Michael,
MPI_Pack_external must convert data to big endian, so it can be dumped into
a file, and be read correctly on big and little endianness arch, and with
any MPI flavor.
if you use only one MPI library on one arch, or if data is never
read/written from/to a file, then it is more efficient to MPI_Pack.
openmpi is optimized and the data is swapped only when needed.
so if your cluster is little endian only, MPI_Send and MPI_Recv will never
byte swap data internally.
if both ends have different endianness, data is sent in big endian format
and byte swapped when received only if needed.
generally speaking, a send/recv requires zero or one byte swap.
fwiw, we previously had a claim that debian nor Ubuntu have a maintainer
for openmpi, which would explain why an obsolete version is shipped. I made
a few researchs and could not find any evidence openmpi is no more
maintained.
Cheers,
Gilles
On Thursday, February 11, 2016, Michael Rezny <michael.re...@monash.edu>
wrote:
> 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
> <javascript:_e(%7B%7D,'cvml','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;
>>>>> }
>>>>>
>>>>>
>>>>>
>>>> _______________________________________________
>>>> devel mailing list
>>>> de...@open-mpi.org
>>>> Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/devel
>>>> Link to this post:
>>>> http://www.open-mpi.org/community/lists/devel/2016/02/18573.php
>>>>
>>>
>>> _______________________________________________
>>> devel mailing list
>>> de...@open-mpi.org
>>> Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/devel
>>> Link to this post:
>>> http://www.open-mpi.org/community/lists/devel/2016/02/18575.php
>>>
>>>
>>>
>>> _______________________________________________
>>> devel mailing list
>>> de...@open-mpi.org
>>> Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/devel
>>> Link to this post:
>>> http://www.open-mpi.org/community/lists/devel/2016/02/18576.php
>>>
>>
>> _______________________________________________
> devel mailing list
> de...@open-mpi.org <javascript:_e(%7B%7D,'cvml','de...@open-mpi.org');>
> Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/devel
> Link to this post:
> http://www.open-mpi.org/community/lists/devel/2016/02/18579.php
>
>
>
