The Structuring of Systems
Using Upcalls
David D. Clark
4/26/2011
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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
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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
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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
6 presentation
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session
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transport
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network
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data link
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physical
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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 Protocol
3 Layers and 3 Tasks
Display
Transport
Network
Create
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Receive
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Send
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Control Flow in Network Protocol
Display
Layer
Transport
Layer
Network
Layer
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Example - Display Layer
Network Protocol
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Example - Transport Layer
Network Protocol
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Example Network Layer
Network 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
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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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