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Subject: Re: [OMPI users] meaning of MPI_THREAD_*
From: Hicham Mouline (hicham_at_[hidden])
Date: 2010-12-06 09:26:42

-----Original Message-----
From: "Tim Prince" [n8tm_at_[hidden]]
Date: 06/12/2010 01:40 PM
To: users_at_[hidden]
Subject: Re: [OMPI users] meaning of MPI_THREAD_*

>On 12/6/2010 3:16 AM, Hicham Mouline wrote:
>> Hello,
>> 1. MPI_THREAD_SINGLE: Only one thread will execute.
>> Does this really mean the process cannot have any other threads at all, even if they doen't deal with MPI at all?
>> I'm curious as to how this case affects the openmpi implementation?
>> Essentially, what is the difference between MPI_THREAD_SINGLE and MPI_THREAD_FUNNELED?
>> 2. In my case, I'm interested in MPI_THREAD_SERIALIZED. However if it's available, I can use MPI_THREAD_FUNNELED.
>> What cmake flags do I need to enable to allow this mode?
>> 3. Assume I assign only 1 thread in my program to deal with MPI. What is the difference between
>> int MPI::Init_thread(MPI_THREAD_SINGLE)
>> int MPI::Init_thread(MPI_THREAD_FUNNELED)
>> int MPI::Init()
>You're question is too broad; perhaps you didn't intend it that way.
>Are you trying to do something which may work only with a specific
>version of openmpi, or are you willing to adhere to portable practice?
>I. tend to believe what it says at

>A call to MPI_INIT has the same effect as a call to MPI_INIT_THREAD with
>a required = MPI_THREAD_SINGLE

>You would likely use one of those if all your MPI calls are from a
>single thread, and you don't perform any threading inside MPI. MPI
>implementations vary on the extent to which a higher level of threading
>than what is declared can be used successfully (there's no guarantee of
>bad results if you exceed what was set by MPI_INIT). There shouldn't
>be any bad effect from setting a higher level of thread support which
>you never use.

>I would think your question about cmake flags would apply only once you
>chose a compiler. I have never seen anyone try mixing
>auto-parallelization with MPI; that would require MPI_THREAD_MULTIPLE
>but still appears unpredictable. MPI_THREAD_FUNNELED is used often with
>OpenMP parallelization inside MPI.

>Tim Prince

Thanks, it is now clarified that a call to MPI_INIT has the same effect as a call to MPI_INIT_THREAD with
a required = MPI_THREAD_SINGLE. Perhaps it should be added here:
as well.
I try to remain portable but am using only openmpi at the moment.

It is still unclear to me the difference between MPI_THREAD_SINGLE and MPI_THREAD_FUNNELED.
Whether a program is or no multi threaded, if using MPI only from 1 thread (the one that calls MPI_INIT) would have no bearing on the mpi implementation, or?