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POEM ID: 038
Title: Raise an error if a user declares a sub-jacobian to have a value of zero.
authors: [Kenneth-T-Moore]
Competing POEMs:
Related POEMs:
Associated implementation PR:

Status:

  • Active
  • Requesting decision
  • Accepted
  • Rejected
  • Integrated

<Note: two space are required after every line of the header to create proper linebreaks in the markdown>

Motivation

In an OpenMDAO component, the declare_partials method is used to declare which input/output pairs have derivatives defined. Any pair that is not declared is assumed to be zero. However, at present, you can also declare a derivative to have a constant zero value like this:

self.declare_partials(of='y', wrt='x', val=0.0)

To the user, this is currently the same as not declaring the derivative. It is also the same mathematically, however, there are some important differences:

  1. The value 0.0 is stored as a sub-jacobian, so there is some memory overhead associated with storing it. If any matrix-vector products are computed, there will also be some unnecessary calculation multiplying by zero.

  2. In order to calculate the coloring for the total derivatives, a jacobian is generated and filled with random values. Every entry that is declared is filled, even ones that are declared as 0.0 constants. This can result in a total derivatives coloring that isn't as efficient as it should be (i.e., requires more colors than it would if the zero-valued derivative was not explicitly declared.)

Description

The following:

self.declare_partials(of='y', wrt='x', val=0.0)

will raise an error in future versions of OpenMDAO.