1. The Structuring of Systems Using Upcalls Paper by David D. Clark Presentation by Emerson Murphy-Hill

2. Objectives Build Swift operating system Develop and describe layered OS methodology that is: –Supportive of highly interactive, parallel execution –Based on layered network protocols –Coherent and easily readable –Simple, short, and efficient

3. General Architecture Tasks Multitask modules (layers) Processes Subroutine Calls Synchronized process calls (IPC) Interprocess (intermodule?) Communication Interprocess Communication Swift Standard Layers (Dijkstra)

4. Layers / Multitask Modules Each layer is an encapsulated module: –Private state –Accessible only through subroutine calls A layer “depends on” the layer below A layer is a coherent piece of functionality Layer “instance of” multitask module (analogously – object instance of class)

5. Call Direction Typically down through layers (example: send data) Can be upcall (example: more data?), but: –Do nice testing properties remain? –What are the semantics of the relationship of a lower to an upper layer? –What about recursion?

6. display-start(): local-port = transport-open(display-receive) end display-receive(char) : //write char to display end transport-open(receive-handler): local-port = net-open (transport-receive) transport-handler-array (local-port)=receive-handler return(localport) end transport-get-port(packet): //upcalled by interruptlayer extract port from packet return(port) end transport-receive(packet,port): //upcalled by net-layer handler=transport-handler-array(port) validate packet header for,each char in packet do handler(char) end net-open(receive-handler): port=generate-uid() task-id = create-task( net-receive( port, receive-handler)) net-task-array(port)=task-ld return(port) end net-receive(port,handler): handler=net-handler-array(port) do forever remove packet from port queue handler(packet,port) block() end net-dispatch(): //upcalled by interrupt handler read packet from device restart device port = transport-get-port(packet) put packet on per port queue task-id = net-task-array(port) wakeup-task(task-ld) end Display Layer Transport Layer Net Layer transport_handler_array[p] = {display-receive} net_task_array[p] = {net-receive(p,transport_receive)}

7. display-start(): local-port = transport-open(display-receive) end display-receive(char) : //write char to display end transport-open(receive-handler): local-port = net-open (transport-receive) transport-handler-array (local-port)=receive-handler return(localport) end transport-get-port(packet): //upcalled by interruptlayer extract port from packet return(port) end transport-receive(packet,port): //upcalled by net-layer handler=transport-handler-array(port) validate packet header for,each char in packet do handler(char) end net-open(receive-handler): port=generate-uid() task-id = create-task( net-receive( port, receive-handler)) net-task-array(port)=task-ld return(port) end net-receive(port,handler): handler=net-handler-array(port)? do forever remove packet from port queue handler(packet,port) block() end net-dispatch(): //upcalled by interrupt handler read packet from device restart device port = transport-get-port(packet) put packet on per port queue task-id = net-task-array(port) wakeup-task(task-ld) end transport_handler_array[p] = {display-receive} net_task_array[p] = {net-receive(p,transport_receive)}

8. Advantages of Methodology Generally, layers have low-coupling Uses subroutine calls: –Substantially cheaper than IPC / RPC (no context switching or message queue management) –Preserves traditional programming semantics –Fast! –No need to implement communication protocol Best suited towards interactive and network-based systems

9. Encountered Problems Local upcall failure. Control propagation through: –Partitioning/recover data –Destroy task stripe Indirect recursive calls corrupt state. Solutions: –Completely reevaluate state on return (expensive/clumsy) –Prohibit recursive downcall while doing upcall (runtime only) Locking problems –Solution: queue work on recursive lock conflict (not implemented) –Not easy to impose locking discipline

10. Other Features Task Scheduling –Dynamic priority based on deadline –Deadline promotion Address Space Management –Compile/runtime checking for pointers, etc –Garbage collection

11. Conclusion Layers with upcalls are suitable for “a large class of programming problems” – if you have good programmers! Layers implemented as: –processes are bad –multitask modules are good (fast and simple)