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

[Lorenzo Botti <bottilorenzo@xxxxxxxxx>]

Ok, thanks! It works. 

The trick is to make the matrix storage consistent to the thread's filling behavior (by row in my case). 

One caveat: I need to recreate the matrix at each assembly, using setZero() does not work.  

Il giorno gio 5 lug 2018 alle ore 19:28 Gael Guennebaud <gael.guennebaud@xxxxxxxxx> ha scritto:

Hi,

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.

gael

On Thu, Jul 5, 2018 at 3:20 PM Christoph Hertzberg <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
re-allocated.

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


Christoph



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