我在将GPU缓冲区传输到CPU以执行排序操作时遇到了一些问题。缓冲区是由300.000个浮点值组成的GL_SHADER_STORAGE_BUFFER
。 glGetBufferSubData
的传送操作大约需要10毫秒,glMapBufferRange
的传送操作需要100毫秒以上。
Im使用的代码如下:
std::vector<GLfloat> viewRow;
unsigned int viewRowBuffer = -1;
int length = -1;
void bindRowBuffer(unsigned int buffer){
glBindBuffer(GL_SHADER_STORAGE_BUFFER, buffer);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 3, buffer);
}
void initRowBuffer(unsigned int &buffer, std::vector<GLfloat> &row, int lengthIn){
// Generate and initialize buffer
length = lengthIn;
row.resize(length);
memset(&row[0], 0, length*sizeof(float));
glGenBuffers(1, &buffer);
bindRowBuffer(buffer);
glBufferStorage(GL_SHADER_STORAGE_BUFFER, row.size() * sizeof(float), &row[0], GL_DYNAMIC_STORAGE_BIT | GL_MAP_READ_BIT | GL_MAP_WRITE_BIT);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0);
}
void cleanRowBuffer(unsigned int buffer) {
float zero = 0.0;
glClearNamedBufferData(buffer, GL_R32F, GL_RED, GL_FLOAT, &zero);
}
void readGPUbuffer(unsigned int buffer, std::vector<GLfloat> &row) {
glGetBufferSubData(GL_SHADER_STORAGE_BUFFER,0,length *sizeof(float),&row[0]);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0);
}
void readGPUMapBuffer(unsigned int buffer, std::vector<GLfloat> &row) {
float* data = (float*)glMapBufferRange(GL_SHADER_STORAGE_BUFFER, 0, length*sizeof(float), GL_MAP_READ_BIT); glBindBuffer(GL_SHADER_STORAGE_BUFFER, 0);
memcpy(&row[0], data, length *sizeof(float));
glUnmapBuffer(GL_SHADER_STORAGE_BUFFER);
}
主要是在做:
bindRowBuffer(viewRowBuffer);
cleanRowBuffer(viewRowBuffer);
countPixs.bind();
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, gPatch);
countPixs.setInt("gPatch", 0);
countPixs.run(SCR_WIDTH/8, SCR_HEIGHT/8, 1);
countPixs.unbind();
readGPUbuffer(viewRowBuffer, viewRow);
其中countPixs是一个计算着色器,但我认为问题不存在,因为如果我注释运行命令,则读取将花费完全相同的时间。
奇怪的是,如果我只执行1个float的getbuffer:
glGetBufferSubData(GL_SHADER_STORAGE_BUFFER,0, 1 *sizeof(float),&row[0]);
需要完全相同的时间...所以我猜一路都有问题...可能与GL_SHADER_STORAGE_BUFFER
有关?
这很可能是GPU-CPU同步/往返引起的延迟。即映射缓冲区后,触摸缓冲区的先前GL命令需要立即完成,从而导致管道停顿。请注意,驱动程序是懒惰的:很可能是GL命令已经甚至没有开始执行。
如果可以:glBufferStorage(..., GL_MAP_PERSISTENT_BIT)
并持久映射缓冲区。这样可以避免完全重新映射和分配任何GPU内存,并且可以通过一些警告将映射的指针保留在绘图调用之上: