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Effects of the Single Address Space Effects of the Single Address Space ParadigmParadigm

on CPU and OS Design for aon CPU and OS Design for aDistributed Computer System Distributed Computer System

Donald S. Miller

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What’s a Paradigm?What’s a Paradigm? Constellation of beliefs, values, techniques and so on shared by

the members of a given community that implicitly define the legitimate problems and methods of a research field for succeeding generations of practitioners1

attracts an enduring group of adherents away from competing modes of scientific activity

sufficiently open-ended to leave all sorts of problems for the group of practitioners to resolve

Includes law theory, application and instrumentation together Examples:

Aristotle’s physics, Ptolemaic astronomy, Galileo’s motion studies, Copernican astronomy, Newtonian mechanics, Einstein's relativity

and Process-Oriented OSs and Single Address Space Operating

Systems

____________________1. The Structure of Scientific Revolutions, 3rd Edition, Thomas S. Kuhn, University of Chicago Press, 1996.

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Process-Oriented ParadigmProcess-Oriented Paradigm

Each process runs in it’s own virtual address space virtual addresses are (process) context dependent

Process boundaries encapsulate and bind temporary virtual address spaces, execution state, objects, protection domains and principals

Persistent storage is in a separate namespace - the file system

Message-based communication or RPC on top of message passing is the most prevalent mechanism for data exchange and synchronization between executing entities

Page-based object granularity Threads encapsulated within a process - domain crossing and

machine migration without an address space and thread context switch essentially impossible

Layered system structure - operating system kernel and services and user-level servers and programs layered on top of each other

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Process-Oriented Paradigm (continued)Process-Oriented Paradigm (continued)

Obtaining operating system services requires a domain crossing AND entry into privileged supervisor mode

Process and data migration difficult - involves creating a new namespace for data (DSM) and dealing with distinct machine namespaces for code

Difficult to directly support OOD and OOP - preprocessors and OS run-time systems are needed

OS support for protection implementation protection policy are intertwined

Access authorization, resource accounting and name services typically require kernel services or intervention

Copy set management is usually page-based and requires broadcasts reducing scalability

Network-wide fault tolerance requires additional mechanisms

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Process-Oriented Paradigm (concluded)Process-Oriented Paradigm (concluded)

Examples of Process-Oriented operating systems Industrial

MULTICS, UNIX, MVS, Mach, Chorus, VMS, NT Research

Exokernel, L4, SPIN

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Single Address Space ParadigmSingle Address Space Paradigm

All programs run in a single virtual address space that covers all levels of the memory hierarchy, all nodes of a network and persists for the life of the computer system. This leads to the following 5 properties.

1. Permanent and unique binding of all data and code to virtual addresses.

2. The ability to freely reassociate memory objects, access rights, protection domains, virtual addresses, principals and threads.

3. A single network-wide namespace backed by the local demand paged memory management on every node and that includes all levels of the memory cache hierarchy.

4. Object, thread and protection domain mobility across the entire network-wide virtual address namespace.

5. Separation of ownership of code and ownership of execution of a computation.

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Single Address Space Paradigm Single Address Space Paradigm (continued)(continued)

All programs run in the same virtual address space virtual addresses are not context dependent

Execution state, objects, protection domains and principals are not bound to virtual addresses and hence not bound to each other

Persistent storage is in the same virtual address namespace

Shared memory is the most prevalent mechanism for data exchange and synchronization

Object granularity can be independent of page granularity Threads can travel throughout the virtual address space

without an address space switch or a thread context switch System architecture can be flat and modular - operating

system services, environment servers and user-level programs can be made accessible to each other and themselves via simple procedure calls and returns

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Single Address Space Paradigm Single Address Space Paradigm (continued)(continued)

Obtaining operating system services is orthogonal to entry into privileged supervisor mode

Process and Data Migration is straight-forward - objects and threads exist at the same virtual addresses on all nodes; kernel structures can be replicated or represented by surrogates on different nodes

OOD and OOP can be directly supported - a protection domain can translate directly into an instantiation of an object class available at the user level - preprocessors and OS run-time systems are not needed; base classes can be extended via overrides at the user level

Clean separation of OS support for protection implementation and user-level definition of protection policy is simple

Access authorization, resource accounting and name services can be done at the user level

The common virtual address space provides simple scaleable per object copy set management and a lowest level cache for fault tolerance

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Single Address Space Paradigm Single Address Space Paradigm (concluded)(concluded)

Examples of Single Address Space Operating Systems

Without additional protection hardware assistance Opal, Mungi, Arius, Carthage (KFRPC)

With additional protection hardware assistance AS/400, MONADS, ToM, Carthage (FRPC), Sombrero

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Sombrero PrincipalsSombrero Principals

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SYSTEM ARCHITECTURESYSTEM ARCHITECTURE Process Oriented OSProcess Oriented OS Single Address Single Address

Space OS Space OS Layered system structure -

operating system kernel and services and user-level servers and programs layered on top of each other

System architecture can be flat and modular - operating system services, environment servers and user-level programs can be made accessible to each other and themselves via simple procedure calls and returns

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SYSTEM ARCHITECTURESYSTEM ARCHITECTURE Process Oriented OSProcess Oriented OS Single Address Single Address

Space OSSpace OS Obtaining operating system

services requires a domain crossing AND entry into privileged supervisor mode

Obtaining operating system services is orthogonal to entry into privileged supervisor mode

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Essence of the Multiple to Single Address SpaceParadigm Shift

Going from multiple namespaces in which object names canvary and are determined by the logical structuresthey are associated with, e.g., process, file orrecord, or by the physical container that holdsthem, e.g., cache, RAM or disk

Going to a single network-wide namespace in which objectnames are invariant and are determined by theunique virtual addresses used to access them

Since VAs are directly operated upon by CPUs, the focusshifts from naming objects by the changing logical orphysical structures associated with them to using VAs as theprimary identifying/locating /accessing mechanism for allobjects wherever they are and whenever they are used.

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SYSTEM ARCHITECTURESYSTEM ARCHITECTURE Process Oriented OSProcess Oriented OS Single Address Single Address

Space OSSpace OS Difficult to directly support

OOD and OOP - preprocessors and OS run-time systems are needed

OOD and OOP can be directly supported - a protection domain can translate directly into an instantiation of an object class available at the user level - preprocessors and OS run-time systems are not needed; base classes can be extended via overrides at the user level

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SYSTEM ARCHITECTURESYSTEM ARCHITECTURE Process Oriented OSProcess Oriented OS Single Address Single Address

Space OSSpace OS OS support for protection

implementation protection policy are intertwined

Clean separation of OS support for protection implementation and user-level definition of protection policy is simple

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SYSTEM ARCHITECTURESYSTEM ARCHITECTURE Process Oriented OSProcess Oriented OS Single Address Single Address

Space OSSpace OS Access authorization,

resource accounting and name services typically require kernel services or intervention

Access authorization, resource accounting and name services can be done at the user level