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在Altera SoC DE1板卡上跑完整的卷积神(🥋)经网络

这次为大家详细展示一个利用卷积神经网络实现图片自动分类(🖖)的例程。

神经网络的优点：自动从数据中学习经验知识，无需复杂的模型和算法。

缺点：有监督学习，需要大量的带标签数据(🍮)；参数量太少时容易过拟合，泛化能力差，参数量太大(🚏)时训练收敛很慢（有可能需要几个月到几年）。

为了克服上述缺点，人们(🦎)发掘了各种计算资源，包(🔄)括多核CPU、GPU、DSP、ASIC、FPGA，甚至使用模拟电路。

使(🚥)用CPU实现卷积神经网络比较方便调试，但性能太差，一般人们都选用更快的GPU实现。目前开源的框架大多都支持GPU，如伯克利大学Caffe和Google Convnet。

微(🈯)软在2015年2月宣布使用Stratix V完成了CNN加速器，处理 CIFAR10 图片速度可达(🔮)每秒2300多张。

这里我们也使用CIFAR10图片数据，在Cyclone V板子上跑一个卷(🐧)积神经网络CNN demo。由于板子上计算(🔘)资源太(🎇)少（DSP Slice只有80多个(🌼)），实(👓)现完整的网络不太现实，只能在FPGA上实现基本计算单元，然后由HPS统一调度。性能预期不会太高，后面给出。

CIFAR10图片(👴)都是什么呢？先来张图！

有兴趣的朋友可以到官网下载（CIFAR10官网）。上面提到过，CNN是有监督学习系统，需要大量带label的数(💋)据，CIFAR10就是这(🌸)样一个开放(🕌)的数据库，提供了60000张不同类别的图片，分(🧑)为10个类（如上图左侧所示），每个类别有600张图。这个数据集不算特别大，适合在嵌入式平台(👙)上实现。而更大的数据集有ImageNet-1000（ImageNet官网），拥有120多万张高(🏢)清无(🌉)码大图，我下载(🧛)到硬盘，占用了近200GB空间（只能忍痛将其他rmvb和avi删掉(🐸)了）！

有朋友会问，不用这些数据行不行，我们的智能手机里面(🗣)照片能不能用于CNN做训练？

答案是可以的，只是你的数据集很不“均匀”，采样不(💁)够“完备”，训练出的模型是真实模型的“有偏估计”，而上述两个数据集经过了种种考验，已经(👼)是学术界公认的优(🕓)质数据集，一年一度的ILSVRC比赛就采用了这些数据集。

说完数据(🌶)，再说模型。先来看一张经典的CNN结构：

这是世界上第(💟)一个将CNN实用化的例子，实现了手写体字母自动识别。在这个(💦)CNN模型中，可以看到输入是一张32 x 32的(🥗)二(🐚)维图像(🔽)，经过卷积层(Convolution)、下采样层（Subsampling，也称Pooling)、全(🈺)连接(🍶)层（Full Connection，也称Inner Product)后，得到一组概率密度，我们选其中概率最大的元素作为该模型对输(💱)入图(🎷)像的分类结果。所以实现CNN时，只需要实现三种基本算法：卷积、下(🌧)采样、矩阵乘。除此之外，每层输出都可选择是否经过非线性变换，常用的非(🐧)线性变换有ReLU和Sigmoid，前者计算较为(🤣)简单，使用(🎲)较为广泛。

Caffe框架中提供了专门为cifar10数据定制的模型，是用proto格式(🛋)写的，我们的demo也基于这个模型。内容如下：

name: "CIFAR10_quick_test"input: "data"input_dim: 1input_dim: 3input_dim: 32input_dim: 32layers {name: "conv1"type: CONVOLUTIONbottom: "data"top: "conv1"blobs_lr: 1blobs_lr: 2convolution_param {num_output: 32pad: 2kernel_size: 5stride: 1}}layers {name: "pool1"type: POOLINGbottom: "conv1"top: "pool1"pooling_param {pool: MAXkernel_size: 3stride: 2}}layers {name: "relu1"type: RELUbottom: "pool1"top: "pool1"}layers {name: "conv2"type: CONVOLUTIONbottom: "pool1"top: "conv2"blobs_lr: 1blobs_lr: 2convolution_param {num_output: 32pad: 2kernel_size: 5stride: 1}}layers {name: "relu2"type: RELUbottom: "conv2"top: "conv2"}layers {name: "pool2"type: POOLINGbottom: "conv2"top: "pool2"pooling_param {pool: AVEkernel_size: 3stride: 2}}layers {name: "conv3"type: CONVOLUTIONbottom: "pool2"top: "conv3"blobs_lr: 1blobs_lr: 2convolution_param {num_output: 64pad: 2kernel_size: 5stride: 1}}layers {name: "relu3"type: RELUbottom: "conv3"top: "conv3"}layers {name: "pool3"type: POOLINGbottom: "conv3"top: "pool3"pooling_param {pool: AVEkernel_size: 3stride: 2}}layers {name: "ip1"type: INNER_PRODUCTbottom: "pool3"top: "ip1"blobs_lr: 1blobs_lr: 2inner_product_param {num_output: 64}}layers {name: "ip2"type: INNER_PRODUCTbottom: "ip1"top: "ip2"blobs_lr: 1blobs_lr: 2inner_product_param {num_output: 10}}layers {name: "prob"type: SOFTMAXbottom: "ip2"top: "prob"}

复制代码(🌚)

可(🎺)见，上述模型经过了3个卷积层（conv1, conv2, conv3），每个卷(➿)积层后面都跟着(🌺)下采样层（pool1, pool2, pool3），之后有两个全连接层(ip1, ip2)，最后一(📂)层prob为SOFTMAX分类层，是计算概率密度的(🌱)，这里我们不需要关心。

下面三张图分别统计了CNN模型各层的参数量、数据量和计算量。

可以看出，卷(🔮)积层的参数量很少，但数据量很大；全(💝)连接层刚好(🦎)相反，参数量较大，但数(🃏)据量很少。

通过计算量统计发现conv2计算量最大，其次是conv3和conv1。全连接层的计算量相对卷积层较小，但不可忽略。其他层（pool1, pool2以及各(🀄)级relu）由(💽)于计算量太小，本设计中没有将其实现为Open CL kernel，而是直接(🍦)CPU端实现。

综上所述，我们重点实现两个算法：卷积和矩阵乘，分别对应卷(🍴)积层、全连接层的实(🌒)现。

在DE1-SOC上我利用了友晶提供的Open CL BSP，支持C语言开发FPGA。

卷积层计算kernel函数如下：

__attribute__((num_compute_units(4)))__kernelvoid conv(__global float * a, __global float * b, __global float * c, const int M, const int N, const int K){int gx = get_global_id(0);int gy = get_global_id(1);float tmp=0.0f;for(int x = 0; x < K; x ++){for(int y = 0; y < K; y ++){tmp += a[(gx + x) * M + (gy + y)] * b[x * K + y];}}

复制代码

全连(⌚)接层计算采用矩(✉)阵乘实现，kernel函数如下：

__attribute__((num_compute_units(4)))__kernelvoid gemm(__global float * a, __global float * b, __global float * c, const int M, const int N, const int K){int gx = get_global_id(0);int gy = get_global_id(1);int sy = get_global_size(1);int sx = get_global_size(0);int s = sx * sy;for(int x = gx; x < M; x += sx){for(int y = gy; y < N; y += sy){float tmp=0.0f;for(int z = 0; z < K; z++){tmp += a[z * M + x] * b[y * K + z];}c[y * M + x] = tmp;}}}

复制代码(🐳)

编译(🥃)kernel函数需要使用Altera SDK for OpenCL，我用的版本是14.0.0.200，申请了两个月的license。编译使用(🔛)命令行aoc，得到*.aocx文件(⏺)。

Open CL编译输出报告中给出(🥓)了资源占用情况：

+--------------------------------------------------------------------+; Estimated Resource Usage Summary ;+----------------------------------------+---------------------------+; Resource + Usage ;+----------------------------------------+---------------------------+; Logic utilization ; 83% ;; Dedicated logic registers ; 46% ;; Memory blocks ; 57% ;; DSP blocks ; 25% ;+----------------------------------------+---------------------------;

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可见，逻辑资源、(📨)存储器资源消耗较为明显，而DSP资源并未用尽，说明还有优化的空间。

编译主程序需要使用SoCEDS，我用的版本为14.0.2.274，也是命(🎴)令行方式，在(😉)工程目录下执行make，结束后得(🈂)到可执行文件cnn。

将这两个文件拷贝到SD卡，按照前面的博客对板子进行设置(🧔)，将CNN的模型、CIFAR10数据也拷贝到SD卡中，板子上电，mount SD卡到/mnt，执行cnn，得到输出如下：

<div class="blockcode"><blockquote>Please input the number of images(1~100):100Loading data...OK!Constructing CNN...OK!Begin calculation...Elapsed Time = 141.861 s.Real Label = 3(cat), Calc Label = 3(cat), error count = 0Real Label = 8(ship), Calc Label = 8(ship), error count = 0Real Label = 8(ship), Calc Label = 8(ship), error count = 0Real Label = 0(airplane), Calc Label = 0(airplane), error count = 0Real Label = 6(frog), Calc Label = 6(frog), error count = 0Real Label = 6(frog), Calc Label = 6(frog), error count = 0Real Label = 1(automobile), Calc Label = 1(automobile), error count = 0Real Label = 6(frog), Calc Label = 6(frog), error count = 0Real Label = 3(cat), Calc Label = 3(cat), error count = 0Real Label = 1(automobile), Calc Label = 1(automobile), error count = 0Real Label = 0(airplane), Calc Label = 0(airplane), error count = 0Real Label = 9(truck), Calc Label = 9(truck), error count = 0Real Label = 5(dog), Calc Label = 5(dog), error count = 0Real Label = 7(horse), Calc Label = 7(horse), error count = 0Real Label = 9(truck), Calc Label = 9(truck), error count = 0Real Label = 8(ship), Calc Label = 8(ship), error count = 0Real Label = 5(dog), Calc Label = 5(dog), error count = 0Real Label = 7(horse), Calc Label = 7(horse), error count = 0Real Label = 8(ship), Calc Label = 8(ship), error count = 0Real Label = 6(frog), Calc Label = 6(frog), error count = 0Real Label = 7(horse), Calc Label = 7(horse), error count = 0Real Label = 0(airplane), Calc Label = 2(bird), error count = 1Real Label = 4(deer), Calc Label = 4(deer), error count = 1Real Label = 9(truck), Calc Label = 9(truck), error count = 1Real Label = 5(dog), Calc Label = 4(deer), error count = 2Real Label = 2(bird), Calc Label = 3(cat), error count = 3Real Label = 4(deer), Calc Label = 4(deer), error count = 3Real Label = 0(airplane), Calc Label = 0(airplane), error count = 3Real Label = 9(truck), Calc Label = 9(truck), error count = 3Real Label = 6(frog), Calc Label = 6(frog), error count = 3Real Label = 6(frog), Calc Label = 6(frog), error count = 3Real Label = 5(dog), Calc Label = 5(dog), error count = 3Real Label = 4(deer), Calc Label = 4(deer), error count = 3Real Label = 5(dog), Calc Label = 5(dog), error count = 3Real Label = 9(truck), Calc Label = 9(truck), error count = 3Real Label = 2(bird), Calc Label = 3(cat), error count = 4Real Label = 4(deer), Calc Label = 7(horse), error count = 5Real Label = 1(automobile), Calc Label = 9(truck), error count = 6Real Label = 9(truck), Calc Label = 9(truck), error count = 6Real Label = 5(dog), Calc Label = 5(dog), error count = 6Real Label = 4(deer), Calc Label = 4(deer), error count = 6Real Label = 6(frog), Calc Label = 6(frog), error count = 6Real Label = 5(dog), Calc Label = 5(dog), error count = 6Real Label = 6(frog), Calc Label = 6(frog), error count = 6Real Label = 0(airplane), Calc Label = 0(airplane), error count = 6Real Label = 9(truck), Calc Label = 9(truck), error count = 6Real Label = 3(cat), Calc Label = 5(dog), error count = 7Real Label = 9(truck), Calc Label = 9(truck), error count = 7Real Label = 7(horse), Calc Label = 7(horse), error count = 7Real Label = 6(frog), Calc Label = 6(frog), error count = 7Real Label = 9(truck), Calc Label = 9(truck), error count = 7Real Label = 8(ship), Calc Label = 8(ship), error count = 7Real Label = 0(airplane), Calc Label = 2(bird), error count = 8Real Label = 3(cat), Calc Label = 3(cat), error count = 8Real Label = 8(ship), Calc Label = 8(ship), error count = 8Real Label = 8(ship), Calc Label = 8(ship), error count = 8Real Label = 7(horse), Calc Label = 7(horse), error count = 8Real Label = 7(horse), Calc Label = 7(horse), error count = 8Real Label = 4(deer), Calc Label = 3(cat), error count = 9Real Label = 6(frog), Calc Label = 3(cat), error count = 10Real Label = 7(horse), Calc Label = 7(horse), error count = 10Real Label = 3(cat), Calc Label = 5(dog), error count = 11Real Label = 6(frog), Calc Label = 6(frog), error count = 11Real Label = 3(cat), Calc Label = 3(cat), error count = 11Real Label = 6(frog), Calc Label = 6(frog), error count = 11Real Label = 2(bird), Calc Label = 2(bird), error count = 11Real Label = 1(automobile), Calc Label = 1(automobile), error count = 11Real Label = 2(bird), Calc Label = 2(bird), error count = 11Real Label = 3(cat), Calc Label = 3(cat), error count = 11Real Label = 7(horse), Calc Label = 9(truck), error count = 12Real Label = 2(bird), Calc Label = 2(bird), error count = 12Real Label = 6(frog), Calc Label = 6(frog), error count = 12Real Label = 8(ship), Calc Label = 8(ship), error count = 12Real Label = 8(ship), Calc Label = 8(ship), error count = 12Real Label = 0(airplane), Calc Label = 0(airplane), error count = 12Real Label = 2(bird), Calc Label = 2(bird), error count = 12Real Label = 9(truck), Calc Label = 0(airplane), error count = 13Real Label = 3(cat), Calc Label = 3(cat), error count = 13Real Label = 3(cat), Calc Label = 2(bird), error count = 14Real Label = 8(ship), Calc Label = 8(ship), error count = 14Real Label = 8(ship), Calc Label = 8(ship), error count = 14Real Label = 1(automobile), Calc Label = 1(automobile), error count = 14Real Label = 1(automobile), Calc Label = 1(automobile), error count = 14Real Label = 7(horse), Calc Label = 7(horse), error count = 14Real Label = 2(bird), Calc Label = 2(bird), error count = 14Real Label = 5(dog), Calc Label = 7(horse), error count = 15Real Label = 2(bird), Calc Label = 2(bird), error count = 15Real Label = 7(horse), Calc Label = 7(horse), error count = 15Real Label = 8(ship), Calc Label = 8(ship), error count = 15Real Label = 9(truck), Calc Label = 9(truck), error count = 15Real Label = 0(airplane), Calc Label = 0(airplane), error count = 15Real Label = 3(cat), Calc Label = 4(deer), error count = 16Real Label = 8(ship), Calc Label = 8(ship), error count = 16Real Label = 6(frog), Calc Label = 6(frog), error count = 16Real Label = 4(deer), Calc Label = 4(deer), error count = 16Real Label = 6(frog), Calc Label = 6(frog), error count = 16Real Label = 6(frog), Calc Label = 6(frog), error count = 16Real Label = 0(airplane), Calc Label = 2(bird), error count = 17Real Label = 0(airplane), Calc Label = 0(airplane), error count = 17Real Label = 7(horse), Calc Label = 7(horse), error count = 17Classify Score = 83 %.

上面的执行流程是这样的，首先输入测试样本数目（(💱)1到100），由于DE1板子FPGA端SDRAM容量较小，难以加载全部(📍)测试数据（10000张图片），故每次最多(📣)装入100张图片。之后载入数据到HPS内存，然(👦)后开始构建CNN模型，构建过程中也实现了Open CL的初始化。构建完毕，将输入图像依次通过CNN，得到一系列分类结果，与标签进行对比，统计错误分类个数，计算分类准确率。

经过测试(💐)，分类准确率达到83%，与Caffe测试结果一致。

经过以上测试，可以得到结论：

（1）(👣)使用Open CL可以很(💺)方便地移植高级语言编写的算法；

（2）CNN在移(📑)植过程中需要考(🍻)虑实际硬件，定制合(😽)适的模型和数据；

（3）Cyclone 5逻辑资源较少（85K，Open CL kernel占用了83%），如果希望进一步提高计算速(🧣)度，一方(🔛)面可以选用高性能(🍮)器件（如Stratix V、Arria 10）(😫)，另一方面(🚓)可以使(⛔)用RTL自己搭建计算系统。

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