Configuration options

NengoDL uses Nengo’s config system to allow users to control more fine-grained aspects of the simulation. In general, most users will not need to worry about these options, and can leave them at their default settings. However, these options may be useful in some scenarios.

configure_settings is a utility function that can be used to set these configuration options. It needs to be called within a Network context, as in:

with nengo.Network() as net:
    nengo_dl.configure_settings(config_option=config_value, ...)

Each call to configure_settings only sets the configuration options specified to that call. That is,


is equivalent to

nengo_dl.configure_settings(option0=val0, option1=val1)

Under the hood, configure_settings is setting config attributes on the top-level network. All of the same effects could be achieved by setting those config attributes directly, configure_settings simply makes this an easier process.


The trainable config attribute can be used to control which parts of a model will be optimized by the Simulator.train process. configure_settings(trainable=None) will add a configurable trainable attribute to the objects in a network. Setting trainable=None will use the default trainability settings, or trainable=True/False can be used to override the default for all objects.

Once the trainable attribute has been added to all the objects in a model, the config system can then be used to control the trainability of individual objects/networks.

For example, suppose we only want to optimize one connection in our network, while leaving everything else unchanged. This could be achieved via

with nengo.Network() as net:
    # this adds the `trainable` attribute to all the trainable objects
    # in the network, and initializes it to `False`

    a = nengo.Node([0])
    b = nengo.Ensemble(10, 1)
    c = nengo.Node(size_in=1)

    nengo.Connection(a, b)

    # make this specific connection trainable
    conn = nengo.Connection(b, c)
    net.config[conn].trainable = True

Or if we wanted to disable training for some subnetwork:

with nengo.Network() as net:
    with nengo.Network() as subnet:
        net.config[subnet].trainable = False

Note that config[nengo.Ensemble].trainable controls both encoders and biases, as both are properties of an Ensemble. However, it is possible to separately control the biases via config[nengo.ensemble.Neurons].trainable or config[my_ensemble.neurons].trainable.

There are two important caveats to keep in mind when configuring trainable, which differ from the standard config behaviour:

  1. trainable applies to all objects in a network, regardless of whether they were created before or after trainable is set. For example,

    with nengo.Network() as net:
        net.config[nengo.Ensemble].trainable = False
        a = nengo.Ensemble(10, 1)

    is the same as

    with nengo.Network() as net:
        a = nengo.Ensemble(10, 1)
        net.config[nengo.Ensemble].trainable = False
  2. trainable can only be set on the config of the top-level network. For example,

    with nengo.Network() as net:
        with nengo.Network() as subnet:
            my_ens = nengo.Ensemble(...)
            # incorrect
            subnet.config[my_ens].trainable = False
            # correct
            net.config[my_ens].trainable = False


This option can be used to change the algorithm used for assigning an order to simulation operations during the graph optimization stage. For example, we could change the planning algorithm to the graph_optimizer.transitive_planner via

from nengo_dl.graph_optimizer import transitive_planner

with nengo.Network() as net:


This option can be used to change the algorithm used for sorting signals/operators during the graph optimization stage. For example, we could disable sorting via

from nengo_dl.graph_optimizer import noop_order_signals

with nengo.Network() as net:


This option can be used to change the simplification transformations applied during the graph optimization stage. This takes a list of transformation functions, where each will be applied in sequence. For example, we could apply only two of the default simplifications via

from nengo_dl.graph_optimizer import remove_identity_muls, remove_zero_incs

with nengo.Network() as net:


TensorFlow has its own configuration options which can control various aspects of the TensorFlow Session. session_config can be used to set those options on the underlying NengoDL simulator Session. These are specified as a dictionary mapping config names to values. For example, if in TensorFlow we wanted to do

config = tf.ConfigProto()
config.gpu_options.allow_growth = True
sess = tf.Session(..., config=config)

the equivalent in NengoDL would be

    session_config={"gpu_options.allow_growth": True})


By default, NengoDL models are built to support both training and inference. However, sometimes we may know that we’ll only be using a simulation for inference (for example, if we want to take advantage of the batching/GPU acceleration of NengoDL, but don’t need the sim.train functionality). In that case we can improve the simulation speed of the model by omitting some of the aspects related to training. Setting nengo_dl.configure_settings(inference_only=True) will cause the network to be built in inference-only mode.


During training, NengoDL automatically replaces the non-differentiable spiking LIF neuron model with the differentiable LIFRate approximation. However, although LIFRate is generally differentiable, it has a sharp discontinuity at the firing threshold. In some cases this can lead to difficulties during the training process, and performance can be improved by smoothing the LIFRate response around the firing threshold. This is known as the SoftLIFRate neuron model.

SoftLIFRate has a parameter sigma that controls the degree of smoothing (SoftLIFRate approaches LIFRate as sigma goes to zero). Setting nengo_dl.configure_settings(lif_smoothing=x) will cause the LIF gradients to be approximated by SoftLIFRate instead of LIFRate, with sigma=x.


This specifies the floating point precision to be used for the simulator’s internal computations. It can be either tf.float32 or tf.float64, for 32 or 64-bit precision, respectively. 32-bit precision is the default, as it is faster, will use less memory, and in most cases will not make a difference in the results of the simulation. However, if very precise outputs are required then this can be changed to tf.float64.


By default, a nengo.Probe stores the probed output from every simulation timestep. However, sometimes in NengoDL we want to add a probe to something for other reasons, and don’t necessarily care about all of that data (which can consume a lot of memory). For example, we might want to apply a probe to some connection weights so that we can apply a regularization penalty, but since the weights aren’t changing during a simulation run we don’t need to keep the value from every simulation step.

The keep_history config option allows Probes to be configured so that they only store the output of the probed signal from the last simulation timestep. Calling


will set the default value for all probes in the simulation, which can then be further configured on a per-probe basis, e.g.

with nengo.Network() as net:
   my_probe = nengo.Probe(...)
   net.config[my_probe].keep_history = False