[Pympi-users] About Gather/Scatter

[Luigi P. <lu...@la...>]

This is a general question about how Gather and Scatter functions work.

I refer to Miller 2002 paper which says that a "simple way to achieve=20
parallelism is with gather/scatter parallelism. A scatter operation will=20
take a container, split it into equal (or nearly equal) parts that are=20
messaged to various slave tasks. A gather reverses that and collects=20
sub-containers together into one larger Python list."

This is the example code:


import mpi
import crypt

if mpi.rank =3D=3D 0:
   words =3D open(=92/usr/dict/words=92).read().split()
else:
   words =3D []

local_words =3D mpi.scatter(words)

target =3D =92xxaGcwiAKoYgc=92
for word in local_words:
   if crypt.crypt(word,target[:2]) =3D=3D target:
     print =92the word is=92,word
     break

I would like to understand whether

a) is the list split into mpi.size groups and then each group sent to a=20
parallel task

b) or is each list entry successively sent to the first parallel task fre=
e?

Example (np=3D2)

local_words =3D ['Luigi', 'Dough', 'Patrick', 'Julian', 'James']

a)

local_words_0 =3D ['Luigi', 'Dough', 'Patrick'] --> to rank 0
local_words_1 =3D ['Julian', 'James'] --> to rank 1


b)

for word in local_words:
   word --> single list element sent to the first "rank" free


Hope I'm clear.

Thanks.

Luigi