Cooperative Task Management without Manual Stack Management Or, Event-driven Programming is not the Opposite of Thread Programming Atul Adya, John Howell, Marvin Theimer, William J. Bolosky, John R. Douceur Microsoft Research  Presented by : Satish Gottimukkala

Outline  Introduction  Definitions  Stack Management  Hybrid Approach  Related Work  Conclusion

Introduction  Two major Concurrency models :  Multithreaded programming   Difficult to handle race conditions, deadlocks (Ousterhout 96)  Event-driven programming   Task explicitly yields control by returning to the scheduler   Task runs serially between yield points Vs

Definitions  Task management  Serial, preemptive, cooperative  Stack management  Automatic, manual  I/O management  Synchronous, asynchronous  Conflict management  With concurrency, need locks, semaphores, monitors  Data partitioning  Shared, task-specific

Cooperative Task Management Cooperative Task Management   Concurrency considered only at I/O yield points  Task must re-validate state after resuming

Task Management Vs Stack Management

Stack Management   Automatic Stack Management (ASM)  Each complete task a procedure  Task state stored on stack  Allows natural code structure   Manual Stack Management (MSM)  Each step an event handler  Event handlers invoked by scheduler  Control flow expressed through continuations  Necessary state + next event handler   Forces programmer to abandon basic programming language features   Mistakenly conflated with cooperative task mgmt

Stack Ripping  Function Scoping  Two or more functions represent a single conceptual function  Automatic variables  From stack to heap  Control structures  Loops can get nasty  Debugging stack  Need to recover call stack

Hidden assumptions  Hidden assumptions become explicit  Concurrency  Static check: yielding, atomic  Dynamic check: startAtomic(), endAtomic(), yield()

Hybrid approach  Separate out concerns of task and stack mgmt  Cooperative task management  Avoid synchronization issues  Allow interactions between manual and automatic stack mgmt code styles

Hybrid approach  Based on Windows fibers  User-level, cooperative threads  Main fiber  Event scheduler  Event handlers  Auxiliary fibers  Blocking code

Manual calling automatic  Set up continuation  Copy result  Invoke original continuation  Set up fiber  Switch to fiber  Issue: I/O  Are we really blocking?  No, we use asynchronous I/O and yield back to main fiber

Automatic calling manual  Set up special continuation  Test whether we actually switched fibers  If not, simply return  Invoke event handler  Return to main fiber  When done with task  Resume fiber

Related work   “Event-driven” to reduce concurrency bugs [Oust96]   Cooperative task management conflated with MSM   “Event-driven” model for performance   Popular for web servers   Inter-stage: each task reads as in MSM   Equivalence of “procedure-oriented” and “message-oriented” systems [Lauer+Needham]   Cooperative task management not considered

Conclusion  Clears debate between event driven versus threaded programming models by separating concerns of task and stack management  Manual stack management leads to poor code structure  Code evolution exacerbates problem  Automatic stack management leads to natural code structure  Adopts hybrid approach which shows that Manual and Automatic Stack Management code can co-exist  Enables cooperation among disparate programmers and software evolution of disparate code bases

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