[Caml-list] Parallel for with exception

[Caml-list] Parallel for with exception

[Christophe Raffalli]

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Hello,

I am trying ocaml 5.0 and Task.parralel_for in
domainlib is not compatible with exception to interrupt the loop.
When an exception is raised by a task, we need to interrupt or wait for the
other tasks to finish before raising the exception.

What is the recommanded solution for that pattern ?

Cheers,
Christophe

--
Christophe Raffalli
tél: +689 87 23 11 48
web: http://raffalli.eu

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Re: [Caml-list] Parallel for with exception

[Gabriel Scherer]

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If I understand correctly, you are asking about cancellation for Domainslib tasks, specifically for Task.parallel_for.
(I would have created an issue against Domainslib to ask. Interestingly, I can find only one vague mention of cancellation there,
  https://github.com/ocaml-multicore/domainslib/pull/51#discussion_r729296762
)

I haven't tried it, but I would think that the simple approach is to turn

  Task.parallel_for ~start ~finish pool ~body:(fun i ->
    ... raise Exit ...
  )

into

  let stop : exn option Atomic.t = Atomic.make None

  Task.parallel_for ~start ~finish pool ~body:(fun i ->
    Option.iter raise (Atomic.get stop);
    .. (Atomic.set stop Exit; raise Exit) ...
  )

which guarantees prompt termination by checking a shared failure value at each iteration of the loop. But of course this adds a small amount of overhead, which may be undesirable if each loop iteration is supposed to be very fast. (Then the simple approach is to change "Option.iter" into "if i mod 100 = 0 then Option.iter ...".)

This is an instance of the general approach of letting users do cancellation explicitly on their side, if they want to.

On Thu, Oct 13, 2022 at 6:01 AM Christophe Raffalli <christophe@raffalli.eu<mailto:christophe@raffalli.eu>> wrote:

Hello,

I am trying ocaml 5.0 and Task.parralel_for in
domainlib is not compatible with exception to interrupt the loop.
When an exception is raised by a task, we need to interrupt or wait for the
other tasks to finish before raising the exception.

What is the recommanded solution for that pattern ?

Cheers,
Christophe

--
Christophe Raffalli
tél: +689 87 23 11 48
web: http://raffalli.eu

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Re: [Caml-list] Parallel for with exception

[Christophe Raffalli]

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On 22-10-14 22:49:46, Gabriel Scherer wrote:
> If I understand correctly, you are asking about cancellation for Domainslib
> tasks, specifically for Task.parallel_for.

Hello,

Not only, parallel_for does not seem to even transmit the exception.
But yes, it should also cancel the useless computation.

In fact, what I really need are parallel exists and for_all.

> (I would have created an issue against Domainslib to ask. Interestingly, I can
> find only one vague mention of cancellation there,
>   https://github.com/ocaml-multicore/domainslib/pull/51#discussion_r729296762
> )

I added an issue...

Cheers,
Christophe



--
Christophe Raffalli
tél: +689 87 23 11 48
web: http://raffalli.eu

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