cython并行中数组的减少

Question

I have an array that needs to contain sum of different things and therefore I want to perform reduction on each of its elements. Here's the code:

cdef int *a=<int *>malloc(sizeof(int) * 3)
for i in range(3):
    a[i]=1*i
cdef int *b
for i in prange(1000,nogil=True,num_threads=10):
    b=res() #res returns an array initialized to 1s
    with gil: #if commented this line gives erroneous results 
        for k in range(3):
            a[k]+=b[k]
for i in range(3):
    print a[i]

Till there is with gil the code runs fine else gives wrong results. How to deal with reductions on each element of array without using gil cause gil i think will block other threads

Answer 1

The way reductions usually work in practice is to do the sum individually for each thread, and then add them together at the end. You could do this manually with something like

cdef int *b
cdef int *a_local # version of a that is duplicated by each thread
cdef int i,j,k

# set up as before
cdef int *a=<int *>malloc(sizeof(int) * 3)
for i in range(3):
    a[i]=1*i

# multithreaded from here
with nogil, parallel(num_threads=10):
    # setup and initialise a_local on each thread
    a_local = <int*>malloc(sizeof(int)*3)
    for k in range(3):
        a_local[k] = 0

    for i in prange(1000):
        b=res() # Note - you never free b
                # this is likely a memory leak....

        for j in range(3):
            a_local[j]+=b[j]

    # finally at the end add them all together.
    # this needs to be done `with gil:` to avoid race conditions 
    # but it isn't a problem
    # because it's only a small amount of work being done
    with gil:
        for k in range(3):
            a[k] += a_local[k]
    free(a_local)