How would I use Dask to perform parallel operations on slices of NumPy arrays?

Question

I have a numpy array of coordinates of size n_slice x 2048 x 3, where n_slice is in the tens of thousands. I want to apply the following operation on each 2048 x 3 slice separately

import numpy as np
from scipy.spatial.distance import pdist

# load coor from a binary xyz file, dcd format

n_slice, n_coor, _ = coor.shape
r = np.arange(n_coor)
dist = np.zeros([n_slice, n_coor, n_coor])

# this loop is what I want to parallelize, each slice is completely independent
for i in xrange(n_slice): 
    dist[i, r[:, None] < r] = pdist(coor[i])

I tried using Dask by making coor a dask.array ,

import dask.array as da
dcoor = da.from_array(coor, chunks=(1, 2048, 3))

but simply replacing coor by dcoor will not expose the parallelism. I could see setting up parallel threads to run for each slice but how do I leverage Dask to handle the parallelism?

Here is the parallel implementation using concurrent.futures

import concurrent.futures
import multiprocessing

n_cpu = multiprocessing.cpu_count()

def get_dist(coor, dist, r):
    dist[r[:, None] < r] = pdist(coor)

# load coor from a binary xyz file, dcd format

n_slice, n_coor, _ = coor.shape
r = np.arange(n_coor)
dist = np.zeros([n_slice, n_coor, n_coor])

with concurrent.futures.ThreadPoolExecutor(max_workers=n_cpu) as executor:
    for i in xrange(n_slice):
        executor.submit(get_dist, cool[i], dist[i], r)

It is possible this problem is not well suited to Dask since there are no inter-chunk computations.

Answer 1

map_blocks

The map_blocks method may be helpful:

dcoor.map_blocks(pdist)

Uneven arrays

It looks like you're doing a bit of fancy slicing to insert particular values into particular locations of an output array. This will probably be awkward to do with dask.arrays. Instead, I recommend making a function that produces a numpy array

def myfunc(chunk):
    values = pdist(chunk[0, :, :])
    output = np.zeroes((2048, 2048))
    r = np.arange(2048)
    output[r[:, None] < r] = values
    return output

dcoor.map_blocks(myfunc)

delayed

Worst case scenario you can always use dask.delayed

from dask import delayed, compute
coor2 = delayed(coor)
slices = [coor2[i] for i in range(coor.shape[0])]
slices2 = [delayed(pdist)(slice) for slice in slices]
results = compute(*slices2)