Convolutional neural networks (CNNs) are attracting much attention by ac- hieving high accuracy in various applications such as image recognition, natural language processing, object detection. CNN is a type of neural networks which employs convolution operation as a feature extraction method, where the weights of the convolution are also self-obtained through the training of the network. This “trainable” feature extraction algorithm is the key of high reco- gnition accuracy of the CNNs.
CNNs are computationally intensive, many kinds of hardware acceleration such as GPGPU computation or ASIC/FPGA-based implementations   have been utilized to process CNNs with an acceptable throughput and efficiency. GPGPU parallel computation  is used for server-side off-line CNN training. The high performance and it programmable feature allows for software-based development, but its large energy consumption is not suitable for mobile ap- plications. ASIC approach can be the most efficient solution with the optimal design for an application. However, the disadvantage of an ASIC solution is the difficulty with adapting to new methods of an ever-evolving deep learning algorithm. FPGA-based implementations are also used to balance the perfor- mance and availability with the optimized architectures for the demand, but its effective power efficiency is not so high.
The difference of computational structures between convolutional and fully- connected layers prevents the accelerators to process the entire CNN efficiently. In addition, the convolutional layers in a single CNN may have quite different memory (data transfer) requirements.
In this paper, we analyze the data access patterns of CNN layers and find out a reconfigurable architecture that has the ability of changing data access/operation patterns layer by layer in order to utilize the locality of the layers. We propose a novel CGRA (Coarse Grained Reconfigurable Array) accelerator with time- domain multithreading, which exploits the locality of the input data and con- ceals the latency of the external memory access. Finally, we evaluate the benefits of the multithreading by estimating the data reusability and the hardware re- quirements for our experimental architecture. As a result, the efficiency and usability of the multithreaded processing in CGRA are proven, and the practical and quantitative methods for building/using multithreaded CGRA neural net- work accelerators are shown.
This paper is based on our previous work  . The followings are the main supplementary contribution of this paper: we considered some practical architec- tures with feasible SRAMs, analyzed the performance and complexity of these configurations, and also indicated the practical design points of the presented architecture based on the application requirements.
The purpose of this paper is to propose a CNN accelerator architectue and design for embedded applications. This paper is organized as follows: in Section 2, we analyze the computation pattern of CNNs. In Section 3, we discuss the localities of the CNN layers and propose an ideal multithreaded architecture to exploit them. In Section 4, we consider two feasible architectures with multith- readed accumulators. And then in Section 5 we evaluate the performances, arithmetic intensities, and hardware requirements of the architectures on several situations. In Section 6, we present some related studies. Then in Section 7, and we finally conclude this paper.
2. Computation Model of CNN Layers
A CNN includes the layers operate 2-dimensional convolution between the inputs and the weights (kernels), called “convolutional” (CONV) layers, which behave as feature extraction that the pixels having the similar pattern to the weight pattern are amplified. The weights of the CNN are trained to have the reasonable patterns for the task, and this self-tuned feature extraction is the secret the CNNs succeed in the natural data recognition/classification. Typically a CNN might have the “fully-connected” (FC) layers after the CONV layers, and these layers operate vector transformation and work as classifier like the classical multi-layer perceptron.
In this section, we discuss the difference and the similarity of the com- putational structures and the data paths of the two types of layers.
First, we take a look at the computation of the CONV layer with single input/output channel.
An output pixel of the CONV layer is calculated using the weights shaped and the input partial area which has the same shape, as shown in Figure 1(a). While the partial input area (sometimes called “window”) slides over the entire input plane with stride, the weight values are common among all output elements. When we denote this 2-d convolution into the classical neural net style, each output neuron has a few connections to the input neurons of the particular location and all the output neurons has the same synapse weight values (Figure 1(b)).
The computation of the CONV layer consists of the same number of the multiplication and summation. Since all the output elements share the weights, each element of the output uses all of the weights. Then, the corresponding input partial area is uniquely determined by the location of the output element and the size of the weight matrix(H). Input pixels used by each output element are determined when we pick one weight element. Therefore, the convolution can be
Figure 1. The structure of the convolutional layer (a) (b) and the data de- pendency (c).
computed by selecting a weight element one by one and multiplying the corresponding input area for all output elements. This one operation requires only one weight value and as many the input pixels as the output elements, as shown Figure 1(c).
From another view, as shown as black solid lines in Figure 2, an input element has inference to elements of all output channels. And, like broken lines in the figure, a weight element is shared among all output/input elements. In a CONV layer, both input and weight elements have forms of the data reusability.
The computation of an FC layer is denoted as the multiplication of the input vector and the weight matrix. The name “fully-connected” layer comes from that an output element is calculated using all of the input elements via the weights (Figure 3(b)). The value of an output element is the dot product of a row of the weight and the input vector, as Figure 3(a).
Figure 2. Data reusability of convolutional layers.
Figure 3. The structure of the fully-connected layer (a) (b) and the data dependency (c).
Similar to the CONV layers, the dominant operation of the FC layers is the multiplication and the addition. An output element is connected to all of the input element via their weight, as we discussed above, therefore an input ele- ment has the influence to all output elements. The computation of the FC layer can be done by picking an input element and the weights corresponding to the input and all output elements. One input element and as many weight values as output elements are needed in one operation cycle (Figure 3(c)).
Here, we look at the data reusability of an FC layer (Figure 4). An input element FC layer is used by all output elements, however, in contrast to CONV layers, weight elements have no chance to be reused. This means that, generally, the data size of the weights of an FC layer is larger than that of the inputs.
3. Multithreaded CGRA Architecture for Input Value Reuse
The discussion of the previous section shows that the dominant operation of the CONV and FC computation is commonly the multiply-add operation but data sizes of the operand are reversed. In this section, we propose the basic idea of the flexible architecture which does not need any difference of the hardware str- ucture through the entire CNN computation.
3.1. Base Architecture
For the multiply-add (MULADD) operation, the main calculation of the CONV and FC layers, we construct simple multiply-accumulator-based array processor. This is an output-parallel system where a processor engine (PE) handles an output element. According to the above discussion, we need to distribute a common value to all PEs and individual data to each of PEs per one clock cycle.
In the CONV processing, we feed multiple input elements and a shared weight value into the PE array (Figure 5(a)). When the output plane is mapped to the array, the input partial area connected to the outputs through a weight is the same shape as the output plane.
The basic processing flow of the single-channel convolution is shown in Figure 6. Typically, the input and output have multiple channel (e.g. RGB color
Figure 4. Data reusability of fully-connected layers.
Figure 5. Data delivery of the convolutional (a) and fully-connected layers (b).
Figure 6. Basic procedure of a convolutional layer.
space for the input image or features for the hidden layers), convolution be- comes multi-channel (that is, although the CONV layers have 2-d topology in the plane the channels of the input and the output are “fully-connected” so as to express the combination of the features), but let us think about the case the input and the output have only one channel here. Let the parallelism of the array , the size of the weight plane , the stride of the convolution s (in this example, .) The operation for one clock cycle is feeding one weight value into the shared input, cropping partial area from the input and feeding it through the individual input. After cycles (one operation per one weight), then the convolution is done. In this example, the processor array covers whole the output plane, but in the case the number of PEs is not sufficient the output plane divided into blocks and the array executes the time-division processing.
On the other hand, in the FC layer an input, not a weight, is used by multiple elements. An input element of the FC layer and the weight values for each PE are provided at one time (Figure 5(b)). The weight values to each output are one row of the weight matrix when an input neuron is picked, we distribute weights to all PEs very similarly to the CONV processing.
The basic computation for the FC layer is shown in Figure 7. The number of weights is , where and is the number of input and output elements, respectively. One PE calculates one output element. We feed an input element into the array via the common input in a clock cycle, while the weights for the input are fed through the individual input. The computation of the FC layer completes cycles.
3.2. Multithreaded Accumulator for Data Reusing
Lower data transfer per operation is the better in point of the computational time and efficiency. There are two opportunities for reducing the data accesses.
First is the sequential locality of the input partial area of CONV layers. A pixel in an input channel is referenced times, the operations of the H weights in each row are consecutive. As shown in Figure 8, partial inputs referenced by the next weight are the same except the last one column. In the same weight row, the input elements required by the PEs are the input values the adjoining PE loaded in the previous cycle. If these already-loaded elements are reused, the data requests decrease to at most (in the case there is no padding opera- tions). Figure 8 indicates this idea, where the grayed input area is the pixels to be reused.
Figure 7. Basic procedure of a fully-connected layer.
Figure 8. Sequential locality of the convolutional layer processing. The light-grayed elements can be reused.
Moreover, the temporal locality of the input of the multiple channel con- volution is also observed. Let the number of input channels , the number of the output channels . One partial input area is used times because all output channels (features) are computed from the combination of the all input channels. According to this, once the input elements are fetched for the com- putation of the first output channel, we can skip loading the same inputs for following output channels, as shown in Figure 9. When we utilize a T-word register file as accumulator in a PE, this data reusing enables the PE array to reduce data accesses at most .
4. Row-Wise Data Feeding
We have explained the computation procedure of CONV and FC layers using an ideal processor array with individual and shared inputs, and the data reusability of CNNs on the array system. In this section, we build a feasible arrayed architecture with the multithreaded accumulator and the row-wise data delivery feature to exploit the locality the layers originally have.
4.1. Shift-Based Data Supply
Although a square partial input area is needed in every clock cycle in the CONV processing, the square area is not memory-friendly because it requires dis- continuous memory access. In the ideal architecture we indicated in the previous
Figure 9. Temporal locality of the multi-channel convolution.
section, we assumed that the input partial area is fetched within a cycle to the PE array. This requires a very wide-band internal buffer. For a practical imple- mentation, we propose a shift-based row-wise memory system, aiming simpler access pattern and a compact architecture design.
Since memory address of the CONV operations with output-parallel spatial array is continuous in the weight row, row-buffers where each buffer retains an input row can be appropriate (Figure 10). With admitting the latency to fill PEs, this idea is light-weighted since only the PEs on the most left column must have the “individual inputs” and the other PEs acquire the input data from their left-side PEs via the “forwarding bus”. Note that the overhead of filling all PEs sequentially through the forwarding bus will be discussed later in Section 4.2.
The row-wise buffers are not sufficient because the CONV operations require H input rows per PE and rows in the entire PE array ( rows (colored input values in Figure 10) are used for the first weight row, and the additional rows (white input cells in Figure 10) are loaded for the re- maining weight rows). Therefore we need a mechanism for selecting which row buffer should be connected to the left column. There are two solutions to this, using multi-port or multi-bank memories.
Figure 11(a) and Figure 12 show a system using the Py-W Py-R multi-port SRAMs, where the inputs can be read from any row. Compared to the basic idea (Figure 5), the “individual” input and output buses moved from PEs to row buffers. This is the solution with the most few internal memory accesses that each input row is loaded to buffer only once and is read by all PE rows using them (at most H rows) (Figure 13(a)), however the multi-port SRAMs cost area.
Figure 11(a) and Figure 12 indicate a solution with the rows of the 1W2R multi-bank SRAMs and the looped-back forwarding bus connected to the next row SRAMs. The input of the row buffers are connected to the external memory and to the forwarding output of the right column of the adjoining row (Figure 13(b)). This costs only resource and remains the minimal ex- ternal memory access, but the internal buffer write accesses increase compared to the multi-port solution, since the same data as the adjoining row used are
Figure 10. Inpout data mapping on the shift-based array.
Figure 11. Shift-based architectures with (a) “multi-port” and (b) “multi-bank”' row buffers.
Figure 12. A PE for shift-based array (Figure 11(a) and Figure 11(b)).
written to the row buffers again from the forwarding output of the most right column.
4.2. Deep-Multithreaded Accumulator with SRAM
When we employ the row-wise buffers and the shift-based data delivery instead of the ideal fully-parallel memory system, the overhead for filling all PE columns occurs prior to the multiplication of the first element of the weight row. To overcome this, the time-domain multithreaded partial sum accumulation could be effective. Generally, many CNN implementations contain many-channel and
Figure 13. Data mapping and row selecting on (a) the “multi-port” and (b) the “multi-bank” systems.
small-plane CONV layers in deeper part of them, the overhead for sequential transfer will be relatively small, because an input connected to more output channels could be used more times and the data load is reduced by multith- reading.
Now, we try to extend the multi-channel accumulator from a register to an SRAM buffer. An SRAM can have more channels than a register array; this helps us to gain the number of operations per row (i.e. arithmetic intensity). Though SRAM accumulators require more area, it is feasible because the most layers of a CNN have smaller plane and more channels, so the number of PEs and threads (accumulators) could be actually less and more respectively.
We evaluate the performance, the arithmetic intensity of the proposed arch- itecture. Then the complexity of SRAM types for row selection is discussed.
In this section, the input plane size is , the output plane is , the kernel size is ( elements for all input/out- put channel), the number of PEs is , and the number of thread is T. The system operates at clock frequency f and the data bit precision is b.
We evaluated the processing time and data rate of the architecture (Table 1).
In the CONV processing, we pick all weights from all input/output channels
Table 1. Processing time [cycles] and data rate [bps] of the architecture. Bit precision b, clock frequency f, array -parallel, accumulator threads T, weight size .
one by one with each PE processing an output element ( cycles; it takes cycles per row for filling the array and cycles for loading all the weight columns). Time-division processing is taken if the output
size exceeds the number of PEs ( cycles). The processing time is:
The data transfer is buffered so as to fetch all pixels required by a row-wise processing block , the input data size is:
The input data rate is denoted as:
The output data size and rates are:
In the FC processing, it takes
(It takes cycles per an input element for weight preloading, and
is the number of time-division processing blocks), requiring
inputs (while weights are needed for each of input elements) and
outputs. The input and output data rates are:
We evaluated the theoretical processing time and data rates of the proposed (“multi-port” and “multi-bank”) architecture with AlexNet  , as shown in Table 2. The number of PEs is 256 ( ), the bitwidth is . The table shows that this system can process AlexNet in 380 msec (2.6 fps) in .
According to these results, the required data rate in CONV layers is 16 MB/s at most. For the embedded systems this is an acceptable result.
“Util.” means “PE Utilization”, the ratio of the averaged number of operating PEs to the number of PEs through the layer processing. Total PE utilization is calculated as:
Table 2. Evaluation of the proposed architecture using AlexNet  .
where the remainder pixels of the time-division blocks are and , as illustrated in Figure 14. If the width/height of the output plane can be divided by the width/height of the PE array, all the PEs are utilized through all processing time of the layer, the utilization ratio is 100%. Therefore, if we design a CNN to run on the CGRA, we should set the number of output elements to a multiple of that of PEs in order to maximize the computational efficiency.
Figure 15 and Figure 16 show the relation between the problem size and processing time/resource, where the number of processors is . The processing time is in proportion to the number of output channels and to the output plane size, while required data rate is almost independent from the problem size. When the problem becomes larger the processing time increases
Figure 14. Example of dividing the output plane into the processing blocks. In the case of and , the remainder pixels are , the number of fully utilized blocks is , total blocks are .
Figure 15. Processing time and output bitrate over number of output channels. ( ).
but required resources such as data rate are independent, the availability is retained over the problem size.
Figure 17 shows the processing time versus the size of the input in CONV processing. There is no difference between the processing times of the “multi- port” and the “multi-bank” row-wise SRAM systems because the processing method that the row SRAMs feed the input data into each row sequentially is common among these two ideas. Compared to the “ideal” system (all data used in the PE array are provided at once, we described in Section 3), row-buffer system take more processing time, however the arithmetic intensities of these systems are the same because the required numbers of operations and input/ weight data are not different (Figure 18). And more, as Figure 19, the required data input rate for row-wise system is lower than the base idea and is constant for the output size (if the output size becomes larger, both the processing time and required data increases, it results in the almost same data rate).
5.3. Arithmetic Intensity
Arithmetic intensities of some situations are indicated in Figure 20. The broken
Figure 16. Processing time and output bitrate over numbers of output rows and columns ( ).
Figure 17. Output size and processing time. .
Figure 18. Output size and arithmetic intensity. .
Figure 19. Output size and input data rate. .
Figure 20. Output size and arithmetic intensity in some situations. .
line is for 1-thread (i.e. non-multithreaded register) accumulator, the dotted line for 8-thread, and the solid lines are for 512-thread memory. The red line is in case, assuming the deeper layer of a CNN, while the black ones are assuming the input layer. This tells us that, highly-multith- readed system works better especially in the layers with more output channels (i.e. the deeper layers) of CNNs, since the multithreaded computation reduces the data load from the external RAMs by exploiting the input data reusability among the output channels. If the problem size is bigger than the spatial array size, the time-divided processing is needed, but the arithmetic intensity is saturated since the required computation and data transfer per output element do not dramatically vary.
We introduced two types of the row-wise SRAM buffers with “multi-port” and “multi-bank” data selection in Sections 4.1, and estimated the performance of the row-wise shift architectures in Section 5.3. Let us discuss the complexity of the ways of data selecting, “multi-port” and “multi-bank” SRAMs.
As described in Section 4.1, a PE row requires H input rows. To select which row buffer to connected to a PE row, two types of the buffer system have been proposed; “multi-port” SRAM that any PE row can have connection to any row buffer, and “multi-bank” one that row selection can be done by moving data internally.
For “multi-port” SRAM, we utilize a -read/write SRAM as the row buffer. The input data for an input row are stored only once and are read by at most H PE rows (i.e. H output rows), so the number of buffer write accesses is minimized (once per input element). However, an n-read SRAM costs hardware resource by , generally. For row buffers, the capacity (costs linearly) and the number of ports (quadratically) are both in , so this costs area.
In the “multi-bank” system, on the other hand, we use a sets (or “banks”) of simple 1-write 1-read SRAM. This costs resource for rows. After finishing loading input data for a CONV process, every clock cycle the data are fed into each row via the read port of SRAM, while the write port of SRAM acquires the data overflowed from the forwarding output of the right PE on the previous row. Therefore, although the input data are transferred from the external memory to the system only once, just like the “multi-port” imple- mentation, the internal memory write is taken for at most H times per input element as the loaded data flows over the array.
Table 3 shows the area [mm2] and energy [nJ] specification of the “multi- port” and “multi-bank” 16-bit SRAMs (512 words per row) calculated using CACTI simulation   . We assume that 2 more ports/banks than are needed for interleaved read access during the multithreaded processing in the array in this simulation. The energy is the simulated value of a CONV operation of , including the internal data moving. We supposed the power consumption of the write accesses of the SRAM is equal to the read accesses  . The actual capacity and number of the ports/banks should be determined considering the array size ( , ), the throuphput/latency of the
Table 3. Comparison of “multi-port” and “multi-bank” implementations.
external memory system, and the target application (mainly H). Seeing the result of this calculation, although the “multi-bank” memories require more write accesses, the energy and area are still lower than “multi-port” memories.
In CONV processing, the array requires input data at the first output channel in the multithreaded accumulators, therefore the input data rate requirement of the array (internal memory bandwidth) is . Note that the data rate may be if the number of output channels is less than that of accumulator threads. And more, if the read interval T or is greater than , the number of input values of the whole array, even a 1W1R buffer with b-bit width and the shift registers for whole the PE array can be used.
The output values of the CONV processing become valid every
cycles, the required output data rate is . For the
weights, every clock cycle the array multiply-accumulates the inputs with one weight value, so weight data rate is constantly. This means that higher input bandwidth is required when the output channels or the threads are fewer, and that the output rate requirement gets severe if the input channels are fewer or the filter kernels are smaller.
Then we discuss the optimal configuration of the internal SRAMs. To get over the latency of loading input data from the external memories, the processing time must exceed the memory access time, we need row SRAMs of enough depth that the solid partial input area can be read from the external memory behind the multithreaded computation. At this point, if the number of threads is large, the SRAM buffer size should be at least words per row (needs “2” for independent read/write accesses). The data rate of the internal read access is , as described above, required data rate of the external bus is also . When this data rate requirement is met, the time for the data acquirement can be covered by the processing time of the array.
The area of the PE array is in proportion to the number of multithreading accumulators T, while the external bandwidth or the area of the row buffers is in proportion to the array height and in inverse proportion to T. The pro- cessing time is in inverse ratio to and direct to T. For planning the practical CGRA system, these observation should be in consideration on de- mand from the applications.
6. Related Work
AlexNet  proved the high accuracy of the modern CNNs on image re- cognition in 2012. This firstly appeared in ILSVRC  , the competition for the accuracy and adaptability of neural networks on image recognition; the com- petition has been held every year, many CNNs are invented.
Various kinds of neural network accelerators have been developed. Zhang et al. analysed the data reusability and the arithmetic intensity of input- and output-parallel architectures built on an FPGA, and provided a method to optimize it when a target application is given  . Tanomoto et al. implemented EMAX  , a CNN on the CGRA with multiple local memory banks; the main difference from our work is the data stationary. FPGA implementations of CNN coprocessors using DSP blocks were also proposed   . LUT-based FPGA implementation  was proposed; this transforms the weight values using a mathematical techniques to replace the multipliers with the LUT and the adders. Chakradhar et al. proposed mapping a CNN into a coprocessor with the dy- namic reconfiguration techniques  .
For an example of purpose-built architecture, MIT Eyeriss  is a CNN- targeted ASIC system with spatial processor array, which employs an NoC multicasting and a hierarchical memory system aiming at maximizing the PE utilization and exploiting the locality of the input data. Shindo et al. model general multicore neural network accelerators with local memories, and discuss the efficient way to map neural networks onto them  . KU Leuven’s ac- celerator  is a SIMD array system with dynamic voltage and bit precision control, aiming for low-power mobile applications. The accelerator proposed by KAIST  is a CNN accelerator which employs principal component analysis for the weights of convolutional layers to minimize the data size read from external memory. ShiDianNao  and its previous work DaDianNao  also retain the weight values in the internal buffers and employ spatial-mapped neural function unit. On the whole, including our work, the trend of the CNN accelerators is reducing external memory access and maximally utilizing the locality of the data for low-power embedded applications.
Some researches are trying to reduce the data transference and the multiply operations. Recently “binarized” neural networks were proposed   , where a weight value is only 1-bit, which replace the multiplications with simple bit- wise operations. Exploiting redundancy of the neural networks, the computation costs are drastically reduced while keeping the recognition/classification accuracy. The hardware binary neural network accelerators with high-throughput mas- sively-paralleled processors have been developed   .
Commercial systems for neural network processing have been working. Google  and Microsoft  developed their own hardware/FPGA neural network processors and they are using them for their commercial services. NVIDIA’s GPUs have become a very popular way to train/examine CNNs on the server with official libraries  both in academic/busuiness purposes.
We proposed a CGRA architecture with time-domain multithreading for ex- ploiting input data locality. This architecture employs the multithreaded PE accumulators for obtaining multiple in-flight operations and the row-wise data feeding for reusing the input of the neighboring PEs. Our evaluation proved that the proposed architecture is suitable for both deeper and shallower layers in general CNNs. The multithreading in the PEs improves the arithmetic intensity by an order of magnitude. We also showed the quantitative way to determine the specification of the architecture on demand from the applications. It indicated that the architecture requires only a little bandwidth of the external memory, this feature is useful for embedded machine learning applications. Though we showed the advantage of the architecture through the evaluation of the per- formance, the energy efficiency evaluation is desired to develop a more efficient architecture. In order to realize more useful accelerators, we should consider the co-design of the CNN algorithms and the architectures.
This work is supported in part by the JST ACCEL program. We thank the editor and the anonymous reviewers for their useful comments.
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