automatically exploiting cross-invocation parallelism using runtime ...
automatically exploiting cross-invocation parallelism using runtime ... automatically exploiting cross-invocation parallelism using runtime ...
1.2 ContributionsFigure 1.5 demonstrates the contribution of this thesis work compared to prior works. Thisthesis work presents two novel automatic parallelization techniques (DOMORE and SPEC-CROSS) that capture dynamic cross-invocation parallelism. Unlike existing techniques,DOMORE and SPECCROSS gather cross-invocation dependence information at runtime.Even for programs with irregular dependence patterns, DOMORE and SPECCROSS synchronizeiterations which depend on each other and allow iterations without dependencesto execute concurrently. As a result, they are able to enable more cross-invocation parallelizationand achieves more scalable performance.As a non-speculative technique, DOMORE first identifies the code region containingthe targeted loop invocations, and then transforms the program by dividing the region intoa scheduler thread and several worker threads. The scheduler thread contains code to detectmemory access conflicts between loop iterations and code to schedule and dispatch loopiterations from different loop invocations to worker threads. In order to detect access violations,the scheduler duplicates loops the instructions used for calculating the addressesof memory locations to be accessed in each loop iteration. As a result, at runtime, it knowswhich iterations access the common memory locations and then coordinates the executionof these conflicting iterations by generating and forwarding synchronization conditions tothe worker threads. A synchronization condition tells the worker thread to wait until anotherworker thread finishes executing the conflicting iteration. Consequently, only threadswaiting on the synchronization conditions must stall, and iterations from consecutive loopinvocations may execute in parallel.SPECCROSS parallelizes independent loops and replaces the barrier synchronizationbetween two loop invocations with its speculative counterpart.Unlike non-speculativebarriers which pessimistically synchronize to enforce dependences, speculative techniquesallow threads to execute past barriers without stalling. Speculation allows programs to optimisticallyexecute potentially dependent instructions and later check for misspeculation.8
LLAutomaticallyLExploitingLCrossFinvocationLParallelismNoYesLUsingLRuntimeLInformationNoYesLUsingLRuntimeLInformationNoYesDOALLL[1]LDOANYL[55]DOACROSSL[15]LOCALWRITEL[26]DSWPL[51]NoIEL[53]SpeculativeYesLRPDL[61]SMTLiteL[42]LoopLFusionL[22]SpeculativeTsengL[72] No YesDOMORESpecCrossThisLThesisLWorkFigure 1.5: Contribution of this thesis workIf misspeculation occurs, the program recovers using checkpointed non-speculative state.Speculative barriers improve performance by synchronizing only on misspeculation.The two techniques are complimentary in the sense that they can parallelize programswith potentially very different characteristics. SPECCROSS, with less runtime overhead,works best when programs’ cross-invocation dependences seldom cause any runtime conflict.While DOMORE has its advantage in handling dependences which cause frequentconflicts. Implementation and evaluation demonstrate that both techniques can achievemuch better scalability compared to existing automatic parallelization techniques. Amongtwenty programs from seven benchmark suites, DOMORE is automatically applied to parallelizesix of them and achieves a geomean speedup of 2.1× over codes without crossinvocationparallelization and 3.2× over the original sequential performance on 24 cores.SPECCROSS is found to be applicable to eight of the programs and it achieves a geomeanspeedup of 4.6× over the best sequential execution, which compares favorably to a 1.3×speedup obtained by parallel execution without any cross-invocation parallelization.9
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LLAutomaticallyLExploitingLCrossF<strong>invocation</strong>LParallelismNoYesLUsingLRuntimeLInformationNoYesLUsingLRuntimeLInformationNoYesDOALLL[1]LDOANYL[55]DOACROSSL[15]LOCALWRITEL[26]DSWPL[51]NoIEL[53]SpeculativeYesLRPDL[61]SMTLiteL[42]LoopLFusionL[22]SpeculativeTsengL[72] No YesDOMORESpecCrossThisLThesisLWorkFigure 1.5: Contribution of this thesis workIf misspeculation occurs, the program recovers <strong>using</strong> checkpointed non-speculative state.Speculative barriers improve performance by synchronizing only on misspeculation.The two techniques are complimentary in the sense that they can parallelize programswith potentially very different characteristics. SPECCROSS, with less <strong>runtime</strong> overhead,works best when programs’ <strong>cross</strong>-<strong>invocation</strong> dependences seldom cause any <strong>runtime</strong> conflict.While DOMORE has its advantage in handling dependences which cause frequentconflicts. Implementation and evaluation demonstrate that both techniques can achievemuch better scalability compared to existing automatic parallelization techniques. Amongtwenty programs from seven benchmark suites, DOMORE is <strong>automatically</strong> applied to parallelizesix of them and achieves a geomean speedup of 2.1× over codes without <strong>cross</strong><strong>invocation</strong>parallelization and 3.2× over the original sequential performance on 24 cores.SPECCROSS is found to be applicable to eight of the programs and it achieves a geomeanspeedup of 4.6× over the best sequential execution, which compares favorably to a 1.3×speedup obtained by parallel execution without any <strong>cross</strong>-<strong>invocation</strong> parallelization.9
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