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

[Brad Bell <bradbell@xxxxxxxxxx>]

I think that one would have to be careful to avoid false sharing
	https://en.wikipedia.org/wiki/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:
> 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 
> <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
>     re-allocated.
>
>     To repeat it more clearly: An Eigen::SparseMatrix is _not_ inherently
>     thread-safe (when values are inserted).
>
>
>     Christoph
>
>
>
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