There is no concept of sparsity in vectors in PETSc. If much of your Vec is zero entries then it is expected that you define subproblems that work with only the relevant part of the vectors in your algorithms and not expect it to come "for free" from the usual PETSc operations. Barry On Feb 9, 2012, at 8:09 AM, Hui Zhang wrote:

Hi,

Is sparsity of Vec exploited in MatMult? I'm implementing some domain decomposition methods. The residual is non-zero or significant only at very few nodes. Does MatMult knows that most entries of the Vec are zeros? Is there a threshold to regard an value as zero?

Thanks, Hui