    allocation, protection and reclamation should only be coarse grained on
    an OS level; finer control should be left to compilers. Finally, having
    spent a great deal of time in the mountains with Jeff Chase while this
    research was underway, I am happy to discover Jeff made such good use of
    both his recreation and work time.


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