Re: [eigen] assembling a sparse matrix from multiple threads

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I think that one would have to be careful to avoid false sharing
to make this efficient; i.e., the memory for each thread would need to be in separate cache.

On 07/05/2018 09:10 AM, Gael Guennebaud wrote:


if you can guaranty that each column* is written by a single thread AND that you can predict a reasonable upper-bound for the number of non-zeros in each column*, then you can preallocate using reserve() and the subsequent writes within different columns will be thread safe.

* you can replace "column" by row if you use a row-majpr storage.


On Thu, Jul 5, 2018 at 3:20 PM Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx <mailto:chtz@xxxxxxxxxxxxxxxxxxxxxxxx>> wrote:

    On 2018-07-05 14:37, Lorenzo Botti wrote:
     > The point here is that I know that the code is thread safe, that is no more
     > than one thread will try write in position row i col j.
     > Accondingly performing the assembly in parallel should not required to lock
     > the insertion operation, am I wrong?

    That would be true for a dense matrix, but for a sparse matrix that
    can't work (in general) since sometimes values are moved or memory gets

    To repeat it more clearly: An Eigen::SparseMatrix is _not_ inherently
    thread-safe (when values are inserted).


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