A Method for Providing Complete Access to the Concurrent Programming Model

1. A Method for Providing Complete Access to the Concurrent Programming Model Jerry James and Douglas Niehaus Information and Telecommunication Technology Center Electrical Engineering and Computer Science Department University of Kansas {james,niehaus}@ittc.ku.edu

2. Overview • Problem • Irreproducible Concurrent Behavior • Background • Definitions and Terms • Solution • Programming Environment Enhancements • Record and Control • Implementation • Conclusions

3. Problem • Concurrent software is important to many people • Wide range of applications and contexts • None of it is subject to scientifically valid experimentation • Irreproducible behavior based on concurrency control • Programming model does not constrain interleaving of concurrent components • Concurrency control errors are often intermittent • Happen in a few of all possible component interleavings • Problematic target for model driven programming

4. Background • Multi-threading is one common form of concurrency • Distribution is another • Thread events include: • Instruction execution (Basic Blocks) • Context Switch, Signals, Read/Write, System Calls • History of a thread is the set of events it experiences • History of a computation is the interleaving of the thread histories • Complete event history would be reproducible because it would include every relevant event • Everything affecting thread behavior and interleaving

5. Solution • Record Computation Event History • All important computation events • Interleaving of computation thread histories • Message arrival for distributed computations • Programming environment support for executing/reproducing a specified history • Recorded, or • Synthesized • BERT: implementation pattern enabling recording and control of all important computation events • Multi-threaded and distributed • Add reproducibility property to programming model

6. Recording an Event History • Thread Events • Gathered by adaptation of GCC code profiling • CPU Instruction Execution is basic thread event • Entry to each Basic Block is an equivalent shorthand • Basic block is a linear sequence of machine instructions • Entered at a label, Terminated by a jump (mostly) • Thread interactions with environment • Signals • Read/Write • System Calls

7. Recording an Event History • A Thread event history is the sequence of events it experiences during execution • Context switches are not events but note where the computation switches from one thread to another • A Computation event history is the concatenated segments of the histories of its component threads punctuated by context switches • Distributed computations also track message delivery

8. History Completeness • A history is complete when it contains everything required to reproduce computation behavior • Behaviors contained in “everything” depend on the semantics of the application computation • Complete set • Instruction execution (Basic Block) • Context Switch • Signals • Read/Write (including messages) • System calls

9. Reproduce an Event History • Clever Insight completes the reproduction environment • Adds ability to attach a TCL/GDB script to a breakpoint • Breakpoints at signal and context switch locations • Clever Insight uses specified history and that produced by executing program to identify proper breakpoint context • Requires the ability, from the programming environment, to context switch and deliver signals on demand • Implemented with breakpoint scripts that take the required action and set the next breakpoint in the history

10. Synthesize an Event History • Test interesting concurrency scenarios • Model generated would be great • Not all history specifications correspond to possible scenarios • Use Clever Insight to guide execution and thus construct a desired event history • Specify the next context switch or signal point • Clever Insight sets a breakpoint • Compares predicted behavior to actual behavior

11. BThreads: Reproducible Environment • Compiler modifications for history recording • BThreads is a user-level library implementing the POSIX multi-thread standard • Provides both recording and control hooks • Encapsulates system calls of interest • Currently limited to CPU-bound computations • Modified debugger (Clever Insight) for following recorded or synthesized event histories • Well behaved target for abstract modeling of concurrent implementations of applications

12. Conclusion • This addresses a long-neglected concurrent programming need • Our approach gives control over concurrency, which is necessary for the reproducibility property • We have constructed a multithreaded example • Another example exists for concurrency arising from distribution, based on single-machine simulation of the distributed environment • KU-PNNI simulator

13. Future Work • Extension to include I/O • Read/Write of files, and reproduction of byte streams • Capture system call information transfer to computation • Message histories for distributed computations • Explicit Modeling support • Execute synthesized event histories • Instrument model specified data sets • Post processing comparison of predicted and produced event histories • Suggestions from modelers ??