The Structuring of SystemsUsing Upcalls

David D. Clark

4/26/2011 1Frank Sliz, CS533, Upcalls

Overview

• A method for implementing layers with synchronous procedure calls between layers

• Upcalls – flow of control by a lower layer calling an upper layer via procedure call

• Multi-task module – subroutines in different tasks that make up the layer

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Outline

• Background• Upcalls and Multi-task Modules• Example• Advantages• Swift Operating System• Problems• Conclusions

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Background

• 1968 – THE operating system

• Great interest in using layers to structure OS

• 1982 – ISO model for network communication

• 1985 – Swift operating system

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Monolithic Operating System

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Layered Operating System

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Advantages of Layered OS

• Modularity• Easier to test and debug• Easier to modify • Acyclic dependency • Verification• Trustworthiness

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7-Layer OSI Model

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application

presentation

session

transport

network

data link

physical

561

7

4

6

2

3

1

Disadvantages of Layered OS

• Performance– Asynchronous communication between layers– Buffering of data between layers– Copying of data for crossing protection boundary– Programming complexity

• Inefficient support for upward flow of control

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Upcalls and Multi-Task Modules

• Upcalls – Flow of control by a lower layer calling an upper

layer via procedure call

• Multi-Task Modules– Subroutines in different tasks that make up the

layer– Subroutines are callable from above or below via

procedure calls– State variables in shared memory

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System Organization

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Example - Network Protocol3 Layers and 3 Tasks

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Display

Transport

Network

Create Receive Send

Control Flow in Network Protocol

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Transport Layer

Display Layer

Network Layer

Example - Display LayerNetwork Protocol

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Example - Transport LayerNetwork Protocol

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Example Network LayerNetwork Protocol

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Example – Create Task

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Example – Receive Task

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Example – Send Task

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Advantages of Upcalls

• Upward, synchronous flow via procedure call is more efficient than IPC

• No need for data buffering between layers• Simplicity of implementation• Lower layer can ask advice from upper layer

resulting in simplification of algorithm

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Advantages of Multi-Task Modules

• Subroutine interfaces are easier to deal with than IPC interfaces

• No system wide IPC message format needed• Decisions about task usage can be made later,

the tasks are not hard coded into the layers• Acknowledgements can be sent with new

data packet - Piggybacking

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Swift Operating System

• Explore upcall and multi-module usage• Single address space with a typesafe language• Monitor locks for access to shared state• Initially used for network protocols• Test to determine if useful in other contexts• Develop solutions for upcall problems

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Problems with Upcall

• Violates basic principle of layering– upcall failure could leave lower layer left in

unstable condition

• Recovery of resources when an upcall fails

• Upcall gets into an infinite loop

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Mitigating Upcall Failures

• 2 classes of data – Client or private data which is expendable– Data shared between tasks must be unlocked and

consistent before upcall

• Expendable tasks• Layer-specific cleanup via system procedures• Timer or User to detect infinite loop

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Solutions to Indirect Recursive Call

• Put state variables in consistent state before upcall and then reevaluate them upon return

• Prohibit recursive downcall – variables can be left locked and inconsistent when upcalling

• Downcall queues the work for later execution• Downcall is restricted in its actions• Downcall is replaced by extra arguments or it

is replaced by a second upcall to query

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Problems with Multi-Task Modules

• Failure to use monitor locks properly which can cause incorrect results

• With single shared memory address space program bugs can corrupt memory

• Need queues for suspended tasks waiting to obtain monitor locks

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Conclusions

• Methodology is useful for operating systems – upcalls and multi-task modules are efficient– Strongly checked typesafe language should be

used in a single shared address space– useful for network protocols, text editors, and I/O

• Bad idea to implement a layer as a process• Parallel systems need shared memory for

efficient communication between processes

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