Re: [Boost-users] mpi isend to group

Subject: Re: [Boost-users] mpi isend to group
From: Philipp Kraus (philipp.kraus_at_[hidden])
Date: 2012-12-29 18:02:29

Am 29.12.2012 um 23:32 schrieb Andreas Schäfer:

> Hi,
> 
> since you mention both, threads and MPI, I might add that the threading
> support of most MPI implementations contains some caveats, to say the
> least. It is certainly possible to emulate asynchronous collectives
> either by isend/irecv (although that's really tough job given the
> level of optimizations at hand in any major MPI implementation) or to
> just use (possibly empty) synchronous broadcasts (as Riccardo
> suggested). But to get a clearer picture I'd like to ask for some
> details:
> 
> - Which MPI are you using?

I use at the moment OpenMPI (but it should be worked also under MS Windows)

> - How many MPI processes do your jobs contain?

The system hast got 64 cores (on each core 2 threads can be created).

> - Which threading level do you request via MPI_Thread_init()?

At my testing I use MPI_THREAD_SERIALIZED

> - How do you ensure asynchronous progress? (i.e.: mostly MPI will only
>  send/receive data when some MPI function is being called. Unless
>  e.g. MPI_Test is being polled or you MPI supports progress threads,
>  the bulk of communication won't be carried out until you call MPI_Wait())
> 

Each MPI process runs in the thread loop some database calls, so after each
database block I will check if there is a message from the MPI core 0 and if it
exists, all cores should be "barried". The core 0 checks after the database calls
is there is any data, if yes, it sends this data to the other cores. So I can not use
a MPI_Wait call, because this creates a blocking communication. 

Thanks

Phil

> 
> 
> On 15:15 Thu 27 Dec     , Philipp Kraus wrote:
>> Hello,
>> 
>> I have got a problem to create a mpi::isend call. I have got a thread loop like:
>> 
>> while (thread_is_running)
>> {
>>      std::size_t id = 0;
>>      if (!mpicom.rank())
>>      {
>>            try {
>>                id = getID();
>>                mpicom.isend(id)
>>            } catch (...) { }
>>     } else {
>>        mpicom.ireceive(id);
>>     }
>> 
>>    if (id > 0)
>>    {
>>           mpi::barrier();
>>           do something with id
>>    }
>> 
>>   boost::thread::yield();
>> }
>> 
>> I would like to create a non-blocking communication over all nodes, so my node with rank 0
>> checks if a dataset exists, if it exists, the id should be send to all other hosts. After all hosts
>> reveived this id, they should start the calculation. The nodes must be synchronized before the
>> "working part" is startet, but if there is no data send to the host, they should do nothing.
>> 
>> I don't know how to use isend to send a message to all hosts. I know only the mpi::any_source flag
>> but I can not find an any_destination. IMHO the ireceive call must be written with iprobe like
>> 
>> if (boost::optional<mpi::status> l_status = mpicom.iprobe( 0, myTag ))
>> {
>> 	std::size_t id = 0;
>>        mpicom.recv( l_status->source(), l_status->tag(), id )
>> }
>> 
>> Is there a way to create a non-blocking group communication? How can I send if data exists, a message
>> to all nodes and receive it on each node also with non-blocking communication?
>> 
>> Thanks for your help
>> 
>> Phil
>> }
>> 
>> _______________________________________________
>> Boost-users mailing list
>> Boost-users_at_[hidden]
>> http://lists.boost.org/mailman/listinfo.cgi/boost-users
> 
> -- 
> ==========================================================
> Andreas Schäfer
> HPC and Grid Computing
> Chair of Computer Science 3
> Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
> [phone]
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