JeremyJ · 2021年12月15日

【模型推理】量化实现分享一:详解 min-max 对称量化算法实现

欢迎关注我的公众号 [极智视界]

  大家好,我是极智视界,本文剖析一下 min-max 对称量化算法实现,以 Tengine 的实现为例。

   Tengine 是 OpenAILab 开源的优秀端侧深度学习推理框架,其核心主要由 C 语言实现,包裹的功能代码嵌套了 C++。量化是推理加速必不可少的优化环节,成熟的推理框架一般会把量化模块剥离出来形成独立的一套工具,如 Tengine、NCNN、昇腾、寒武纪都这么做,这主要是因为量化过程和硬件非强相关,解耦开来能干更多的事。

   min-max 和 kl 量化算法是硬件厂商适配推理引擎的基础和标配, 其中 kl 量化深受用户喜爱,如英伟达的 TensorRT 也正是采用了 kl 量化策略;而这里要介绍的 min-max 的特点是逻辑简单、效果良好,作为量化实现分享系列的开篇比较合适,这里带大家一起研究一下 Tengine 中 minx-max 量化策略的具体实现。

1、量化使用

   量化主要分为激活值(动态)量化、权值&偏置(静态)量化,而权值&偏置的量化是对精度影响比较大的,激活值的量化对整体影响较小,但也需要量化,才有可能协同达到整体满意的效果。对于一般量化来说,权值&偏置的量化会采用逐通道 perChannel 的方式,而激活值的量化一般是逐层 perLayer 的方式。解释一下为啥会这样,对于量化来说,卷积肯定是大头,对于卷积来说,若激活值量化采用逐通道方式,这和卷积核参数共享是相悖的,所以一般激活值就用逐层量化,以契合卷积参数共享。

   这里主要看一下 Tengine 量化需要的传参:

  • Input model:传入的 fp32 tmfile 模型文件;
  • Output model:生成的 int8 tmfile 模型文件;
  • Calib images:传入的激活值量化校准图片;
  • Scale file:生成的校准表文件;
  • Agorithm:量化算法,可选 MIN-MAX、KL、ACIQ、DFQ、EQ;
  • Dims:输入校准图的 shape,这里传三维 c h w,n 在代码中写死 n = 1;
  • Mean:图像预处理均值;
  • Scale:图像预处理缩放尺度;
  • BGR2RGB:通道转换;
  • Center crop:图像预处理,裁剪;
  • Letter box:图像预处理,保持横纵比的前提下对图像做 resize;
  • YOLOv5 focus:类似 yolov5 的预处理注意力机制;
  • Thread num:量化多线程设置;

2、min-max 量化

  min-max 是最简单的量化算法,主要逻辑如下:

   在 Tengine 中实现 min-max 方法的主要代码如下:

case ALGORITHM_MIN_MAX:{
    if (quant_tool.scale_file.empty()){
        quant_tool.scale_file = "table_minmax.scale";
        quant_tool.activation_quant_tool();
    }
    save_graph_i8_perchannel(quant_tool.model_file.c_str(), quant_tool.scale_file.c_str(), quant_tool.output_file, quant_tool.inplace, false);
    /* Evaluate quantitative losses */
    if (quant_tool.evaluate){
        fprintf(stderr, "[Quant Tools Info]: Step Evaluate, evaluate quantitative losses\n");
        quant_tool.assess_quant_loss(0);
    }
    break;
}

   其中最主要的量化搜索策略接口是 quant_tool.activation_quant_tool()save_graph_i8_perchannel,对于 min-max 来说这两个接口分别做了两件事:

   (1) 激活值量化,生成 table_minmax.scale

   (2) 权值&偏置量化,生成 scale_weight.txtscale_bias.txt

2.1 激活值量化

   看 Tengine 源码一定要抓住 struct graph* ir_graph,graph 这个结构体是精髓。

   激活值量化是个动态的过程,需要动态的获取每层的数据分布,这也就是为啥需要你喂一定数量校准图片的原因。

   先说一下预处理模块,这个其他量化算法是通用的:

// 将 input_tensor 和 input_data 地址绑定,而 input_tensor=>ir_graph->tensor_list。注意:这一步一定要看到,不然后续代码很难看懂
tensor_t input_tensor = get_graph_input_tensor(ir_graph, 0, 0);

if (set_tensor_shape(input_tensor, dims, 4) < 0){
    fprintf(stderr, "Set input tensor shape failed\n");
    return -1;
}

if (set_tensor_buffer(input_tensor, input_data.data(), img_size * sizeof(float)) < 0){
    fprintf(stderr, "Set input tensor buffer failed\n");
    return -1;
}

// prerun graph,做一些初始化配置
if (prerun_graph_multithread(ir_graph, this->opt) < 0){
    fprintf(stderr, "Prerun multithread graph failed.\n");
    return -1;
}

// 图像预处理,传出 input_data,这个和前面的 input_tensor & ir_graph->tensor_list[0] 输入参 绑定,修改了 input_data 即修改了 ir_graph.tensor_list,这样就能看懂
get_input_data_cv(imgs_list[nums].c_str(), input_data.data(), img_c, img_h, img_w, mean, scale, sw_RGB, center_crop, letterbox_rows, letterbox_cols, focus);

   然后 run 一下,把中间激活值记录到 ir_graph->tensor_list[i] 里:

if (run_graph(ir_graph, 1) < 0){
    fprintf(stderr, "Run graph failed\n");
    return -1;
}

   激活激活值的 min、max 值:

/* get the min/max value of activation tensor */
for (int i = 0; i < ir_graph->tensor_num; i++){
    struct tensor* act_tensor = ir_graph->tensor_list[i];
    if (act_tensor->tensor_type == TENSOR_TYPE_VAR || act_tensor->tensor_type == TENSOR_TYPE_INPUT){
        float* start_addr = (float*)act_tensor->data;
        float* end_addr = (float*)act_tensor->data + act_tensor->elem_num;
        max_activation[i] = std::max(max_activation[i], *std::max_element(start_addr, end_addr));
        min_activation[i] = std::min(min_activation[i], *std::min_element(start_addr, end_addr));}
}

  计算激活值量化尺度,对于 softmax 层 scale 默认为 1 / 127.f

/* save the calibration file with min-max algorithm */
FILE* fp_minmax = fopen("table_minmax.scale", "wb");
for (int i = 0; i < ir_graph->tensor_num; i++){
    struct tensor* t = ir_graph->tensor_list[i];
    if (t->tensor_type == TENSOR_TYPE_VAR || t->tensor_type == TENSOR_TYPE_INPUT){
        float act_scale = 1.f;
        int act_zero_point = 0;

        act_scale = std::max(std::abs(max_activation[i]), std::abs(min_activation[i])) / 127.f;

        /* the scale of softmax is always scale = 1 / 127.f */
        for (int j = 0; j < ir_graph->node_num; j++){
            struct node* noden = ir_graph->node_list[j];
            struct tensor* tensor_tmp = get_ir_graph_tensor(ir_graph, noden->output_tensors[0]);

            if (!(tensor_tmp->tensor_type == TENSOR_TYPE_INPUT || tensor_tmp->tensor_type == TENSOR_TYPE_VAR))
                continue;

            std::string tmp_op_name = get_op_name_from_type(noden->op.type);
            std::string cur_name = t->name;
            std::string tmp_name = tensor_tmp->name;

            if ((cur_name == tmp_name) && tmp_op_name == "Softmax"){
                act_scale = 1 / 127.f;
                break;}
        }

        fprintf(fp_minmax, "%s %f %d\n", ir_graph->tensor_list[i]->name, act_scale, act_zero_point);}
}

2.2 权值 & 偏置量化

  权值 & 偏置量化和激活值量化不太一样,激活值量化需要校准图片推理以获得输入数据的动态分布,而权值 & 偏置是静态的,单纯的量化过程不需执行前向推理。

2.2.1 创建 graph

   加载 tmfile,构建 graph:

struct graph* ir_graph = (struct graph*)create_graph(nullptr, "tengine", model_file);
if (nullptr == ir_graph){
fprintf(stderr, "Create graph failed.\n");
return -1;}

2.2.2 优化激活值量化 scale

   这里主要做一个 quant.inplace 的优化,这是针对非卷积算子的量化处理策略。

if (inplace == 0){
    for (int i = 0; i < ir_graph->tensor_num; i++){
        struct tensor* ir_tensor = ir_graph->tensor_list[i];
        if (ir_tensor->tensor_type == TENSOR_TYPE_VAR || ir_tensor->tensor_type == TENSOR_TYPE_INPUT){
            ir_tensor->scale = layer_scale[ir_tensor->name];
            ir_tensor->zero_point = layer_zeropoint[ir_tensor->name];}}
    }
    else{
        std::tr1::unordered_map<std::string, bool> layer_pass;
        for (int i = ir_graph->tensor_num - 1; i >= 0; i--){
            struct tensor* ir_tensor = ir_graph->tensor_list[i];
            if (ir_tensor->tensor_type == TENSOR_TYPE_VAR || ir_tensor->tensor_type == TENSOR_TYPE_INPUT){
                if (layer_pass[ir_tensor->name] == false){
                    uint32_t ir_node_idx = ir_tensor->producer;
                    struct node* t_node = ir_graph->node_list[ir_node_idx];

                    std::string op_name = get_op_name_from_type(t_node->op.type);

                    bool poolTrue = false;
                    bool reluTrue = false;
                    if (op_name == "Pooling"){
                        struct pool_param* pool_param = (struct pool_param*)t_node->op.param_mem;
                        if (pool_param->pool_method == 0)
                            poolTrue = true;
                    }
                    else if (op_name == "ReLU"){
                        struct relu_param* relu_param = (struct relu_param*)t_node->op.param_mem;
                        if (relu_param->negative_slope == 0.f)
                            reluTrue = true;
                    }
                    if (op_name == "Flatten" || op_name == "Reshape" || op_name == "Squeeze" || op_name == "Clip" || op_name == "Slice" || poolTrue || reluTrue){
                        struct tensor* t_in_tensor = ir_graph->tensor_list[t_node->input_tensors[0]];
                        if (layer_scale[ir_tensor->name] != 0){
                            ir_tensor->scale = layer_scale[ir_tensor->name];
                            ir_tensor->zero_point = layer_zeropoint[ir_tensor->name];

                            if (t_in_tensor->tensor_type == TENSOR_TYPE_VAR || t_in_tensor->tensor_type == TENSOR_TYPE_INPUT){
                                recursion_pass_through(ir_graph, ir_tensor->name, t_in_tensor, layer_used, layer_scale, layer_zeropoint, layer_pass);}}
                    }
                    else{
                        ir_tensor->scale = layer_scale[ir_tensor->name];
                        ir_tensor->zero_point = layer_zeropoint[ir_tensor->name];
                    }
                    layer_pass[ir_tensor->name] = true;}}}
}

2.2.3 权值 & 偏置量化

  量化的整个过程和激活值量化类似,即先搜索 min、max 值,后做截断缩放处理。这里不仅需要计算 scale,而且还要做截断缩放处理的原因是需要生成 int8 tmfile 量化模型文件。这里还有一点需要注意的是权值量化精度为 int8,偏置量化精度为 int32,因为权值做完矩阵乘后值很有可能就会溢出 int8,所以需要权值矩阵乘后的值用 int32 存储,然后与 int32 的偏置做加法。

   除了以上这些,和激活值量化还有个区别是,激活值量化是 perLayer 的,而权值 & 偏置量化是 perChannel 的。

   权值 int8 量化:

/* quantize the weight data from fp32 to int8 */
if (op_name == "Convolution" || op_name == "FullyConnected" || op_name == "Deconvolution"){
    struct tensor* weight_tensor = ir_graph->tensor_list[noden->input_tensors[1]];

    int channel_num = weight_tensor->dims[0];
    int cstep = int(weight_tensor->elem_num / channel_num);
    float* weight_data = (float*)weight_tensor->data;
    int8_t* i8_weight_data = (int8_t*)sys_malloc(weight_tensor->elem_num * sizeof(int8_t));

    float* weight_scale_list = (float*)sys_malloc(channel_num * sizeof(float));
    int* weight_zp_list = (int*)sys_malloc(channel_num * sizeof(int));

    fprintf(fp_weight, "%s ", weight_tensor->name);
    /* calculate the quant scale value of weight perchannel, scale = abs(min, max) / 127 */
    if (internal){
        // TODO
        for (int ch = 0; ch < channel_num; ch++){
            weight_scale_list[ch] = weight_tensor->scale_list[ch];
            weight_zp_list[ch] = 0;}
    }
    else{
        for (int ch = 0; ch < channel_num; ch++){
            float* weight_data_ch_start = weight_data + ch * cstep;
            float* weight_data_ch_end = weight_data + (ch + 1) * cstep;
            float weight_max = *std::max_element(weight_data_ch_start, weight_data_ch_end);
            float weight_min = *std::min_element(weight_data_ch_start, weight_data_ch_end);

            weight_scale_list[ch] = std::max(std::abs(weight_max), std::abs(weight_min)) / 127.f;
            weight_zp_list[ch] = 0;
            fprintf(fp_weight, "%8.8f ", weight_scale_list[ch]);
        }
        fprintf(fp_weight, "\n");
    }

    /* quantize the value of weight from Float32 to Int8, value_i8 = (value_fp32 / scale).round().clip(-127, 127) */
    for (int ch = 0; ch < channel_num; ch++){
        for (int j = 0; j < cstep; j++){
            if (weight_data[ch * cstep + j] == 0 || weight_scale_list[ch] == 0)
                i8_weight_data[ch * cstep + j] = 0;
            else{
                float int8_data = round(weight_data[ch * cstep + j] / weight_scale_list[ch]);
                int8_data = int8_data > 127.f ? 127.f : int8_data;
                int8_data = int8_data < -127.f ? -127.f : int8_data;
                i8_weight_data[ch * cstep + j] = int8_t(int8_data);}}
    }

    weight_tensor->scale_list = weight_scale_list;
    weight_tensor->zp_list = weight_zp_list;
    weight_tensor->data_type = TENGINE_DT_INT8;
    weight_tensor->elem_size = sizeof(int8_t); // int8, signed char
    weight_tensor->data = i8_weight_data;
    weight_tensor->quant_param_num = channel_num;
}

   偏置 int32 量化:

/* quantize the weight data from fp32 to int32 */if (noden->input_num > 2){    struct tensor* input_tensor = ir_graph->tensor_list[noden->input_tensors[0]];    struct tensor* bias_tensor = ir_graph->tensor_list[noden->input_tensors[2]];    float* bias_scale_list = (float*)sys_malloc(bias_tensor->dims[0] * sizeof(float));    int* bias_zp_list = (int*)sys_malloc(bias_tensor->dims[0] * sizeof(int32_t));    float* bias_data = (float*)bias_tensor->data;    int* int32_bias_data = (int*)sys_malloc(bias_tensor->elem_num * sizeof(int32_t));    int bstep = int(bias_tensor->elem_num / channel_num);    fprintf(fp_bias, "%s ", bias_tensor->name);    /* calculate the quant scale value of bias perchannel, scale = scale_weight * scale_in */    for (int ch = 0; ch < channel_num; ch++){        bias_scale_list[ch] = weight_scale_list[ch] * input_tensor->scale;        bias_zp_list[ch] = 0;        fprintf(fp_bias, "%8.8f ", bias_scale_list[ch]);    }    fprintf(fp_bias, "\n");    /* quantize the value of bias from Float32 to Int32, value_i32 = (value_fp32 / scale).round() */    for (int ch = 0; ch < channel_num; ch++){        for (int bi = 0; bi < bstep; bi++){            if (bias_data[ch * bstep + bi] == 0 || bias_scale_list[ch] == 0)                int32_bias_data[ch * bstep + bi] = 0;            else                int32_bias_data[ch * bstep + bi] = int(round(bias_data[ch * bstep + bi] / bias_scale_list[ch]));}    }    bias_tensor->scale_list = bias_scale_list;    bias_tensor->zp_list = bias_zp_list;    bias_tensor->data_type = TENGINE_DT_INT32;    bias_tensor->elem_size = sizeof(int32_t); // int32, signed int    bias_tensor->data = int32_bias_data;    bias_tensor->quant_param_num = channel_num;}

  到这里权值 & 偏置的量化就介绍的差不多咯。

   以上详细介绍了 min-max 量化算法的实现,主要以 Tengine 为例进行代码说明,希望我的分享能对你的学习有一点帮助。

【公众号传送】
【模型推理】量化实现分享一:详解 min-max 对称量化算法实现

推荐阅读
关注数
5
内容数
25
目录
极术微信服务号
关注极术微信号
实时接收点赞提醒和评论通知
安谋科技学堂公众号
关注安谋科技学堂
实时获取安谋科技及 Arm 教学资源
安谋科技招聘公众号
关注安谋科技招聘
实时获取安谋科技中国职位信息