[GRASS-dev] multiprocessing problem

[Stefan Blumentrath]

Ciao Luca,

Yes, you could also consider looping over e.g. rows (maybe in combination with "np.apply_along_axis") so you could put results easier back together to a map if needed at a later stage.

In addition, since you use multiprocessing.Manager, you may try to use multiprocessing.Array: https://docs.python.org/3/library/multiprocessing.html#multiprocessing.Array

E.g. here:
https://github.com/lucadelu/grass-addons/blob/5ca56bdb8b3394ebeed23aa5b3240bf6690e51bf/src/raster/r.raoq.area/r.raoq.area.py#L81

According to the post here: https://medium.com/analytics-vidhya/using-numpy-efficiently-between-processes-1bee17dcb01
multiprocessing.Array is needed to put the numpy array into shared memory and avoid pickling.

I have not tried or investigated myself, but maybe worth a try...

Cheers
Stefan

-----Original Message-----
From: grass-dev <grass-dev-bounces at lists.osgeo.org> On Behalf Of Luca Delucchi
Sent: fredag 8. april 2022 10:46
To: Moritz Lennert <mlennert at club.worldonline.be>
Cc: GRASS-dev <grass-dev at lists.osgeo.org>
Subject: Re: [GRASS-dev] multiprocessing problem

On Fri, 8 Apr 2022 at 09:14, Moritz Lennert <mlennert at club.worldonline.be> wrote:
>
> Hi Luca,
>

Hi Moritz,

> Just two brainstorming ideas:
>
> - From a rapid glance at the code it seems to me that you create a separate worker for each row in the raster. Correct ? AFAIR, spawning workers does create quite a bit of overhead. Depending on the row to column ratio of your raster, maybe you would be better off sending larger chunks to workers ?
>

right now I creating a worker for each pixel to be checked against all the other pixels, yes it could be and idea to send larger chunks, I could split the array vertically according to the number of processor

> - Depending on the number of parallel jobs, disk access can quickly become the bottleneck on non parallelized file systems. So it would be interesting to see if using fewer processes might actually speed up things. Then it is a question of finding the equilibrium.
>

ok, this make sense
thanks for your support

> Moritz
>

--
ciao
Luca

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