[mpich-discuss] What is the difference between MPI_THREAD_SINGLE and MPI_THREAD_FUNNELED

[rreddypsc at gmail.com]

Sorry about resurrecting a topic that is more than a month old.

I was also under the impression that as long as all MPI calls are done 
outside of the threaded regions, using MPI_THREAD_SINGLE was sufficient. 
Based on the discussion below it looks like even in those situations one 
has to use MPI_THREAD_FUNNELED.

For my understanding is it possible to briefly explain why this is the 
case?  Is this still required, for example, if there is no MPI activity 
happening at all (for example no asynchronous communication is happening 
when any part of the threaded code is active)?

In particular, I'm thinking about:

	MPI comm start
	MPI comm end
	
	threaded work (OpenMP for example with no MPI of any kind)

	MPI comm start
	MPI comm end
	...etc

The reason for this question is that I have a feeling that a lot of other 
users may be of the same opinion and would benefit from clarification.

TIA for clarification!

Raghu


--On Monday, June 2, 2014 12:58 AM +0000 "Balaji, Pavan" <balaji at anl.gov> 
wrote:

>
> On May 30, 2014, at 12:30 PM, Boisvert, Sebastien <boisvert at anl.gov>
> wrote:
>> On Friday, May 30, 2014 12:26 PM, discuss-bounces at mpich.org
>> [discuss-bounces at mpich.org] on behalf of Rajeev Thakur
>> [thakur at mcs.anl.gov] wrote:
>>> To: discuss at mpich.org
>>> Subject: Re: [mpich-discuss] What is the        difference      between
>>> MPI_THREAD_SINGLE and MPI_THREAD_FUNNELED
>>>
>>> It shouldn't, but what does it return in your program?
>>
>> I am currently using MPI_Init (I thought it was OK if only the main
>> thread calls MPI functions).
>>
>> Code->
>> https://github.com/GeneAssembly/biosal/blob/master/engine/node.c#L33
>
> If you initialize with MPI_INIT, the MPI implementation can choose to
> provide whatever thread-level it likes.  There are two options for you:
>
> Option 1:
>
> MPI_INIT  /* tell MPI to give you what it can */
> MPI_QUERY_THREAD /* ask for what you got */
> if (less than FUNNELED) abort;
>
> Option 2:
>
> MPI_INIT_THREAD(FUNNELED, &provided); /* tell MPI to give you FUNNELED */
> if (provided < FUNNELED) abort;
>
> Option 2 is cleaner since you gave MPI a hint that you would like to have
> FUNNELED level.
>
>   — Pavan
>
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