# speculative

Compute speculative gradients

**Topics**

method_independent_controls

**Specification**

*Alias:*None*Arguments:*None*Default:*no speculation

**Description**

When performing gradient-based optimization in parallel,
`speculative`

gradients can be selected to address the load imbalance
that can occur between gradient evaluation and line search phases. In
a typical gradient-based optimization, the line search phase consists
primarily of evaluating the objective function and any constraints at
a trial point, and then testing the trial point for a sufficient
decrease in the objective function value and/or constraint
violation. If a sufficient decrease is not observed, then one or more
additional trial points may be attempted sequentially. However, if the
trial point is accepted then the line search phase is complete and the
gradient evaluation phase begins. By speculating that the gradient
information associated with a given line search trial point will be
used later, additional coarse grained parallelism can be introduced by
computing the gradient information (either by finite difference or
analytically) in parallel, at the same time as the line search phase
trial-point function values. This balances the total amount of
computation to be performed at each design point and allows for
efficient utilization of multiple processors. While the total amount
of work performed will generally increase (since some speculative
gradients will not be used when a trial point is rejected in the line
search phase), the run time will usually decrease (since gradient
evaluations needed at the start of each new optimization cycle were
already performed in parallel during the line search phase). Refer to
[BSS88] for additional details. The
speculative specification is implemented for the gradient-based
optimizers in the DOT, CONMIN, and OPT++ libraries, and it can be used
with dakota numerical or analytic gradient selections in the responses
specification (refer to `responses`

gradient section for information on these
specifications). It should not be selected with vendor numerical
gradients since vendor internal finite difference algorithms have not
been modified for this purpose. In full-Newton approaches, the Hessian
is also computed speculatively. NPSOL and NLSSOL do not support
speculative gradients, as their gradient-based line search in
user-supplied gradient mode (dakota numerical or analytic gradients)
is a superior approach for load-balanced parallel execution.

The `speculative`

specification enables speculative computation of
gradient and/or Hessian information, where applicable, for parallel
optimization studies. By speculating that the derivative information
at the current point will be used later, the complete data set (all
available gradient/Hessian information) can be computed on every
function evaluation. While some of these computations will be wasted,
the positive effects are a consistent parallel load balance and
usually shorter wall clock time. The `speculative`

specification is
applicable only when parallelism in the gradient calculations can be
exploited by Dakota (it will be ignored for `vendor`

`numerical`

gradients).