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Bug 1384

Summary: SparseMatrix division by scalar seems to be using dense math
Product: Eigen Reporter: Alec Jacobson <alecjacobson>
Component: SparseAssignee: Nobody <eigen.nobody>
Status: RESOLVED FIXED    
Severity: Performance Problem CC: chtz, gael.guennebaud
Priority: Normal    
Version: 3.3 (current stable)   
Hardware: All   
OS: All   
Whiteboard:

Description Alec Jacobson 2017-01-25 16:37:20 UTC
In Eigen 3.2.9 the follow code is fast/instantaneous:

  Eigen::SparseMatrix<double> I = Eigen::SparseMatrix<double>(100000,100000);
  Eigen::SparseMatrix<double> B = I / 100.0;

this suggests that Eigen was only doing the division on the non-zeros (and there aren't any so it's fast).

However, in Eigen 3.3.1 and 3.3.90 the code takes _a very long time_ and uses >1GB in memory. This suggests that its using dense rather than sparse math.

In my library we're doing operations like this to sparse matrices all of the time. Is this sparse matrix divided by scalar no longer supported?
Comment 1 Gael Guennebaud 2017-01-30 12:39:53 UTC
Thank you for the report.

https://bitbucket.org/eigen/eigen/commits/9fa03a189815/ (devel)
https://bitbucket.org/eigen/eigen/commits/d0f3c1ef1112/ (3.3)
Summary:     Bug 1384: fix evaluation of "sparse/scalar" that used the wrong evaluation path.
Comment 2 Nobody 2019-12-04 16:45:46 UTC
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