51 lines
2.7 KiB
Markdown
51 lines
2.7 KiB
Markdown
# Tutorials
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As demonstrated on MNIST in `mnist_dpsgd_tutorial.py`, the easiest to use
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a differentially private optimizer is to modify an existing training loop
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to replace an existing vanilla optimizer with its differentially private
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counterpart implemented in the library.
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## Parameters
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All of the optimizers share some privacy-specific parameters that need to
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be tuned in addition to any existing hyperparameter. There are currently three:
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* num_microbatches (int): The input data for each step (i.e., batch) of your
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original training algorithm is split into this many microbatches. Generally,
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increasing this will improve your utility but slow down your training in terms
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of wall-clock time. The total number of examples consumed in one global step
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remains the same. This number should evenly divide your input batch size.
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* l2_norm_clip (float): The cumulative gradient across all network parameters
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from each microbatch will be clipped so that its L2 norm is at most this
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value. You should set this to something close to some percentile of what
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you expect the gradient from each microbatch to be. In previous experiments,
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we've found numbers from 0.5 to 1.0 to work reasonably well.
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* noise_multiplier (float): This governs the amount of noise added during
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training. Generally, more noise results in better privacy and lower utility.
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This generally has to be at least 0.3 to obtain rigorous privacy guarantees,
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but smaller values may still be acceptable for practical purposes.
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## Measuring Privacy
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Differential privacy is measured by two values, epsilon and delta. Roughly
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speaking, they mean the following:
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* epsilon gives a ceiling on how much the probability of a change in model
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behavior can increase by including a single extra training example. This is
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the far more sensitive value, and we usually want it to be at most 10.0 or
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so. However, note that this is only an upper bound, and a large value of
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epsilon may still mean good practical privacy.
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* delta bounds the probability of an "unconditional" change in model behavior.
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We can usually set this to a very small number (1e-7 or so) without
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compromising utility. A rule of thumb is to set it to the inverse of the
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order of magnitude of the training data size.
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To find out the epsilon given a fixed delta value for your model, follow the
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approach demonstrated in the `compute_epsilon` of the `mnist_dpsgd_tutorial.py`
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where the arguments used to call the RDP accountant (i.e., the tool used to
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compute the privacy guarantee) are:
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* q : The sampling ratio, defined as (number of examples consumed in one
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step) / (total training examples).
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* stddev_to_sensitivity_ratio : The noise_multiplier from your parameters above.
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* steps : The number of global steps taken.
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