python - python 中的 numba CUDA 非常慢

Question

我在 numba cuda 中运行这个简单的代码，发现速度非常慢。知道瓶颈吗？

   @cuda.jit('int32(float64,  float64, int32)', device=True)
   def mandelbrot_numbagpu(creal,cimag,maxiter):
       real = creal
       imag = cimag
       for n in range(maxiter):
           real2 = real*real
           imag2 = imag*imag
           if real2 + imag2 > 4.0:
               return n
           imag = 2* real*imag + cimag
           real = real2 - imag2 + creal
       return 0



   @cuda.jit
   def        mandelbrot_set_numbagpu(xmin,xmax,ymin,ymax,width,height,maxiter,n3,r1,r2):
      for i in range(width):
         for j in range(height):
            n3[i,j] = mandelbrot_numbagpu(r1[i],r2[j],maxiter)


   r1 = np.linspace(-2.0,0.5,1000, dtype=np.float )
   r2 = np.linspace(-1.25,1.25,1000, dtype=np.float)
   n3 = np.zeros((1000,1000),  dtype=np.uint8)

   %timeit mandelbrot_set_numbagpu(-2.0,0.5,-1.25,1.25,1000,1000,80,n3,r1,r2)
   #1 loops, best of 3: 4.84 s per loop

如果我在 JIT 上运行，速度会快 10 倍!......

Answer 1

一般而言，对于 Numba/CUDA(我认为对于 CUDA)来说，您的函数不应该迭代数组。相反，它们应该处理单个数组元素，并且 Numbda/CUDA 处理程序将一大堆数组元素分配给一大堆 GPU 核心，因此一切都会快速并行地发生。这是all documented .

不幸的是，这意味着您不能只是将 @jit 更改为 @cuda.jit，而是必须对其进行调整。

以下作品:

# mandelbrot_numbagpu as before...

# I've removed some of the useless arguments for simplicity
@cuda.jit
def mandelbrot_set_numbagpu(n3,r1,r2,maxiter):
    # numba provides this function for working out which element you're
    # supposed to be accessing
    i,j = cuda.grid(2)
    if i<n3.shape[0] and j<n3.shape[1]: # check we're in range
        # do work on a single element
        n3[i,j] = mandelbrot_numbagpu(r1[i],r2[j],maxiter)

然后您将其称为

# you assign a number of threads, and split it into blocks
# this is all in the documentation!
import math
threadsperblock = (16,16)
blockspergrid_x = math.ceil(n3.shape[0] / threadsperblock[0])
blockspergrid_y = math.ceil(n3.shape[1] / threadsperblock[1])
blockspergrid = (blockspergrid_x, blockspergrid_y)

mandelbrot_set_numbagpu2[blockspergrid,threadsperblock](n3,r1,r2,80)
# n3, r1 and r2 are defined as before

在我的 PC 上，速度提高了 3800 倍。我不知道这与同等的 CPU 程序相比如何。