[mpich-discuss] How to use non-blocking send/receive without calling MPI_Wait

[Jeff Hammond]

As far as I know, Ethernet is not good at making asynchronous progress in
hardware the way e.g. InfiniBand is.  I would have thought that a dedicated
progress thread would help, but it seems you tried that.  Did you use your
own progress thread or MPICH_ASYNC_PROGRESS=1?

Jeff

On Fri, Apr 3, 2015 at 10:10 AM, Lei Shi <lshi at ku.edu> wrote:

> Huiwei,
>
> Thanks for your email. Your answer leads to my another question about
> asynchronous MPI communication.
>
> I'm trying to do an overlapped communication/computing to speedup my MPI
> code. I read some papers comparing some different approaches to do the
> overlapped communication. The "naive" overlapped communication
> implementation, which only use non-blocking mpi Isend/Irecv and the hybrid
> approach using OpenMP and MPI together. In the hybrid approach, a separated
> thread is use to do all non-blocking communications. Just exactly as you
> said, the results indicate that current MPI implementations do not
> support true asynchronous communication.
>
> If I use the naive approach, my code with non-blocking or blocking
> send/recv gives me almost the same performance in term of Wtime. All
> communications are postponed to MPI_Wait.
>
> I have tried calling mpi_test to push library to do communication during
> iterations. And try to use a dedicated thread to do communication and the
> other thread to do computing only. However, the performance gains are very
> small or no gain at all. I'm wondering it is due to the hardware. The
> cluster I tested uses 10G Ethernet card.
>
>
> Best,
>
> Lei Shi
>
>
>
>
>
> On Fri, Apr 3, 2015 at 8:49 AM, Huiwei Lu <huiweilu at mcs.anl.gov> wrote:
>
>> Hi Lei,
>>
>> As far as I know, all current MPI implementations do not support true
>> asynchronous communication for now. i.e., If there is no MPI calls in your
>> iterations, MPICH will not be able to make progress on communication.
>>
>> One solution is to poll the MPI runtime regularly to make progress by
>> inserting MPI_Test to your iteration (even though you do not want to check
>> the data).
>>
>> Another solution is to enable MPI's asynchronous progress thread to make
>> progress for you.
>>
>> --
>> Huiwei
>>
>> On Thu, Apr 2, 2015 at 11:44 PM, Lei Shi <lshi at ku.edu> wrote:
>>
>>> Hi Junchao,
>>>
>>> Thanks for your reply. For my case, I don't want to check the data have
>>> been received or not. So I don't want to call MPI_Test or any function to
>>> verify it. But my problem is like if I ignore calling the MPI_Wait, just
>>> call Isend/Irev, my program freezes for several sec and then continues to
>>> run. My guess is probably I messed up the MPI library internal buffer by
>>> doing this.
>>>
>>> On Thu, Apr 2, 2015 at 7:25 PM, Junchao Zhang <jczhang at mcs.anl.gov>
>>> wrote:
>>>
>>>> Does MPI_Test fit your needs?
>>>>
>>>> --Junchao Zhang
>>>>
>>>> On Thu, Apr 2, 2015 at 7:16 PM, Lei Shi <lshi at ku.edu> wrote:
>>>>
>>>>> I want to use non-blocking send/rev MPI_Isend/MPI_Irev to do
>>>>> communication. But in my case, I don't really care what kind of data I get
>>>>> or it is ready to use or not. So I don't want to waste my time to do any
>>>>> synchronization  by calling MPI_Wait or etc API.
>>>>>
>>>>> But when I avoid calling MPI_Wait, my program is freezed several secs
>>>>> after running some iterations (after multiple MPI_Isend/Irev callings),
>>>>> then continues. It takes even more time than the case with MPI_Wait.  So my
>>>>> question is how to do a "true" non-blocking communication without waiting
>>>>> for the data ready or not. Thanks.
>>>>>
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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