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Subject: Re: [OMPI users] About MPI_TAG_UB
From: Iliev, Hristo (iliev_at_[hidden])
Date: 2012-09-28 03:50:23


Hello,

MPI_TAG_UB in Open MPI is INT_MAX == 2^31-1 == 2147483647. The value of
17438272 (0x10A1640) is a bit strange for MPI_TAG_UB. I would rather say
that it is the value of a pointer to someplace in the heap, i.e. you have
missed the fact that the attribute value as returned by MPI_Comm_get_attr /
MPI_Attr_get is a pointer to the actual value (for MPI_TAG_UB it is a
pointer to int).

MPI_TAG_UB is predefined attribute and according to §8.1.2 of the MPI
standard its value cannot be changed by the user.

You have to find another solution, e.g. reduce the tag space to 7 bits or
put the routing info inside the message payload.

Best regards,
Hristo Iliev

--
Hristo Iliev, Ph.D. -- High Performance Computing
RWTH Aachen University, Center for Computing and Communication
Rechen- und Kommunikationszentrum der RWTH Aachen
Seffenter Weg 23,  D 52074  Aachen (Germany)
> -----Original Message-----
> From: users-bounces_at_[hidden] [mailto:users-bounces_at_[hidden]]
> On Behalf Of Sébastien Boisvert
> Sent: Friday, September 28, 2012 1:22 AM
> To: users_at_[hidden]
> Subject: [OMPI users] About MPI_TAG_UB
> 
> Hello,
> 
> I am running Ray (a distributed software in genomics) with Open-MPI on
> 2048 processes and everything runs fine. Ray has a any-to-any
> communication pattern.
> To avoid using too much memory, I implemented a virtual message router.
> 
> Without the virtual message router, I get messages like these:
> 
> [cp2558][[30209,1],0][connect/btl_openib_connect_oob.c:490:qp_create_o
> ne] error creating qp errno says Cannot allocate memory
> 
> We did some tests on the Cray XE6 on 4096 processing elements (4096 MPI
> ranks) without the virtual message router and everything runs fine as is.
So
> using the virtual message router is not required.
> 
> The real message tag, the real source and the real destination are stored
in
> the MPI tag. I know, this is ugly, but it works. I can not store this
information
> in the message buffer because the buffer can be NULL.
> 
> bits 0 to 7: tag (8 bits, values from 0 to 255, 256 possible values) bits
8 to 19:
> true source (12 bits, values from 0 to 4095, 4096 possible values) bits 20
to 31:
> true destination (12 bits, values from 0 to 4095, 4096 possible values)
> 
> Without the virtual router, my code is compliant with the fact that
> MPI_Comm_get_attr(MPI_COMM_WORLD, MPI_TAG_UB,...) is at least
> 32767 (my tags are <= 255).
> 
> 
> When I try jobs with 4096 processes with the virtual message router, I get
the
> error:
> 
>     MPI_ERR_TAG: invalid tag.
> 
> Without the virtual message router I get:
> 
> [cp2558][[30209,1],0][connect/btl_openib_connect_oob.c:490:qp_create_o
> ne] error creating qp errno says Cannot allocate memory
> 
> With Open-MPI 1.5.4, the upper bound is 17438272 (at least in our build).
> That explains MPI_ERR_TAG.
> 
> 
> My 2 questions:
> 
> 1. Is there a better way to store routing information ?
> 
> 2. Can I create my own communicator and set its MPI_TAG_UB to whatever I
> want ?
> 
> 
> Thanks !
> 
> ***
> Sébastien Boisvert
> Ph.D. student
> http://boisvert.info/
> _______________________________________________
> users mailing list
> users_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/users


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