Tips and tricks

This page includes some tips and tricks to get the most out of NengoDL, or answers to common questions we see from users. If you have a question that isn’t answered here, come post it on the forums and we will do our best to help.

Speed improvements

There are a number of options within NengoDL that can customize the behaviour for a particular model in order to improve the training or inference speed.

First is the Simulator unroll_simulation option. This unrolls the simulation loop, which can improve the speed of models that are simulated over time. The optimal value for this parameter will depend on the size of the model; there will likely be some sweet spot in the middle that offers the best performance, with performance decreasing if unroll_simulation is too high or too low. In general, this parameter just needs to be determined experimentally, by running with different unroll_simulation values and seeing which one is fastest.

If you know that you will only ever be running your model for a fixed number of timesteps, you can build the model without the outer simulation loop by setting nengo_dl.configure_settings(use_loop=False). See the documentation for more details.

If you know that you will never need to preserve simulator state between runs, you can avoid building and running the state-saving parts of the model by setting nengo_dl.configure_settings(stateful=False). In particular, if you see warnings like “Method (on_train_batch_end) is slow compared to the batch update”, then this setting may alleviate that issue. See the documentation for more details.

If you know that you will never be calling, you can disable the training parts of the model with nengo_dl.configure_settings(inference_only=True). See the documentation for more details.

For large models with a complex architecture, modifying the graph optimization settings may offer speed improvements. For example, you could increase the planning search depth by setting

import functools

from nengo_dl import graph_optimizer

with nengo.Network():
        graph_optimizer.tree_planner, max_depth=4))

See the documentation for more details.

Training a spiking deep network

Training a spiking version of a deep network comes with some important differences compared to training the corresponding standard deep network. In general, we cannot expect to simply copy code that works for a standard deep network, add spiking neurons, and have everything work the same. The process of successfully training a spiking network will depend on the particular task, but here are some general tips and things to think about.

First, begin with a standard, non-spiking network, and then gradually add complexity. For example, following the steps in this example, we can begin by adding the source network to Nengo completely unchanged, and verify that the performance of the network hasn’t changed. Then we can separate the model into individual layers (and again verify that the performance hasn’t changed). Finally, we can begin changing some of the nonlinearities to spiking neurons, and see how that impacts performance. This process allows us to debug any issues uncovered at each stage one at a time, rather than attempting to jump into a fully redesigned spiking network.

When debugging spiking performance issues, here are somethings to think about:

  1. Connection/Probe synapses. Spiking neurons produce discrete, “spiky” output signals. Synapses smooth that output, making it look more like the continuous signal we would get from a non-spiking nonlinearity. So by adding synaptic filters into our network architecture, we can make the spiking network more like a standard rate network. However, this comes at the cost of introducing temporal filter effects (so, for example, we will need to run the network for multiple timesteps in order to allow the output to stabilize). We may also want to train the network without synapses, and then add them in for evaluation/inference (see this example).

  2. Neuron model. There are many kinds of spiking neurons, just as there are many kinds of non-spiking nonlinearities. The default in Nengo is LIF neurons, which have a very nonlinear response curve (in addition to being spiking). It may be better to start with nengo.SpikingRectifiedLinear, which will behave more similarly to the standard relu nonlinearity.

  3. Ensemble parameterization. The default parameters in Nengo are often different than the typical defaults in deep learning. In particular, in deep learning applications it is often useful to change the Nengo defaults to use a constant max_rate and zero intercepts. We also typically set the amplitude parameter on the neurons to be equal to 1/max_rate (so that the overall output of the neuron will be around the 0–1 range). See this example where we use both of these techniques. Again, however, as with any hyperparameters these will likely need to be adjusted depending on the application if we want to maximize performance.

  4. Firing rates. Non-spiking neurons output continuous values every timestep, so it doesn’t make much difference whether they are outputting a value of 1 or 100. However, spiking neurons communicate via discrete events, and the rate of those events is proportional to the continuous output value of the corresponding non-spiking counterpart. So a spiking neuron emitting spikes at 1Hz is very different than one emitting spikes at 100Hz. Imagine we’re simulating the model for 100 timesteps with a simulation timestep of 0.001s. The 1Hz neuron is only expected to spike once every 1000 timesteps, so it may not spike at all in our 100 timestep window, meaning that we really have no information about what value that neuron is outputting. Even if a neuron spiked 1 or 2 times, that still doesn’t provide much information. The 100Hz neuron, on the other hand, would spike about 10 times in our 100 timestep window, allowing us to estimate its firing rate fairly accurately. In conclusion, it is important to look at the firing rates of neurons in your model, and make sure they are spiking fast enough to provide useful information. If they are not spiking fast enough, consider adjusting Ensemble parameterizations (before or after training) or adding regularization terms during training to encourage higher firing rates.