Hi Gilles,
thanks for thinking about this in more detail.
I understand what you are saying, but your comments raise some questions in my
mind:
If one is in a homogeneous cluster, is it important that, in the case of
little-endian, that the data be
converted to extern32 format (big-endian), only to be always converted at the
receiving rank
back to little-endian?
This would seem to be inefficient, especially if the site has no need for
external MPI access.
So, does --enable-heterogeneous do more than put MPI routines using "extern32"
into straight pass-through?
Back in the old days of PVM, all messages were converted into network order.
This had severe performance impacts
on little-endian clusters.
So much so that a clever way of getting around this was an implementation of
"receiver makes right" in which
all data was sent in the native format of the sending rank. The receiving rank
analysed the message to determine if
a conversion was necessary. In those days with Cray format data, it could be
more complicated than just byte swapping.
So in essence, how is a balance struck between supporting heterogenous
architectures and maximum performance
with codes where message passing performance is critical?
As a follow up, since I am now at home, this same problem also exists with the
Ubuntu 15.10 OpenMP packages
which surprisingly are still at 1.6.5, same as 14.04.
Again, downloading, building, and using the latest stable version of OpenMP
solved the problem.
kindest regards
Mike
On 11/02/2016, at 7:31 PM, Gilles Gouaillardet wrote:
> Michael,
>
> I think it is worst than that ...
>
> without --enable-heterogeneous, it seems the data is not correctly packed
> (e.g. it is not converted to big endian), at least on a x86_64 arch.
> unpack looks broken too, but pack followed by unpack does work.
> that means if you are reading data correctly written in external32e format,
> it will not be correctly unpacked.
>
> with --enable-heterogeneous, it is only half broken
> (I do not know yet whether pack or unpack is broken ...)
> and pack followed by unpack does not work.
>
> I will double check that tomorrow
>
> Cheers,
>
> Gilles
>
> On Thursday, February 11, 2016, Michael Rezny <michael.re...@monash.edu>
> wrote:
> Hi Ralph,
> you are indeed correct. However, many of our users
> have workstations such as me, with OpenMPI provided by installing a package.
> So we don't know what has been configured.
>
> Then we have failures, since, for instance, Ubuntu 14.04 by default appears
> to have been built
> with heterogeneous support! The other (working) machine is a large HPC, and
> it seems OpenMPI was built
> without heterogeneous support.
>
> Currently we work around the problem for packing and unpacking by having a
> compiler switch
> that will switch between calls to pack/unpack_external and pac/unpack.
>
> It is only now we started to track down what the problem actually is.
>
> kindest regards
> Mike
>
> On 11 February 2016 at 15:54, Ralph Castain <r...@open-mpi.org> wrote:
> Out of curiosity: if both systems are Intel, they why are you enabling
> hetero? You don’t need it in that scenario.
>
> Admittedly, we do need to fix the bug - just trying to understand why you are
> configuring that way.
>
>
>> On Feb 10, 2016, at 8:46 PM, Michael Rezny <michael.re...@monash.edu> wrote:
>>
>> Hi Gilles,
>> I can confirm that with a fresh download and build from source for OpenMPI
>> 1.10.2
>> with --enable-heterogeneous
>> the unpacked ints are the wrong endian.
>>
>> However, without --enable-heterogeneous, the unpacked ints are correct.
>>
>> So, this problem still exists in heterogeneous builds with OpenMPI version
>> 1.10.2.
>>
>> kindest regards
>> Mike
>>
>> On 11 February 2016 at 14:48, Gilles Gouaillardet
>> <gilles.gouaillar...@gmail.com> wrote:
>> Michael,
>>
>> does your two systems have the same endianness ?
>>
>> do you know how openmpi was configure'd on both systems ?
>> (is --enable-heterogeneous enabled or disabled on both systems ?)
>>
>> fwiw, openmpi 1.6.5 is old now and no more maintained.
>> I strongly encourage you to use openmpi 1.10.2
>>
>> Cheers,
>>
>> Gilles
>>
>> On Thursday, February 11, 2016, Michael Rezny <michael.re...@monash.edu>
>> wrote:
>> Hi,
>> I am running Ubuntu 14.04 LTS with OpenMPI 1.6.5 and gcc 4.8.4
>>
>> On a single rank program which just packs and unpacks two ints using
>> MPI_Pack_external and MPI_Unpack_external
>> the unpacked ints are in the wrong endian order.
>>
>> However, on a HPC, (not Ubuntu), using OpenMPI 1.6.5 and gcc 4.8.4 the
>> unpacked ints are correct.
>>
>> Is it possible to get some assistance to track down what is going on?
>>
>> Here is the output from the program:
>>
>> ~/tests/mpi/Pack test1
>> send data 000004d2 0000162e
>> MPI_Pack_external: 0
>> buffer size: 8
>> MPI_unpack_external: 0
>> recv data d2040000 2e160000
>>
>> And here is the source code:
>>
>> #include <stdio.h>
>> #include <mpi.h>
>>
>> int main(int argc, char *argv[]) {
>> int numRanks, myRank, error;
>>
>> int send_data[2] = {1234, 5678};
>> int recv_data[2];
>>
>> MPI_Aint buffer_size = 1000;
>> char buffer[buffer_size];
>>
>> MPI_Init(&argc, &argv);
>> MPI_Comm_size(MPI_COMM_WORLD, &numRanks);
>> MPI_Comm_rank(MPI_COMM_WORLD, &myRank);
>>
>> printf("send data %08x %08x \n", send_data[0], send_data[1]);
>>
>> MPI_Aint position = 0;
>> error = MPI_Pack_external("external32", (void*) send_data, 2, MPI_INT,
>> buffer, buffer_size, &position);
>> printf("MPI_Pack_external: %d\n", error);
>>
>> printf("buffer size: %d\n", (int) position);
>>
>> position = 0;
>> error = MPI_Unpack_external("external32", buffer, buffer_size, &position,
>> recv_data, 2, MPI_INT);
>> printf("MPI_unpack_external: %d\n", error);
>>
>> printf("recv data %08x %08x \n", recv_data[0], recv_data[1]);
>>
>> MPI_Finalize();
>>
>> return 0;
>> }
>>
>>
>>
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>
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