Question

我试图在Python中使用MPI（mpi4py）进行一些图像处理。到目前为止，这是我的代码：

from mpi4py import MPI
from PIL import Image

comm = MPI.COMM_WORLD
size = comm.size #number of processors
rank = comm.rank #calling process rank
root = 0
if rank == 0:
    print "rank = 0 \n"
    img = Image.open('dog.jpg')
    width, height = img.size
    pix = list(img.getdata())
    for i in range(len(pix)):
        pix[i] = list(pix[i])
    chunks = [[] for _ in range(size)]
    for i, chunk in enumerate(pix):
        chunks[i % size].append(chunk)
else:
    print "rank != 0"

data = comm.scatter(chunks, root)
#do some pixel manipulations
data = comm.gather(data, root)
for i in range(len(pix)):
    pix[i] = tuple(data[0][i])
image = Image.new("RGB", (width, height))
image.putdata(pix)
image.save("test.jpg")

当我尝试使用

运行它时

mpirun -np 1 python parallel.py

它运作正常。但如果我尝试增加处理器数量，我会收到以下错误

mpirun -np 3 python parallel.py
rank = 1
rank = 2
rank = 0 

Traceback (most recent call last):
  File "parallel.py", line 21, in <module>
    data = comm.scatter(chunks, root)
NameError: name 'chunks' is not defined
Traceback (most recent call last):
  File "parallel.py", line 21, in <module>
    data = comm.scatter(chunks, root)
NameError: name 'chunks' is not defined

我正在使用Virtual Box在Ubuntu虚拟机上运行代码。感谢您的帮助！

Answer 1

这不是MPI问题。 chunks在条件的else:分支的过程中未定义，即在除了等级0之外的所有过程中。只需在那里添加chunks = None：

...
if rank == 0:
    ...
else:
    print "rank != 0"
    chunks = None
...

它适用于一个进程，因为在这种情况下只有一个等级（0），而且从不采用else分支。

使用MPI在Python中进行图像处理

1 个答案: