Parmap.pariter and side effects

Question

Parmap.pariter and side effects

william3 opened this issue 10 years ago · comments

The following program :

let i = ref 0
let _ =
Parmap.pariter ~ncores:2 (fun k ->
incr i;
Printf.printf "loop %i : i:%i\n%!" k !i;
) (Parmap.L [1;2;3;4]);
Printf.printf "end : i:%i\n%!" !i;

returns :
./pb_parmap.native
loop 1 : i:1
loop 2 : i:2
loop 3 : i:1
loop 4 : i:2
end : i:0

which is not so straightforward when I read the presentation...

The paragraph : "By forking the parent process on a sigle machine, the children get access, for free, to all the data structures already built, even the imperative ones, and as far as your computation inside the map/fold does not produce side effects that need to be preserved, the final result will be the same as performing the sequential operation, the only difference is that you might get it faster" miss some warning that updates of imperative values accessed out of the parmap.par* will not be updated for other processes...

Thanks for this interesting library

william3 · Answer 1 · Wed Jun 04 2014 01:32:27 GMT+0800 (China Standard Time)

another question related to this : what is the purpose of Parmap.pariter, if one can not use a function with a side effect ?

Roberto Di Cosmo · Answer 2 · Wed Jun 04 2014 21:45:12 GMT+0800 (China Standard Time)

Parmap does not implement shared memory: the reference i is "copied" to the
process space of each of the subprocesses, and then each copie evolves
independently.

You can think of this as 'each worked get a copy of the data of the parent
process, and any side effect is local to its own copy'; this is a consequence
of the underlying unix fork.

The result you see is perfectly logical once this notion is well understood: on
2 cores, you get two processes, and each of them works on 2 elements, so i is
incremented twice in each subprocess.

Notice that the ideal application for Parmap is when you do not have side effects in the workers, or only "local" side effects, that do not escape to global structures.

On Tue, Jun 03, 2014 at 10:19:29AM -0700, william3 wrote:

The following program :

let i = ref 0
let _ =
Parmap.pariter ~ncores:2 (*~chunksize:1 *) (fun k ->
incr i;
Printf.printf "loop %i : i:%i\n%!" k !i;
) (Parmap.L [1;2;3;4]);
Printf.printf "end : i:%i\n%!" !i;

returns :
./pb_parmap.native
loop 1 : i:1
loop 2 : i:2
loop 3 : i:1
loop 4 : i:2
end : i:0

which is not so straightforward when I read the presentation...
The paragraph : "By forking the parent process on a sigle machine, the children
get access, for free, to all the data structures already built, even the
imperative ones, and as far as your computation inside the map/fold does not
produce side effects that need to be preserved, the final result will be the
same as performing the sequential operation, the only difference is that you
might get it faster" miss some warning that updates of imperative values
accessed out of the parmap.par* will not be updated for other processes...

Thanks for this interesting library

—
Reply to this email directly or view it on GitHub.*

Roberto Di Cosmo

Professeur En delegation a l'INRIA
PPS E-mail: roberto@dicosmo.org
Universite Paris Diderot WWW : http://www.dicosmo.org
Case 7014 Tel : ++33-(0)1-57 27 92 20
5, Rue Thomas Mann
F-75205 Paris Cedex 13 Identica: http://identi.ca/rdicosmo

FRANCE. Twitter: http://twitter.com/rdicosmo

Attachments:
MIME accepted, Word deprecated

http://www.gnu.org/philosophy/no-word-attachments.html

Office location:

Bureau 3020 (3rd floor)
Batiment Sophie Germain
Avenue de France

Metro Bibliotheque Francois Mitterrand, ligne 14/RER C

GPG fingerprint 2931 20CE 3A5A 5390 98EC 8BFC FCCA C3BE 39CB 12D3

Roberto Di Cosmo · Answer 3 · Wed Jun 04 2014 21:46:13 GMT+0800 (China Standard Time)

there are many kinds of side effects... for example, each worker
may launch a complex simulation on data stored in different files,
and write the result in some other different files.

On Tue, Jun 03, 2014 at 10:32:27AM -0700, william3 wrote:

another question related to this : what is the purpose of Parmap.pariter, if
one can not use a function with a side effect ?

—
Reply to this email directly or view it on GitHub.*