[Coin-discuss] CoinPackedMatrix memory management

[Jörg Herbers]

When building a sparse matrix with several ten thousand non-zero entries 
with CoinPackedMatrix, I have run into performance problems: When I 
successively added columns without manual actions wrt. memory management 
(matrix is column-ordered), it took several minutes to build the matrix. 
When reserving memory before (via CoinPackedMatrix::reserve), initialization 
times reduce to fractions of a second.

I have found that a reallocation is done every time a column is added (in 
resizeForAddingMajorVectors) if memory is not allocated before. Effectively, 
all data vectors are newly allocated and copied.

Clearly, manual allocation via reserve() is always possible, but the 
calculation of the number of non-zeros can be cumbersome. Instead, it would 
be better to have an enhanced memory management, e.g. by using "realloc" 
style memory management (which does not always copy the memory blocks) and 
by reserving extra space upon each resize operation (i.e. always 
reallocating for a given "delta" of additional elements). Is there anything 
planned in that direction?

Jörg

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