[mpich-discuss] Failed to allocate memory for an unexpected message

Wed Oct 23 16:22:23 CDT 2013

Hi Antonio,

Thanks for your quick reply.

I confess I have to study a bit more about this, but I think I understood
your suggestion.
After a little research, I understood that when I use MPI_IRecv I am
performing a non-blocking operation to just inform that I have a message to
receive and then after MPI_Waitany returns I can access data to write it on
the file. In this kind of implementation, I inform that I have a bunch of
messages to receive from worker processes and, because of that, I would
have to have a buffer in the writer process to receive messages
asynchronously. The higher efficiency is archived because, while the writer
process is performing the IO operation (writing to hard disk), MPI can
transfer the received data in the meanwhile to my pre-allocated buffer in
my application. Is all this right?

But, what if I don't know the number of messages I will receive?
That's my case, actually... I don't even know which worker process will
send me data and I also don't know how many times each one of them will
send me messages. I use a master-slaves scheme to distribute tasks, so the
number of calculations done in each worker process (and thus, data send to
writer process) depends on the speed each worker and how many times they
ask for tasks. It is easy to deal with messages coming from a unknown
sender, but I don't know how to deal with an unknown number of messages
(i.e., how many MPI_IRecv I have to call?). Any idea?

As I said before, I considered having more than one writer process, but I
can't see how this would solve the problem, that seems to be related to IO
disk speed. In other words, why it would be worth having more than one
writer process if, at the end, I have only one hard disk to perform IO
operations?

Thanks again.

Regards, Luiz

On 23 October 2013 17:42, Antonio J. Peña <apenya at mcs.anl.gov> wrote:

> **
>
>
>
> Hi Luiz,
>
>
>
> Your error trace indicates that the receiver went out of memory due to a
> too large amount (261,895) of eager unexpected messages received, i.e.,
> small messages received without a matching receive operation. Whenever this
> happens, the receiver allocates a temporary buffer to hold the received
> message. This exhausted the available memory in the computer where the
> receiver was executing.
>
>
>
> To avoid this, try to pre-post receives before messages arrive. Indeed,
> this is far more efficient. Maybe you could do an MPI_IRecv per worker in
> your writer process, and process them after an MPI_Waitany. You may also
> consider having multiple writer processes if your use case permits and the
> volume of received messages is too high to be processed by a single writer.
>
>
>
> Antonio
>
>
>
>
>
> On Wednesday, October 23, 2013 05:27:27 PM Luiz Carlos da Costa Junior
> wrote:
>
> Hi,
>
>
> I am getting the following error when running my parallel application:
>
>
> MPI_Recv(186)......................: MPI_Recv(buf=0x125bd840, count=2060,
> MPI_CHARACTER, src=24, tag=94, comm=0x84000002, status=0x125fcff0) failed
>
> MPIDI_CH3I_Progress(402)...........:
>
> MPID_nem_mpich2_blocking_recv(905).:
>
> MPID_nem_tcp_connpoll(1838)........:
>
> state_commrdy_handler(1676)........:
>
> MPID_nem_tcp_recv_handler(1564)....:
>
> MPID_nem_handle_pkt(636)...........:
>
> MPIDI_CH3_PktHandler_EagerSend(606): Failed to allocate memory for an
> unexpected message. 261895 unexpected messages queued.
>
> Fatal error in MPI_Send: Other MPI error, error stack:
>
> MPI_Send(173)..............: MPI_Send(buf=0x765d2e60, count=2060,
> MPI_CHARACTER, dest=0, tag=94, comm=0x84000004) failed
>
> MPID_nem_tcp_connpoll(1826): Communication error with rank 1: Connection
> reset by peer
>
>
> I went to MPICH's FAQ (
> http://wiki.mpich.org/mpich/index.php/Frequently_Asked_Questions#Q:_Why_am_I_getting_so_many_unexpected_messages.3F).
>
>
> It says that most likely the receiver process can't cope to process the
> high number of messages it is receiving.
>
>
> In my application, the worker processes perform a very large number of
> small computations and, after some computation is complete, they sent the
> data to a special "writer" process that is responsible to write the output
> to disk.
>
> This scheme use to work in a very reasonable fashion, until we faced some
> new data with larger parameters that caused the problem above.
>
>
> Even though we can redesign the application, for example, by creating a
> pool of writer process we still have only one hard disk, so the bottleneck
> would not be solved. So, this doesn't seem to be a good approach.
>
>
> As far as I understood, MPICH saves the content of every MPI_SEND in a
> internal buffer (I don't know where the buffer in located, sender or
> receiver?) to allow asynchronous sender's computation while the messages
> are being received.
>
> The problem is that buffer has been exhausted due some resource limitation.
>
>
> It is very interesting to have a buffer but if the buffer in the writer
> process is close to its limit the workers processes should stop and wait
> until it frees some space to restart sending new data to be written to
> disk.
>
>
> Is it possible to check this buffer in MPICH? Or is it possible to check
> the number of messages to be received?
>
> Can anyone suggest a better (easy to implement) solution?
>
>
> Thanks in advance.
>
>
> Regards,
>
> Luiz
>
>
>
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