Jerry James and Douglas Niehaus

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_at_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

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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 ??