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Hi,

I am using the latest release of TBB for parallelizong my c++ finite element analysis application. I am facing trouble with the matrix assembly part, where each thread calculates a component of the global matrix and adds it to the global matrix (i.e., each thread is writing to a global variable). To facilitate this, I am using a spin_mutex lock. Unfortunately, every once in a while I miss one of the thread's contributions to the matrix, and the numerical computations blow-up.

Following is the structure of my code. I think I am using the spin_mutex correctly, but the the problem is random. Sometimes it happens after two iterations, and sometimes it takes 100 or so (and all possibilities in between). I would appreciate if someone could comment on whether there is an error in my use of the mutex, or if this is a known issue and there is a workaround.

I am using this on Mac OS 10.8.2. The hardware is: MacBook Air with 1.7 GHz Intel Core i5 with 4GB 1333 MHz DDR3 RAM.

Thanks,

Manav

Update: Found this to be an error in my code. Fixed and working fine now!

tbb::spin_rw_mutex assembly_mutex;

class AssembleElementMatrices

{

public:

AssembleElementMatrices(const std::vector<FESystem::Mesh::ElemBase*>& e,

FESystem::Numerics::VectorBase<FESystemDouble>& r,

FESystem::Numerics::MatrixBase<FESystemDouble>& stiff):

elems(e),

residual(r),

global_stiffness_mat(stiff)

{ }

void operator() (const tbb::blocked_range<FESystemUInt>& r) const

{

FESystem::Numerics::DenseMatrix<FESystemDouble> elem_mat;

FESystem::Numerics::LocalVector<FESystemDouble> elem_vec;

for (FESystemUInt i=r.begin(); i!=r.end(); i++)

{

// code to calculate elem_vec and elem_mat

{

tbb::spin_rw_mutex::scoped_lock my_lock(assembly_mutex, true);

dof_map.addToGlobalVector(*(elems[i]), elem_vec, residual); // adds elem_vec to appropriate locations in residual vector

dof_map.addToGlobalMatrix(*(elems[i]), elem_mat, global_stiffness_mat); // adds elem_mat to appropriate locations in the global_stiffness_mat matrix

}

}

}

}

protected:

const std::vector<FESystem::Mesh::ElemBase*>& elems;

FESystem::Numerics::VectorBase<FESystemDouble>& residual;

FESystem::Numerics::MatrixBase<FESystemDouble>& global_stiffness_mat;

};

void calculateQuantities()

{

const std::vector<FESystem::Mesh::ElemBase*>& elems = mesh.getElements();

tbb::parallel_for(tbb::blocked_range<FESystemUInt>(0, elems.size()), AssembleElementMatrices(elems, residual, global_stiffness_mat));

}

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*), elem_vec, residual); // adds elem_vec to appropriate locations in residual vector } { tbb::spin_rw_mutex::scoped_lock my_lock(stiffness_mutex, true); dof_map.addToGlobalMatrix(*(elems*

*), elem_mat, global_stiffness_mat); // adds elem_mat to appropriate locations in the global_stiffness_mat matrix } Jim Dempsey*- Subscribe to RSS Feed
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