1. Dependable computing needs pervasive debugging Tim Harris tim.harris@cl.cam.ac.uk

2. Outline  Motivation  Problems with existing debuggers  Pervasive debugging  System architecture  Demo

3. Motivation  One of the projects we’ve been working on is building lock-free data structures  Safe for multi-threaded use, but don’t use locks  Built from atomic read, write and CAS – lots of subtle interactions  Algorithms are short, but formal methods hard to apply here

4. Debugger problems  Usually only forward execution  Probe effects / heisenbugs  e.g. relaxed memory orderings or genuine-parallelism versus time-sliced  Support for concurrency  e.g. deterministic re-execution  Support for distribution  usually one debugger needed for each process  sometimes support for tracing across RPCs  network monitoring software, tcpdump, nProbe

5. Pervasive debugging  Present the user with a single interface to the entire system  All of the processes involved  All of the threads within them  All of the communication links  Execute the application in a virtualized and completely deterministic environment  At least we’d have bohrbugs rather than heisen- ones  Challenges  Correspondence between the real and virtual environments  Design of internal interfaces  Implementation efficiency

6. System architecture Currently GDB Remote stub Execution driver Instruction virtualizer Scheduler definition Instruction annotations Virtualized libraries Pervasive debugger User interface Program files

7. Demo CPU Write buffer Shared memory CPU Write buffer