An Case for an Interleaving Constrained Shared-Memory Multi- Processor CS6260 Biao xiong, Srikanth Bala.

Why is Parallel Programming Hard? Parallel programming is harder than single-threaded programming relatively easy? – Threads interleave in so many ways we can not decide the order of the execution of all the threads – some threads remain untested impossible to test all the interleavings.

Why is Parallel Programming Hard? Legal Thread Interleavings Incorrect interleavings found during testing Untested interleavings - cause for concurrency bugs

Solution Synchronization operation: semaphores locks condition variable transaction. Memeory consistancy model it reduces the number of legal thread interleavings. Using PSet to test correct interleavings in program’s execution. Avoid untested interleavings occur infrequently.

Challenges How to encode tested interleavings in a program’s binary? – Predecessor Set (PSet) interleaving constraints How to efficiently enforce interleaving constraints at runtime? Detect violations of PSet constraints Avoid violations by stalling or using rollback-and- re-execution support

Constraining Interleavings A majority of the concurrency bugs are avoidable – Data races, atomicity violations, and also order violations Performance overhead is low – Untested interleavings in well-tested programs are likely to manifest rarely

Data Race A pair of memory accesses to the same memory location, at least one is write, neither one happens before the other

Data race detectors Happens-before Only data race in a given program execution Lockset-based Predict data races. Not occur in a program’s execution.

Benign data race Not all data races are harmful data races, programmers allow data races to optimize performance.

Atomicity Violation Atomicity is a guarantee of isolation from cuncurrent processes x++? mov eax,dword ptr [x] add eax,1 mov dword ptr [x],eax

Atomicity Violation Detectors AVIO Analyse atomic region and detect atomicity violations.

Order violation Thread should be invoked only after thread2 executes the wait().

Deterministic Multi-threading Any execution of a multi-threaded program would yield the same output as long as the input remains the same.

PSet Defined for each static memory operation. Consider true(read after write) as well as false(write after write and write after read)

PSet P in PSet of M iff 1.either P or M should be a write 2. P, M executed in two different threads (T1, T2) 3.M was immediately dependent on P 4. neither T1 nor T2 executed a read or a write between P and M.

PSet Example

PSet It can capture the fact that a benign data race interleaving is a correct interleaving, it will not hurt the performance.

PSet Instruction with Pset information for a 32-bit ISA

Thank You