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

[Lei Shi]

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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