PRES: Probabilistic Replay with Execution Sketching on Multiprocessors Soyeon Park and Yuanyuan Zhou (UCSD) Weiwei Xiong, Zuoning Yin, Rini Kaushik, Kyu H.Lee and Shan Lu (UIUC) SOSP 2009 LBA Reading Group 9/15/09 Presented by: Michelle Goodstein

Outline Motivation PRES Architecture Capturing Sketches Replaying Intelligently Evaluation Conclusion

Motivation Concurrency bugs are hard… Deterministic Replay can help, but… Deterministic Replay can be expensive What if only record partial information? –Good enough to reproduce bug vs actual execution –Reproduce bug in small (5-50) number of replays rather than first attempt?

PRES Probabilistic Replay via Execution Sketching –Records partial ordering during production run –Intelligently explores space of partial orderings Use feedback from failed attempts to reproduce bug in subsequent explorations

PRES Architecture Sketch Recorders Partial Information based Replayer (PI- Replayer) Replay Recorder Monitor Feedback Generator

PRES Architecture Sketch Recorders –During production run –Captures partial ordering of events –Balance of efficiency and usefulness in replay

PRES Architecture Partial Information based Replayer (PI-Replayer) –During bug reproduction phase –Consults with sketch, feedback from attempts to reproduce bug Sketch specifies ordering  do what sketch proscribes Feedback says ordering did not produce bug  do something else No info available – execute however desired

PRES Architecture Replay Recorder –Deterministic replay recorder –Necessary to produce feedback –When bug reproduced, have a deterministic record of how to repeat with 100% probability

PRES Architecture Monitor: –Tracks replays and detects: Deviations from sketch (new replay necessary) Bug reproduced (success!)

PRES Architecture Feedback Generator –Uses info from recorder to provide feedback for future replay attempts –Try to figure out why bug not discovered

Sketch Recorders Baseline (Base) –Everything necessary for det. replay on uniprocessor Synchronization recorder (Sync) –Above + global order at high-level synch ops System call recorder (Sys) –Above + global order of syscalls Function call recorder (Func) –Global order of all function calls (Michelle: also + above???) Nth-Basic block recorder (BB-n) –Records the nth basic block executed, (count is global) Basic Block recorder (BB) –Global order of all basic blocks Shared reads/writes (RW) –Standard deterministic replay

Sketch Recorders

Replaying Intelligently Monitor observes currently replay –Compares current replay to sketch to notice when to abort Inconsistent or off-sketch Bug reproduced –Operates only on visible events Exceptions, timer signals, outputs

Replaying Intelligently Unsuccessful replays –Sketches that are not RW miss some shared memory data races –If race occurs in certain orders, bug may not manifest –Idea: use info (feedback) from prior runs to guide choice of ordering in next replay attempt

Replaying Intelligently: Generating Feedback Need to do full RW recording of replay attempt Using failed replay recordings, identify data races Filter out data races where sketch implies ordering Select a data race to invert ordering of –Heuristic, chooses a replay recording and then the race closest to fault On next replay, execute deterministically until data race encountered, flip order Then, default PI-Replayer behavior takes over

Evaluation

Conclusion Interesting use of partial orders as compromise between efficiency and replay Partial information often sufficient to recover buggy ordering Similarities to the CHESS paper presented earlier