automatically exploiting cross-invocation parallelism using runtime ...
automatically exploiting cross-invocation parallelism using runtime ... automatically exploiting cross-invocation parallelism using runtime ...
Algorithm 1: Pseudo-code for scheduler synchronizationInput: iterNum : global iteration numberaddrSet ← computeAddr(iterNum)tid ← schedule(iterNum, addrSet)foreach addr ∈ addrSet do< depTid, depIterNum > ← shadow[addr]if depIterNum ≠ −1 thenif depTid ≠ tid thenproduce(tid, < depTid, depIterNum >)shadow[addr] ← < tid, iterNum >produce(tid, < NO SYNC, iterNum >)thread finds no additional dependences, it will send (NO SYNC,i) to thread T1.3.2.3 Synchronizing IterationsWorkers receive synchronization conditions and coordinate with each other to respect dynamicdependences. A status array is used to assist this: latestFinished recordsthe latest iteration finished by each thread. A worker thread waiting on synchronizationcondition (depId, depIterNum) will stall until latestFinished[depId] ≥depIterNum. After each worker thread finishes an iteration, it needs to update its statusin latestFinished to allow threads waiting on it to continue executing.Synchronization conditions are forwarded using produce and consume primitivesprovided by a lock-free queue design [31], which provides an efficient way to communicateinformation between scheduler and worker threads.3.2.4 Walkthrough ExampleThe program CG illustrates DOMORE’s synchronization scheme. Figure 3.5(a) shows theaccess pattern (value j) in each iteration for two invocations. Iterations are scheduled totwo worker threads in round-robin order. Figure 3.5(b) shows the change of the helper datastructures throughout the execution.30
InputSequentialProgramCompile TimeDetect Target Loop NestPartitionerCode GenerationOutputSchedulerCodeWorkerCodeRuntimeSchedulerThreadWorkerThreadsSynchronization SchemeFigure 3.4: Overview of DOMORE compile-time transformation and runtime synchronizationIteration I1 accesses array element A1, the scheduler finds shadow[A1] = meaning no dependence exists. It constructs a synchronization condition (NO SYNC,I1)and produces it to worker thread T1.It then updates shadow[A1] to be ,implying thread T1 has accessed array element A1 in iteration I1.consumes the condition and executes iteration I1 without waiting.Worker thread T1After it finishes, itupdates latestFinished[T1] to be I1. Iteration I2 accesses array element A3 andno dependence is detected.A synchronization condition (NO SYNC,I2) is producedto worker thread T2, and worker thread T2 consumes the condition and executes iterationI2 immediately.Iteration I1 in the second invocation accesses element A3 again.Since shadow[A3] = , a dependence is detected.So the scheduler produces(T2,I2) and (NO SYNC,I3) to worker thread T1. Worker thread T1 then waits31
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InputSequentialProgramCompile TimeDetect Target Loop NestPartitionerCode GenerationOutputSchedulerCodeWorkerCodeRuntimeSchedulerThreadWorkerThreadsSynchronization SchemeFigure 3.4: Overview of DOMORE compile-time transformation and <strong>runtime</strong> synchronizationIteration I1 accesses array element A1, the scheduler finds shadow[A1] = meaning no dependence exists. It constructs a synchronization condition (NO SYNC,I1)and produces it to worker thread T1.It then updates shadow[A1] to be ,implying thread T1 has accessed array element A1 in iteration I1.consumes the condition and executes iteration I1 without waiting.Worker thread T1After it finishes, itupdates latestFinished[T1] to be I1. Iteration I2 accesses array element A3 andno dependence is detected.A synchronization condition (NO SYNC,I2) is producedto worker thread T2, and worker thread T2 consumes the condition and executes iterationI2 immediately.Iteration I1 in the second <strong>invocation</strong> accesses element A3 again.Since shadow[A3] = , a dependence is detected.So the scheduler produces(T2,I2) and (NO SYNC,I3) to worker thread T1. Worker thread T1 then waits31
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