Structuring The of Systems Using Upcalls
By David D. Clark Presented by Samuel Moffatt
Layering
Application Organization Technique
Separate functional components that interact in some hierarchical manner. There is typically only an interface to the layer above. Top down flow. Client requests services of the layer below Using a process to implement a layer is standard at this time and requires expensive interprocess communications to communicate between layers.
Layering in Operating Systems
Pros
Cons
Verification Organization Upward flow not well supported
Asynchronous mechanisms, extra overhead
Upcall
A procedure call from a lower layer into the layer above.
Causes control flow to go "up" Breaks "depends on" or "using" relationship of traditional layering Network protocols are designed with an upflow. Not efficiently supported at the time using interprocess communications.
Multitask Module
"Subroutines in different tasks that collaborate with each other through shared state." A layer. A task is a single thread of execution with subroutine calls for up and downcalls. Shared memory access controlled via locks. "Intertask" communication occurs only within a layer.
Multitask Module
Network Protocol Implementation Example
Upcall Benefits
Synchronous subroutine calls instead of asynchronous signals Helps to support "piggybacking" of information in network protocols. Simplicity of implementation
Eliminates buffering code for signals Lower layer can ask advice of upper layer. Programmer familiarity with subroutines Hide implementation details
Upcall Problems
A failed upcall can corrupt the layer and clients of that layer. Solution
Separate data. Local data and shared data. Shared must data be made consistent and unlocked before a call. If there is an error, may destroy a task (vertical stripe) in the system but resources can be recovered and if shared data is consistent then other tasks are not affected. Cleanup methods to recover resources.
Upcall Problems
Indirect recursive call Solutions
1. Put all variables in a consistent state before making an upcall then reevaluate state on return. 2. Prohibit recursive downcalls. 3. Have downcall perform action directly or queue execution by task holding a required lock. Good for interrupt handlers. 4. Downcalls never perform significant actions. Only simple things, like set flags. 5. Use a special upcall to collect data instead of a downcall.
Task Scheduling
Priority of a task
If a long deadline task is blocking a short deadline task the long task will be temporarily promoted.
Uses a deadline time instead of a number sequence. Easier to understand. Mutable. Assigned each time the process is scheduled.
Single Address Space
Single address space for efficiency. Programmed with CLU which provides compile time and runtime checks to protect the address space from corruption. Garbage collection instead of user destroying objects. No longer valid pointer protection. The above mitigate damage on failure so debugger can still run.
Conclusions
Upcalls are useful Efficiency and simpler code. Multitask modules are useful in organizing the system. "Strongly checked typesafe languages" is useful in a single address space. Decomposing the operating system into vertical stripes representing user requests is better than using a process as a layer. Shared memory is a good communication tool. Not using shared memory would require a task to manage state variables that were in the shared memory.