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Built-in Assessors

NNI provides state-of-the-art tuning algorithm in our builtin-assessors and makes them easy to use. Below is the brief overview of NNI current builtin Assessors:

Note: Click the Assessor's name to get a detailed description of the algorithm, click the corresponding Usage to get the Assessor's installation requirements, suggested scenario and using example.

Currently we support the following Assessors:

Assessor Brief Introduction of Algorithm
Medianstop Medianstop is a simple early stopping rule. It stops a pending trial X at step S if the trial’s best objective value by step S is strictly worse than the median value of the running averages of all completed trials’ objectives reported up to step S. Reference Paper
Curvefitting Curve Fitting Assessor is a LPA(learning, predicting, assessing) algorithm. It stops a pending trial X at step S if the prediction of final epoch's performance worse than the best final performance in the trial history. In this algorithm, we use 12 curves to fit the accuracy curve. Reference Paper

Usage of Builtin Assessors

Use builtin assessors provided by NNI SDK requires to declare the builtinAssessorName and classArgs in config.yml file. In this part, we will introduce the detailed usage about the suggested scenarios, classArg requirements, and example for each assessor.

Note: Please follow the format when you write your config.yml file.

Median Stop Assessor

Builtin Assessor Name: Medianstop

Suggested scenario

It is applicable in a wide range of performance curves, thus, can be used in various scenarios to speed up the tuning progress.

Requirement of classArg

  • optimize_mode (maximize or minimize, optional, default = maximize) - If 'maximize', assessor will stop the trial with smaller expectation. If 'minimize', assessor will stop the trial with larger expectation.
  • start_step (int, optional, default = 0) - A trial is determined to be stopped or not, only after receiving start_step number of reported intermediate results.

Usage example:

# config.yml
assessor:
    builtinAssessorName: Medianstop
    classArgs:
      optimize_mode: maximize
      start_step: 5

Curve Fitting Assessor

Builtin Assessor Name: Curvefitting

Suggested scenario

It is applicable in a wide range of performance curves, thus, can be used in various scenarios to speed up the tuning progress. Even better, it's able to handle and assess curves with similar performance.

Requirement of classArg

  • epoch_num (int, required) - The total number of epoch. We need to know the number of epoch to determine which point we need to predict.
  • optimize_mode (maximize or minimize, optional, default = maximize) - If 'maximize', assessor will stop the trial with smaller expectation. If 'minimize', assessor will stop the trial with larger expectation.
  • start_step (int, optional, default = 6) - A trial is determined to be stopped or not, we start to predict only after receiving start_step number of reported intermediate results.
  • threshold (float, optional, default = 0.95) - The threshold that we decide to early stop the worse performance curve. For example: if threshold = 0.95, optimize_mode = maximize, best performance in the history is 0.9, then we will stop the trial which predict value is lower than 0.95 * 0.9 = 0.855.
  • gap (int, optional, default = 1) - The gap interval between Assesor judgements. For example: if gap = 2, start_step = 6, then we will assess the result when we get 6, 8, 10, 12...intermedian result.

Usage example:

# config.yml
assessor:
    builtinAssessorName: Curvefitting
    classArgs:
      epoch_num: 20
      optimize_mode: maximize
      start_step: 6
      threshold: 0.95
      gap: 1
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