MueLu: Cut Drop Behaving Differently on Entries of Equal Value

[NyquilDreams]

Bug Report

@jhux2 @cgcgcg @csiefer2

Description

While testing the Cut Drop Algorithm in MueLu_CoalesceDropFactory_def.hpp using the unit tests in CoalesceDropFactory.cpp, I noticed that some entries with a value of -1 were being kept while other entries that also have a value of -1 were being dropped in the same row. This is caused by a combination of the fact that at least one off diagonal entry must be kept and a corner case when threshold is less than 1 which causes only the first off diagonal entry to be kept. A check for this scenario needs to be added so that entries of the same value are either all kept or all dropped. The unit test should also be modified accordingly.

Trilinos/packages/muelu/test/unit_tests/CoalesceDropFactory.cpp

Lines 1410 to 1440 in bf922e7

	TEUCHOS_UNIT_TEST_TEMPLATE_4_DECL(CoalesceDropFactory, ClassicalUnScaledCut, Scalar, LocalOrdinal, GlobalOrdinal, Node) {
	#include <MueLu_UseShortNames.hpp>
	typedef Teuchos::ScalarTraits<SC> STS;
	typedef typename STS::magnitudeType real_type;
	typedef Xpetra::MultiVector<real_type, LO, GO, NO> RealValuedMultiVector;
	MUELU_TESTING_SET_OSTREAM;
	MUELU_TESTING_LIMIT_SCOPE(Scalar, GlobalOrdinal, Node);
	out << "version: " << MueLu::Version() << std::endl;
	RCP<const Teuchos::Comm<int> > comm = Parameters::getDefaultComm();
	Level fineLevel;
	TestHelpers::TestFactory<SC, LO, GO, NO>::createSingleLevelHierarchy(fineLevel);
	RCP<Matrix> A = TestHelpers::TestFactory<SC, LO, GO, NO>::Build1DPoisson(36);
	fineLevel.Set("A", A);
	Teuchos::ParameterList galeriList;
	galeriList.set("nx", Teuchos::as<GlobalOrdinal>(36));
	RCP<RealValuedMultiVector> coordinates = Galeri::Xpetra::Utils::CreateCartesianCoordinates<SC, LO, GO, Map, RealValuedMultiVector>("1D", A->getRowMap(), galeriList);
	fineLevel.Set("Coordinates", coordinates);
	CoalesceDropFactory coalesceDropFact;
	coalesceDropFact.SetDefaultVerbLevel(MueLu::Extreme);
	// We're dropping all the interior off-diagonal entries.
	// dx = 1/36
	// L_ij = -36
	// L_ii = 72
	// criterion for dropping is |L_ij|^2 <= tol^2 * |L_ii*L_jj|
	coalesceDropFact.SetParameter("aggregation: drop tol", Teuchos::ParameterEntry(0.51));

Trilinos/packages/muelu/src/Graph/MatrixTransformation/MueLu_CoalesceDropFactory_def.hpp

Lines 577 to 593 in bf922e7

	for (size_t i = 1; i < n; ++i) {
	if (!drop) {
	auto const& x = drop_vec[i - 1];
	auto const& y = drop_vec[i];
	auto a = x.val;
	auto b = y.val;
	if (a > realThreshold * b) {
	drop = true;
	#ifdef HAVE_MUELU_DEBUG
	if (distanceLaplacianCutVerbose) {
	std::cout << "DJS: KEEP, N, ROW: " << i + 1 << ", " << n << ", " << row << std::endl;
	}
	#endif
	}
	}
	drop_vec[i].drop = drop;
	}

[github-actions]

Automatic mention of the @trilinos/muelu team

[cgcgcg]

Maybe it's easier to not allow the user to set a threshold <= 1?