RESISTIVE PROCESSING UNIT WITH MULTIPLE WEIGHT READERS

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First Claim
1. A crossbar array comprising:
 a resistive processing unit (RPU) device at each of a plurality of crosspoints, the RPU device comprising;
a single weight storage element; and
a plurality of weight reader elements comprising;
a first weight reader element that is coupled with a first row wire, the first weight reader configured to compute a first matrix product value using a first value and a stored value, the first value being transmitted via the first row wire and the stored value being stored in the single weight storage element; and
a second weight reader element that is coupled with a second row wire to compute a second matrix product value of a second value and said stored value, the second value being transmitted via the second row wire.
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Abstract
Embodiments of the present invention include a crossbar array that includes a resistive processing unit (RPU) device at each crosspoint in the crossbar array. The RPU device includes a single weight storage element, and multiple weight reader elements. A first weight reader element is coupled with a first row wire to compute a first matrix product value using a first value and a stored value, the first value being transmitted via the first row wire and the stored value being stored in the single weight storage element. A second weight reader element is coupled with a second row wire to compute a second matrix product value of a second value and said stored value, the second value being transmitted via the second row wire.
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20 Claims
 1. A crossbar array comprising:
a resistive processing unit (RPU) device at each of a plurality of crosspoints, the RPU device comprising; a single weight storage element; and a plurality of weight reader elements comprising; a first weight reader element that is coupled with a first row wire, the first weight reader configured to compute a first matrix product value using a first value and a stored value, the first value being transmitted via the first row wire and the stored value being stored in the single weight storage element; and a second weight reader element that is coupled with a second row wire to compute a second matrix product value of a second value and said stored value, the second value being transmitted via the second row wire.  View Dependent Claims (2, 3, 4, 5, 6, 7)
 8. A system comprising:
a controller; and a crossbar array coupled with the controller, the crossbar array configured to perform a matrixmatrix multiplication, the crossbar array receives a first input matrix and a second input matrix from the controller, the crossbar array comprising; a resistive processing unit (RPU) device at each of a plurality of crosspoints, the RPU device comprising; a single weight storage element; and a plurality of weight reader elements comprising; a first weight reader element that is coupled with a first row wire, the first weight reader configured to compute a first matrix product value using a first value and a stored value, the first value being transmitted via the first row wire and the stored value being stored in the single weight storage element; and a second weight reader element that is coupled with a second row wire to compute a second matrix product value of a second value and said stored value, the second value being transmitted via the second row wire.  View Dependent Claims (9, 10, 11, 12, 13, 14)
 15. A method for performing matrixmatrix multiplication, the method comprising:
receiving, by a crossbar array, a first input matrix and a second input matrix for performing the matrixmatrix multiplication; and performing the matrixmatrix multiplication by the crossbar array in parallel using a resistive processing unit (RPU) device at each of a plurality of crosspoints, the RPU device comprising; a single weight storage element; and a plurality of weight reader elements;
wherein performing the matrixmatrix multiplication comprises;computing, by a first weight reader element from the plurality of weight readers, the first weight reader element is coupled with a first row wire, a first matrix product value using a first value and a stored value, the first value being transmitted via the first row wire and the stored value being stored in the single weight storage element; and computing, by a second weight reader element from the plurality of weight readers, the second weight reader element is coupled with a second row wire, a second matrix product value of a second value and said stored value, the second value being transmitted via the second row wire.  View Dependent Claims (16, 17, 18, 19, 20)
1 Specification
The present invention relates in general to configurations of trainable resistive crosspoint devices, which are referred to herein as resistive processing units (RPUs). More specifically, the present invention relates to artificial neural networks (ANNs) formed from crossbar arrays of resistive processing units (RPUs) that provide local data storage and local data processing without the need for additional processing elements beyond the RPU, thereby accelerating the ANN'"'"'s ability to implement algorithms such as matrix multiplication, matrix decomposition and the like.
Technical problems such as character recognition and image recognition by a computer are known to be well handled by machinelearning techniques. “Machine learning” is used to broadly describe a primary function of electronic systems that learn from data. In machine learning and cognitive science, ANNs are a family of statistical learning models inspired by the biological neural networks of animals, and in particular the brain. ANNs can be used to estimate or approximate systems and functions that depend on a large number of inputs and are generally unknown. Crossbar arrays are high density, low cost circuit architectures used to form a variety of electronic circuits and devices, including ANN architectures, neuromorphic microchips and ultrahigh density nonvolatile memory. A basic crossbar array configuration includes a set of conductive row wires and a set of conductive column wires formed to intersect the set of conductive row wires. The intersections between the two sets of wires are separated by socalled crosspoint devices.
Embodiments of the present invention include a crossbar array that includes a resistive processing unit (RPU) device at each crosspoint in the crossbar array. The RPU device includes a single weight storage element, and multiple weight reader elements. A first weight reader element is coupled with a first row wire to compute a first matrix product value using a first value and a stored value, the first value being transmitted via the first row wire and the stored value being stored in the single weight storage element. A second weight reader element is coupled with a second row wire to compute a second matrix product value of a second value and said stored value, the second value being transmitted via the second row wire.
According to one or more embodiments of the present invention a system includes a controller, and a crossbar array coupled with the controller. The crossbar array performs a matrixmatrix multiplication, wherein the crossbar array receives a first input matrix and a second input matrix from the controller. The crossbar array includes a resistive processing unit (RPU) device at crosspoint. The RPU device includes a single weight storage element, and multiple weight reader elements. A first weight reader element is coupled with a first row wire to compute a first matrix product value using a first value and a stored value, the first value being transmitted via the first row wire and the stored value being stored in the single weight storage element. A second weight reader element is coupled with a second row wire to compute a second matrix product value of a second value and said stored value, the second value being transmitted via the second row wire.
According to one or more embodiments of the present invention a method for performing matrixmatrix multiplication includes receiving, by a crossbar array, a first input matrix and a second input matrix for performing the matrixmatrix multiplication. The method further includes performing the matrixmatrix multiplication by the crossbar array in parallel using a resistive processing unit (RPU) device at each crosspoint. The RPU device includes a single weight storage element, and multiple weight reader elements. Performing the matrixmatrix multiplication includes computing, by a first weight reader element from the multiple weight readers, the first weight reader element is coupled with a first row wire, a first matrix product value using a first value and a stored value, the first value being transmitted via the first row wire and the stored value being stored in the single weight storage element. The matrixmatrix multiplication further includes computing, by a second weight reader element from the multiple weight readers, the second weight reader element is coupled with a second row wire, a second matrix product value of a second value and said stored value, the second value being transmitted via the second row wire.
Additional technical features and benefits are realized through the techniques of the present invention. Embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed subject matter. For a better understanding, refer to the detailed description and to the drawings.
The examples described throughout the present document will be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale. Moreover, in the figures, likereferenced numerals designate corresponding parts throughout the different views.
The diagrams depicted herein are illustrative. There can be many variations to the diagram or the operations described therein without departing from the spirit of the invention. For instance, the actions can be performed in a differing order or actions can be added, deleted or modified. Also, the term “coupled” and variations thereof describes having a communications path between two elements and does not imply a direct connection between the elements with no intervening elements/connections between them. All of these variations are considered a part of the specification.
Described herein are technical solutions for accelerating training of convolutional neural networks. The technical solutions include using RPUs, such as those configured in an RPU array for training convolutional neural networks. The RPU array can be a crosspoint arrays. As such the technical solutions are rooted in and/or tied to computer technology in order to overcome a problem specifically arising in the realm of computers, specifically training convolutional neural networks, such as by using matrix operations like matrixmatrix multiplication.
Various embodiments of the invention are described herein with reference to the related drawings. Alternative embodiments of the invention can be devised without departing from the scope of this invention. Various connections and positional relationships (e.g., over, below, adjacent, etc.) are set forth between elements in the following description and in the drawings. These connections and/or positional relationships, unless specified otherwise, can be direct or indirect, and the present invention is not intended to be limiting in this respect. Accordingly, a coupling of entities can refer to either a direct or an indirect coupling, and a positional relationship between entities can be a direct or indirect positional relationship. Moreover, the various tasks and process steps described herein can be incorporated into a more comprehensive procedure or process having additional steps or functionality not described in detail herein.
The following definitions and abbreviations are to be used for the interpretation of the claims and the specification. As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” “contains” or “containing,” or any other variation thereof, are intended to cover a nonexclusive inclusion. For example, a composition, a mixture, process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such composition, mixture, process, method, article, or apparatus.
Additionally, the term “exemplary” is used herein to mean “serving as an example, instance or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs. The terms “at least one” and “one or more” can be understood to include any integer number greater than or equal to one, i.e. one, two, three, four, etc. The terms “a plurality” can be understood to include any integer number greater than or equal to two, i.e. two, three, four, five, etc. The term “connection” can include both an indirect “connection” and a direct “connection.”
The terms “about,” “substantially,” “approximately,” and variations thereof, are intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. For example, “about” can include a range of ±8% or 5%, or 2% of a given value.
For the sake of brevity, conventional techniques related to making and using aspects of the invention may or may not be described in detail herein. In particular, various aspects of computing systems and specific computer programs to implement the various technical features described herein are well known. Accordingly, in the interest of brevity, many conventional implementation details are only mentioned briefly herein or are omitted entirely without providing the wellknown system and/or process details.
It is understood in advance that although one or more embodiments are described in the context of biological neural networks with a specific emphasis on modeling brain structures and functions, implementation of the teachings recited herein are not limited to modeling a particular environment. Rather, embodiments of the present invention are capable of modeling any type of environment, including for example, weather patterns, arbitrary data collected from the internet, and the like, as long as the various inputs to the environment can be turned into a vector.
Artificial neural networks (ANNs) can be used to estimate or approximate systems and functions that depend on a large number of inputs and are generally unknown. Neural networks use a class of algorithms based on a concept of interconnected “neurons.” In a typical neural network, neurons have a given activation function that operates on the inputs. By determining proper connection weights (a process also referred to as “training”), a neural network achieves efficient recognition of a desired patterns, such as images and characters. Oftentimes, these neurons are grouped into “layers” in order to make connections between groups more obvious and to each computation of values. Training the neural network is a computationally intense process.
ANNs are often embodied as socalled “neuromorphic” systems of interconnected processor elements that act as simulated “neurons” and exchange “messages” between each other in the form of electronic signals. Similar to the socalled “plasticity” of synaptic neurotransmitter connections that carry messages between biological neurons, the connections in ANNs that carry electronic messages between simulated neurons are provided with numeric weights that correspond to the strength or weakness of a given connection. The weights can be adjusted and tuned based on experience, making ANNs adaptive to inputs and capable of learning. For example, an ANN for handwriting recognition is defined by a set of input neurons which can be activated by the pixels of an input image. After being weighted and transformed by a function determined by the network'"'"'s designer, the activations of these input neurons are then passed to other downstream neurons, which are often referred to as “hidden” neurons. This process is repeated until an output neuron is activated. The activated output neuron determines which character was read.
Crossbar arrays, also known as crosspoint arrays or crosswire arrays, are high density, low cost circuit architectures used to form a variety of electronic circuits and devices, including ANN architectures, neuromorphic microchips and ultrahigh density nonvolatile memory. A basic crossbar array configuration includes a set of conductive row wires and a set of conductive column wires formed to intersect the set of conductive row wires. The intersections between the two sets of wires are separated by socalled crosspoint devices, which can be formed from thin film material.
Crosspoint devices, in effect, function as the ANN'"'"'s weighted connections between neurons. Nanoscale devices, for example memristors having “ideal” conduction state switching characteristics, are often used as the crosspoint devices in order to emulate synaptic plasticity with high energy efficiency. The conduction state (e.g., resistance) of the ideal memristor material can be altered by controlling the voltages applied between individual wires of the row and column wires. Digital data can be stored by alteration of the memristor material'"'"'s conduction state at the intersection to achieve a high conduction state or a low conduction state. The memristor material can also be programmed to maintain two or more distinct conduction states by selectively setting the conduction state of the material. The conduction state of the memristor material can be read by applying a voltage across the material and measuring the current that passes through the target crosspoint device. So far, however, there have been some practical drawbacks in memristor and other resistiverandomaccessmemory (RRAM) based crosspoint devices that are detrimental to ANN applications; significant devicetodevice variability and the asymmetry between “set (i.e. to increment resistance)” and “reset (i.e. to decrement resistance)” operations are two such main limitations.
In order to limit power consumption, the crosspoint devices of ANN chip architectures are often designed to utilize offline learning techniques, wherein the approximation of the target function does not change once the initial training phase has been resolved. Offline learning allows the crosspoint devices of crossbartype ANN architectures to be simplified such that they draw very little power.
Notwithstanding the potential for lower power consumption, executing offline training can be difficult and resource intensive because it is typically necessary during training to modify a significant number of adjustable parameters (e.g., weights) in the ANN model to match the inputoutput pairs for the training data. Accordingly, simplifying the crosspoint devices of ANN architectures to prioritize powersaving, offline learning techniques typically means that training speed and training efficiency are not optimized.
Providing simple crosspoint devices that keep power consumption within an acceptable range, as well as accelerate the speed and efficiency of training ANN architectures, would improve overall ANN performance and allow a broader range of ANN applications. Accordingly, described herein are technical solutions that facilitate using and training crosspoint arrays that facilitate a parallelized matrixmatrix multiplication. Typically, RPU performs series of vectormatrix multiplication to achieve a matrixmatrix multiplication, where a first matrix is decomposed into multiple vectors. The multiple vectors in the first matrix are then multiplied by the second matrix using an RPU array, in a serial manner. Such a matrix multiplication operation, which is a frequently used operation in CNNs and other types of ANNs, reduces the overall throughput and efficiency of the system. The technical solutions described herein address such a technical challenge by facilitating the RPU array to read a weight stored in a weight storage element of a crosspoint in multiple rows of the RPU array simultaneously, and consequently facilitating multiple calculations using the weight. The technical solutions therefore facilitate a higher throughput of the RPU array during operations like matrix multiplication (and other operations). The technical solutions described herein, accordingly, improves performance of computer technology, particularly RPU arrays.
In one or more examples, weight elements are stored in a weight storage element such as a capacitor and read by a weight reader, such as a field effect transistor (FET). Alternatively, or in addition, the weight storage elements can be digital counters (e.g. JK flipflop based counters), a memory storage device, or any other electronic circuit that can be used for storing the weight. Here, “weight” refers to a computational value being used during computations of an ANN as described further.
Although embodiments of the present invention are directed to electronic systems, for ease of reference and explanation various aspects of the electronic systems are described using neurological terminology such as neurons, plasticity and synapses, for example. It will be understood that for any discussion or illustration herein of an electronic system, the use of neurological terminology or neurological shorthand notations are for ease of reference and are meant to cover the neuromorphic, ANN equivalent(s) of the described neurological function or neurological component.
Instead of utilizing the traditional digital model of manipulating zeros and ones, ANNs create connections between processing elements that are substantially the functional equivalent of the core system functionality that is being estimated or approximated. For example, IBM™'"'"'s SyNapse™ computer chip is the central component of an electronic neuromorphic machine that attempts to provide similar form, function and architecture to the mammalian brain. Although the IBM SyNapse computer chip uses the same basic transistor components as conventional computer chips, its transistors are configured to mimic the behavior of neurons and their synapse connections. The IBM SyNapse computer chip processes information using a network of just over one million simulated “neurons,” which communicate with one another using electrical spikes similar to the synaptic communications between biological neurons. The IBM SyNapse architecture includes a configuration of processors (i.e., simulated “neurons”) that read a memory (i.e., a simulated “synapse”) and perform simple operations. The communications between these processors, which are typically located in different cores, are performed by onchip network routers.
A general description of how a typical ANN operates will now be provided with reference to
Biological neuron 102 is modeled in
Similar to the functionality of a human brain, each input layer node 302, 304, 306 of ANN 300 receives inputs x1, x2, x3 directly from a source (not shown) with no connection strength adjustments and no node summations. Accordingly, y1=f(x1), y2=f(x2) and y3=f(x3), as shown by the equations listed at the bottom of
ANN model 300 processes data records one at a time, and it “learns” by comparing an initially arbitrary classification of the record with the known actual classification of the record. Using a training methodology knows as “backpropagation” (i.e., “backward propagation of errors”), the errors from the initial classification of the first record are fed back into the network and used to modify the network'"'"'s weighted connections the second time around, and this feedback process continues for many iterations. In the training phase of an ANN, the correct classification for each record is known, and the output nodes can therefore be assigned “correct” values. For example, a node value of “1” (or 0.9) for the node corresponding to the correct class, and a node value of “0” (or 0.1) for the others. It is thus possible to compare the network'"'"'s calculated values for the output nodes to these “correct” values, and to calculate an error term for each node (i.e., the “delta” rule). These error terms are then used to adjust the weights in the hidden layers so that in the next iteration the output values will be closer to the “correct” values.
There are many types of neural networks, but the two broadest categories are feedforward and feedback/recurrent networks. ANN model 300 is a nonrecurrent feedforward network having inputs, outputs and hidden layers. The signals can only travel in one direction. Input data is passed onto a layer of processing elements that perform calculations. Each processing element makes its computation based upon a weighted sum of its inputs. The new calculated values then become the new input values that feed the next layer. This process continues until it has gone through all the layers and determined the output. A threshold transfer function is sometimes used to quantify the output of a neuron in the output layer.
A feedback/recurrent network includes feedback paths, which mean that the signals can travel in both directions using loops. All possible connections between nodes are allowed. Because loops are present in this type of network, under certain operations, it can become a nonlinear dynamical system that changes continuously until it reaches a state of equilibrium. Feedback networks are often used in associative memories and optimization problems, wherein the network looks for the best arrangement of interconnected factors.
The speed and efficiency of machine learning in feedforward and recurrent ANN architectures depend on how effectively the crosspoint devices of the ANN crossbar array perform the core operations of typical machine learning algorithms. Although a precise definition of machine learning is difficult to formulate, a learning process in the ANN context can be viewed as the problem of updating the crosspoint device connection weights so that a network can efficiently perform a specific task. The crosspoint devices typically learn the necessary connection weights from available training patterns. Performance is improved over time by iteratively updating the weights in the network. Instead of following a set of rules specified by human experts, ANNs “learn” underlying rules (like inputoutput relationships) from the given collection of representative examples. Accordingly, a learning algorithm can be generally defined as the procedure by which learning rules are used to update and/or adjust the relevant weights.
The three main learning algorithm paradigms are supervised, unsupervised and hybrid. In supervised learning, or learning with a “teacher,” the network is provided with a correct answer (output) for every input pattern. Weights are determined to allow the network to produce answers as close as possible to the known correct answers. Reinforcement learning is a variant of supervised learning in which the network is provided with only a critique on the correctness of network outputs, not the correct answers themselves. In contrast, unsupervised learning, or learning without a teacher, does not require a correct answer associated with each input pattern in the training data set. It explores the underlying structure in the data, or correlations between patterns in the data, and organizes patterns into categories from these correlations. Hybrid learning combines supervised and unsupervised learning. Parts of the weights are usually determined through supervised learning, while the others are obtained through unsupervised learning.
The use of neural networks, particularly with convolutional layers, has driven progress in deep learning. Such neural networks are referred to as convolutional neural networks (CNN). In a CNN, kernels convolute overlapping regions in a visual field, and accordingly emphasize the importance of spatial locality in feature detection. Computing the convolutional layers of the CNN, typically, encompasses more than 90% of computation time in neural network training and inference. The training for example, depending on the size of the training dataset that is being used, can be a week or longer. Thus, accelerating the CNN training, as described by the examples of the technical solutions herein, is a desirable improvement.
Referring to
The data values for each layer in the CNN is typically represented using matrices (or tensors in some examples) and computations are performed as matrix computations. The indexes (and/or sizes) of the matrices vary from layer to layer and network to network, as illustrated in
While fullyconnected neural networks are able, when properly trained, to recognize input patterns, such as handwriting, they can fail to take advantage of shape and proximity when operating on input. For example, because every pixel is operated on independently, the neural network can ignore adjacent pixels. A CNN, in comparison, operates by associating an array of values, rather than a single value, with each neuron. Conceptually, the array is a subset of the input pattern, or other parts of the training data. The transformation of a neuron value for the subsequent layer is generated using convolution. Thus, in a CNN the connection strengths are convolution kernels rather than scalar values as in a fullnetwork.
For example, the input maps 510 are convolved with each filter bank to generate a corresponding output map. For example, in case the CNN is being trained to identify handwriting, the input maps 510 are combined with a filter bank that includes convolution kernels representing a vertical line. The resulting output map identifies vertical lines that are present in the input maps 510. Further, another filter bank can include convolution kernels representing a diagonal line, such as going up and to the right. An output map resulting from a convolution of the input maps 510 with the second filter bank identifies samples of the training data that contain diagonal lines. The two output maps show different information for the character, while preserving pixel adjacency. This can result in more efficient character recognition.
In one or more embodiments, the CNN training is performed using batches. Accordingly, a batch of the input data to be used for training is selected, as shown at block 608. Using the input maps 410 and the convolutional kernels 420, the output maps 430 are generated as described herein, as shown at block 610. Generating the output maps 330 is commonly referred to as a “forward pass.” Further, the method includes using the output maps 430, to determine how close or far off of the expected character recognition and the CNN was, as shown at block 615. A degree of error with relation to each of the matrices, which include the CNN is determined, such as using a gradient descent. Determining the relative errors is referred to as a “backward pass.” The method further includes modifying or updating the matrices to adjust for the error, as shown at block 625. The adjusting the convolution kernels 420 based on the output error information and using it to determine modifications for each neural network matrix, is referred to as an “update pass.” The gradient function, in an example, includes partial derivatives for each entry of each neural network matrix with respect to the error. The gradient function represents how much to adjust each matrix according to the gradient descent method. The processor subsequently modifies the matrices, including the convolutional kernels and the biases, according to the gradient function, as shown at block 625. The processor ensures that all batches of the data are used for the training, as shown at block 628.
The modified convolutional kernels 420 after being adjusted can be used for further training of the CNN, unless the training is deemed completed, as shown at block 630. For example, the training can be deemed completed if the CNN identifies the inputs according to the expected outputs with a predetermined error threshold. If the training is not yet completed, another iteration, or training epoch is performed using the modified convolutional kernels from the most recent iteration.
Thus, according to the CNN training above, the CNN learns to model a dependency between the inputs and the expected outputs in the training data.
Mathematically, for a vector of input maps S and a vector of outputs X, the CNN learns a model to reduce an error E between S and X. One such error function is the mean square error between S and X, for example:
E=Σ_{t}∥ƒ(S(t))−X(t)∥^{2 }
Other error functions can include, for example, crossentropy or logistic loss.
The CNN, for each layer, adapts a matrix of weights A and a vector of biases a to optimize E. To this end, in the forward pass, a value for each value of a next layer (B, b) is calculated using values of the current layer (A, a). For example, the computations in the forward pass for a layer can be represented as X=f(S)=Bφ(AS+a)+b, where, A is the matrix of weights of a current layer, a is a bias vector of the current layer, and B and b are weight matrix and bias of the next layer of the CNN. The function φ represents an elementwise nonlinear relation. In the forward pass, the predicted outputs corresponding to the inputs are evaluated according to the above equation. In the backward pass, partial derivatives of the cost function (E) with respect to the different parameters are propagated back through the CNN. The network weights are then be updated using a gradientbased optimization algorithm, such as the gradient descent. The whole process is iterated until the weights have converged. This approach is computationally rather intensive. Training a CNN in the above method is a time consuming process because the forward, backward, and update passes involve convolution operations.
where V is the input voltage from the input neuron 702 and r is the set resistance of the weight 704. The current from each weight adds columnwise and flows to a hidden neuron 706.
The hidden neurons 706 use the currents from the array of weights 704 to perform some calculation. The hidden neurons 706 then output a voltage of their own to another array of weights 704′. This array performs in the same way, with a column of weights 704′ receiving a voltage from their respective hidden neuron 706 to produce a weighted current output that adds rowwise and is provided to the output neuron 708.
It should be understood that any number of these stages can be implemented, by interposing additional layers of arrays and hidden neurons 706.
During back propagation, the output neurons 708 provide a voltage back across the array of weights 704′. The output layer compares the generated network response to training data and computes an error. The error is applied to the array as a voltage pulse, where the height and/or duration of the pulse is modulated proportional to the error value. In this example, a row of weights 704′ receives a voltage from a respective output neuron 708 in parallel and converts that voltage into a current which adds columnwise to provide an input to hidden neurons 706. The hidden neurons 706 combine the weighted feedback signal with a derivative of its feedforward calculation and stores an error value before outputting a feedback signal voltage to its respective column of weights 704. It should be noted that the weights 704′ operate in the same manner as the weights 704; the labeling is provided to indicate that weights 704 are between layers of neurons 702 and neurons 706, and weights 704′ are between layers of neurons 706 and 708. This back propagation travels through the entire network 700 until all hidden neurons 706 and the input neurons 702 have stored an error value.
During weight updates, the input neurons 702 and hidden neurons 706 apply first weight update voltages to the crosspoint array of weights 704 and the output neurons 708 and hidden neurons 706 apply second weight update voltages to the crosspoint array of weights 704′ through the network 700. Further, during the update phase of the weights 704, the input layer 702 applies voltage pulses (rowwise) proportional to the input values, and the layer 706 applies voltage pulses proportional to the error values of the hidden layer 706 (columnwise). Further yet, during update phase of the neurons 704′, hidden layer 706 applies voltage pulses proportional to its feedforward output values (columnwise), and the output layer 708 applies voltage pulses proportional to the error of the output layer 708 (rowwise). The combinations of these voltages create a state change within each weight 704, 704′, causing the weight 704, 704′ to take on a new countervalue, which in turn changes the resistance value. In this manner, the weights 704, 704′ can be trained to adapt the neural network 700 to errors in its processing. It should be noted that the three modes of operation, feed forward, back propagation, and weight update, do not overlap with one another.
Turning now to an overview of the present invention, one or more embodiments are directed to a programmable resistive crosspoint component referred to herein as a crosspoint device, or a resistive processing unit (RPU), which provides local data storage functionality and local data processing functionality. In other words, when performing data processing, the value stored at each RPU is updated in parallel and locally, which eliminate the need to move relevant data in and out of a processor and a separate storage element. Additionally, the local data storage and local data processing provided by the described RPUs accelerate the ANN'"'"'s ability to implement algorithms such as matrix inversion, matrix decomposition and the like. Accordingly, implementing a machine learning ANN architecture having the described RPU enables the implementation that optimize the speed, efficiency and power consumption of the ANN. The described RPU and resulting ANN architecture improve overall ANN performance and enable a broader range of practical ANN applications.
The described RPU can be implemented as twoterminal resistive crosspoint devices, wherein their switching characteristics have a nonlinearity that can be used for processing data. Thus, the described RPU can be implemented by a twoterminal device having an appropriate update characteristic that can be used to perform analog multiplication and update. For example, the described RPU device can be implemented with resistive random access memory (RRAM), phase change memory (PCM), programmable metallization cell (PMC) memory, memristive systems, or any other twoterminal device that has resistive switching characteristics. The twoterminal RPUs provide local data storage functionality and local data processing functionality without the necessity of extra circuit elements such as transistors and offchip storage and/or processing components.
In forward matrix multiplication, the stored weight (e.g. conduction state) of the RPU device 820 can be read by applying a voltage across the RPU and measuring the current that passes through the RPU device 820. Input voltages V_{1}, V_{2}, V_{3 }are applied to row wires 802, 804, 806, respectively. Each column wire 808, 810, 812, 814 sums the currents I_{1}, I_{2}, I_{3}, I_{4 }generated by each RPU device 820 along the particular column wire. For example, as shown in
Continuing with the diagram of
A row voltage sequence or bit stream 830, which is applied to row wire 806, is shown as a sequence of voltage pulses representing weight updates having a voltage of zero or a voltage of +0.5V_{SET}. A column voltage sequence or bit stream 832, which is applied to column wire 814, is shown as a sequence of voltage pulses also representing weight updates having either a voltage of zero or a voltage of −0.5V_{SET}. In example of
Voltage sequence 834 is the voltages applied to RPU device 820 resulting from the difference between row voltage sequence 830 and column voltage sequence 832. Voltage sequence 834 has 3 voltage steps at 0V, 0.5V_{SET }and V_{SET}. However, because the resistance σ_{43 }of RPU device 820 only changes for device voltages reaching V_{SET}, a single pulse sent either through a column wire or through a row wire is not enough to change the resistance state of RPU device 820. When a column wire sends a voltage at 0.5V_{SET}, and a row wire sends a voltage at −0.5V_{SET}, the resulting V_{SET }pulse applied to the relevant RPU will cause an incremental change in the resistance of the device. Accordingly, the voltage pulses applied to RPU device 820 utilize the nonlinear switching characteristic of RPU device 820 in order to perform a bit wise stochastic AND operation (e.g., as shown in
For example, consider that a matrixmatrix multiplication P×Q. For sake of example, consider that P and Q are 3×3 matrices, however the technical solutions described herein can be used for any sized matrices. The matrix P is decomposed into multiple input vectors 905, in this case, into 3 vectors P1, P2, and P3. The decomposition can be selection of the columns in the matrix P. The matrix Q is stored in the RPU array 800, the weight in each RPU device 820 of the array 800 being the values of the matrix Q. For computing the matrixmatrix multiplication P×Q, the vectors 905 P1, P2, and P3, are sequentially input via the row wires of the RPU array 800. Each RPU device 820, upon receiving the value from the input vectors 905, performs the multiplication operation as described herein and the value is accumulated into the corresponding element in the output array 915. Each input vector 905 generates a corresponding vector 915 upon being multiplied with the matrix stored in the RPU array 800.
Each vector multiplication is sequential. Accordingly, the total time taken to complete the matrixmatrix multiplication P×Q is the sum of each vector multiplication. For example, if each vector multiplication takes 80 nanoseconds, the total time for the matrix multiplication in the above example is 3×80=240 nanoseconds, because there are 3 total vectors. It should be noted that the time for each vector can be different in other examples, and also that the total time can be different based on the number of vectors.
The multiple weight readers 930 in the RPU device 820 facilitates the matrixmatrix multiplication to be parallelized without storing/copying the weights in multiple physical storages. The throughput of the crosspoint array 800 increases n times, n being the number of weight readers 930 in the RPU device 820, because each of the weight readers 930 operates substantially concurrently. For example, in the ongoing example with 3 weight readers,
The array 800 includes the RPU device 820 with the multiple n weight readers 930 at each crosspoint. Each crosspoint has n (e.g. 3) row wires and n (e.g. 3) column wires, the RPU device 820 providing the stored weight to each pair of rowcolumn wires via the n weight readers 930 of the RPU device 820. The multiple vector inputs 905 are supplied simultaneously on respective set of row wires of the array 800. Here, consider that the RPU subset 800A includes row and column wires that receive the vector P1, the RPU subset 800B includes row and column wires that receive the vector P2, and the RPU subset 800C includes row and column wires that receive the vector P3. As depicted in
Accordingly, the RPU array 800, using the RPU devices 820 with the multiple weight readers computes the matrixmatrix multiplication n times faster, as the n column vectors of the P matrix are used concurrently.
The method includes inputting a set of n column vectors from a first input matrix, say P, from the input matrices to the RPU array 800, at 1120. The set of n column vectors are input simultaneously, a value from each of the n vectors being input on a respective row wire of an RPU device 820 (see
The results of the operation from each RPU device 820 are stored concurrently in the output vectors 915, at 1150. The storing includes adding the multiplication values from each column wire.
Here, by storing the values of the matrix Q in the RPU array 800 and inputting a new matrix P multiple times facilitates maximized benefit of the technical solutions herein. For example, in case of the ongoing example with three columns (P1, P2, and P3) of matrix P being used simultaneously, the matrix multiplication using the RPU array 800 can expressed as a loop shown in Table 1.
The column vectors of matrix A are used as input vectors 905, which are input concurrently to the RPU devices 820. For example, row wires 802 and 802′ of the RPU device 820 receive values a11 and a21 from the two column vectors of matrix A. The values of matrix B are stored in the weight storage elements 920 of the RPU devices 820. As can be seen, the result of the matrixmatrix multiplication is obtained in the output vectors 915.
The technical solutions described herein improve matrixmatrix multiplication performed by RPU arrays by using RPU devices with multiple weight readers shared across a single weight storage element. The technical solutions improves a matrixmatrix computation speed by n times, where n is the number of multiple weight readers in each RPU device. The technical solutions accordingly improve computational technology, and particularly RPU arrays and ANNs implemented using such RPU arrays.
The present invention may be a system, a method, and/or a computer program product at any possible technical detail level of integration. The computer program product may include a computer readable storage medium (or media) having computer readable program instructions thereon for causing a processor to carry out aspects of the present invention.
The computer readable storage medium can be a tangible device that can retain and store instructions for use by an instruction execution device. The computer readable storage medium may be, for example, but is not limited to, an electronic storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. A nonexhaustive list of more specific examples of the computer readable storage medium includes the following: a portable computer diskette, a hard disk, a random access memory (RAM), a readonly memory (ROM), an erasable programmable readonly memory (EPROM or Flash memory), a static random access memory (SRAM), a portable compact disc readonly memory (CDROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanically encoded device such as punchcards or raised structures in a groove having instructions recorded thereon, and any suitable combination of the foregoing. A computer readable storage medium, as used herein, is not to be construed as being transitory signals per se, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through a waveguide or other transmission media (e.g., light pulses passing through a fiberoptic cable), or electrical signals transmitted through a wire.
Computer readable program instructions described herein can be downloaded to respective computing/processing devices from a computer readable storage medium or to an external computer or external storage device via a network, for example, the Internet, a local area network, a wide area network and/or a wireless network. The network may comprise copper transmission cables, optical transmission fibers, wireless transmission, routers, firewalls, switches, gateway computers and/or edge servers. A network adapter card or network interface in each computing/processing device receives computer readable program instructions from the network and forwards the computer readable program instructions for storage in a computer readable storage medium within the respective computing/processing device.
Computer readable program instructions for carrying out operations of the present invention may be assembler instructions, instructionsetarchitecture (ISA) instructions, machine instructions, machine dependent instructions, microcode, firmware instructions, statesetting data, configuration data for integrated circuitry, or either source code or object code written in any combination of one or more programming languages, including an object oriented programming language such as Smalltalk, C++, or the like, and procedural programming languages, such as the “C” programming language or similar programming languages. The computer readable program instructions may execute entirely on the user'"'"'s computer, partly on the user'"'"'s computer, as a standalone software package, partly on the user'"'"'s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user'"'"'s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). In some embodiments, electronic circuitry including, for example, programmable logic circuitry, fieldprogrammable gate arrays (FPGA), or programmable logic arrays (PLA) may execute the computer readable program instruction by utilizing state information of the computer readable program instructions to personalize the electronic circuitry, in order to perform aspects of the present invention.
Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.
These computer readable program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. These computer readable program instructions may also be stored in a computer readable storage medium that can direct a computer, a programmable data processing apparatus, and/or other devices to function in a particular manner, such that the computer readable storage medium having instructions stored therein comprises an article of manufacture including instructions which implement aspects of the function/act specified in the flowchart and/or block diagram block or blocks.
The computer readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable apparatus or other device to produce a computer implemented process, such that the instructions which execute on the computer, other programmable apparatus, or other device implement the functions/acts specified in the flowchart and/or block diagram block or blocks.
The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of instructions, which comprises one or more executable instructions for implementing the specified logical function(s). In some alternative implementations, the functions noted in the blocks may occur out of the order noted in the Figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardwarebased systems that perform the specified functions or acts or carry out combinations of special purpose hardware and computer instructions.
The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments described herein.