Debugging Threaded Applications By Andrew Binstock CMPS Parallel Processing Halverson

Why? PP difficult due to complexity of threads Concurrent threads = Speedup Correct design of threads –Orderly –Well designed –Predictable Interference among threads causes problems

Threading Bugs When 2 threads interact –One sends a result to another When 2 threads share a common data variable –Updating a shared array

Common Violations TRY TO AVOID Data Races Deadlocks Live Locks

Data Race Occurs: When 2 threads share variable but don’t guard against simultaneous access E.G. Selecting seat on an airplane Recognize: Inconsistent results using same data Cause: No mutual exclusion on shared variable Prevention: Use mutual exclusion (lock)

Deadlock Occurs: Two or more threads blocked as each waits on a lock held by the other E.G. Combat game – clearing street & removing snipers Recognize: Threads don’t advance; program may freeze Cause: Failure to identify dependence; acquiring locks in wrong order –E.G. Game record score – display & record

Deadlock (cont’d) Avoid: –Required order of acquiring locks –Release one lock of second is not available

Live Lock Occurs: Multiple threads do not advance though actively working –Often - Algorithm flaw, not implementation E.G. Dining Philosophers Problem –If all 5 act synchronously Recognize: Multiple threads and/or program freeze or infinitely loop

Live Lock (cont’d) Hard to reproduce –May be hardware dependent 4 cores vs. 2 cores vs. more Prevention: Consider what happens if all threads run simultaneously

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