Hybrid digital training-analog inferencing AI

Read an article from IBM Research, Iso-accuracy DL inferencing with in-memory computing, the other day that referred to an article in Nature, Accurate DNN inferencing using computational PCM (phase change memory or memresistive technology) which discussed using a hybrid digital-analog computational approach to DNN (deep neural network) training-inferencing AI systems. It’s important to note that the PCM device is both a storage device and a computational device, thus performing two functions in one circuit.

In the past, we have seenPCM circuitry used in neuromorphic AI. The use of PCM here is not that (see our Are neuromorphic chips a dead end? post).

Hybrid digital-analog AI has the potential to be more energy efficient and use a smaller footprint than digital AI alone. Presumably, the new approach is focused on edge devices for IoT and other energy or space limited AI deployments.

Whats different in Hybrid digital-analog AI

As researchers began examining the use of analog circuitry for use in AI deployments, the nature of analog technology led to inaccuracy and under performance in DNN inferencing. This was because of the “non-idealities” of analog circuitry. In other words, analog electronics has some intrinsic capabilities that induce some difficulties when modeling digital logic and digital exactitude is difficult to implement precisely in analog circuitry.

The caption for Figure 1 in the article runs to great length but to summarize (a) is the DNN model for an image classification DNN with fewer inputs and outputs so that it can ultimately fit on a PCM array of 512×512; (b) shows how noise is injected during the forward propagation phase of the DNN training and how the DNN weights are flattened into a 2D matrix and are programmed into the PCM device using differential conductance with additional normalization circuitry

As a result, the researchers had to come up with some slight modifications to the typical DNN training and inferencing process to improve analog PCM inferencing. Those changes involve:

  • Injecting noise during DNN neural network training, so that the resultant DNN model becomes more noise resistant;
  • Flattening the resultant DNN model from 3D to 2D so that neural network node weights can be implementing as differential conductance in the analog PCM circuitry.
  • Normalizing the internal DNN layer outputs before input to the next layer in the model

Analog devices are intrinsically more noisy than digital devices, so DNN noise sensitivity had to be reduced. During normal DNN training there is both forward pass of inputs to generate outputs and a backward propagation pass (to adjust node weights) to fit the model to the required outputs. The researchers found that by injecting noise during the forward pass they were able to create a more noise resistant DNN.

Differential conductance uses the difference between the conductance of two circuits. So a single node weight is mapped to two different circuit conductance values in the PCM device. By using differential conductance, the PCM devices inherent noisiness can be reduced from the DNN node propagation.

In addition, each layer’s outputs are normalized via additional circuitry before being used as input for the next layer in the model. This has the affect of counteracting PCM circuitry drift over time (see below).

Hybrid AI results

The researchers modeled their new approach and also performed some physical testing of a digital-analog DNN. Using CIFAR-10 image data and the ResNet-32 DNN model. The process began with an already trained DNN which was then retrained while injecting noise during forward pass processing. The resultant DNN was then modeled and programed into a PCM circuit for implementation testing.

Part D of Figure 4 shows the Baseline which represents a completely digital implementation using FP32 multiplication logic; Experiment which represents the actual use of the PCM device with a global drift calibration performed on each layer before inferencing; Mode which represents theira digital model of the PCM device and its expected accuracy. Blue band is one standard-deviation on the modeled result.

One challenge with any memristive device is that over time its functionality can drift. The researchers implemented a global drift calibration or normalization circuitry to counteract this. One can see evidence of drift in experimental results between ~20sec and ~60 seconds into testing. During this interval PCM inferencing accuracy dropped from 93.8% to 93.2% but then stayed there for the remainder of the experiment (~28 hrs). The baseline noted in the chart used digital FP32 arithmetic for infererenci and achieved ~93.9% for the duration of the test.

Certainly not as accurate as the baseline all digital implementation, but implementing DNN inferencing model in PCM and only losing 0.7% accuracy seems more than offset by the clear gain in energy and footprint reduction.

While the simplistic global drift calibration (GDC) worked fairly well during testing, the researchers developed another adaptive (batch normalization statistical [AdaBS]) approach, using a calibration image set (from the training data) and at idle times, feed these through the PCM device to calculate an average error used to adjust the PCM circuitry. As modeled and tested, the AdaBS approach increased accuracy and retained (at least modeling showed) accuracy over longer time frames.

The researchers were also able to show that implementing part (first and last layers) of the DNN model in digital FP32 and the rest in PCM improved inferencing accuracy even more.

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As shown above, a hybrid digital-analog PCM AI deployment can provide similar accuracy (at least for CIFAR-10/ResNet-24 image recognition) to an all digital DNN model but due to the efficiencies of the PCM analog circuitry allowed for a more energy efficient DNN deployment.

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