Re: shared variables in parfor()

["Ruud H.G. van Tol" <rvtol@isolution.nl>]

On 2025-01-17 21:31, Max Alekseyev wrote:
> I naively thought that the following code would print a few zeros, 
> which would then follow by all ones:
>
> my(q=0); parfor(i=1,10^6, print(q); 0, R, q=1)
>
> but it constantly prints zeros. So, the value of q is not shared 
> between threads.
> What is the right way to create a shared variable (with pthreads if 
> that matters)?

A shared value is a synchronization point, so that would need locks.
This results in waiting threads, which likely slows things down.

Sometimes a memory-mapped file is used for shared values.

In general, prepare an environment for each thread with start values (Map),
then let the concurrent threads run (Reduce),
then post-process their results (Merge).

Essential is that all Reducing writers are independent ("lock-free").
Such a Map-Reduce-Merge "unit" can be nested, with proper care.

For what case did you want to use a shared variable?
Then we could try and untangle it.

-- Ruud