Saliya,
On Tue, Jun 30, 2015 at 10:50 AM, Saliya Ekanayake <esal...@gmail.com>
wrote:
> Hi,
>
> I am experiencing some bottleneck with allgatherv routine in one of our
> programs and wonder how it works internally. Could you please share some
> details on this?
>
Open MPI has a tunable approach to all the collective algorithms. In case
you have the tuned collective enabled (--mca coll tuned,inter,self,basic as
an example) you do have access to the pipelined ring version you made
reference to. However, in addition to that particular version you also have
access to other, sometimes faster, algorithms such as Bruck.
Do a quick "ompi_info --param coll tuned -l 9" to see all the tuned
collective options. You can alter the selection of a particular AllgatherV
algorithm in Open MPI by adding the 2 following lines in yout
${HOME}/.openmpi/mca-params.conf file.
coll_tuned_use_dynamic_rules = 1
coll_tuned_allgatherv_algorithm = 3
With the above 2 lines I force the Bruck algorithm (which has ID 3 in the
output of ompi_info) for all allgathev collectives.
You can benchmark the MPI_Allgatherv for your particular case and then
force the selection of the right algorithm.
George.
>
> I found this [1] paper from Gropp discussing an efficient implementation.
> Is this similar to what we get in OpenMPI?
>
>
>
> [1]
> http://www.researchgate.net/profile/William_Gropp/publication/221597354_A_Simple_Pipelined_Algorithm_for_Large_Irregular_All-gather_Problems/links/00b49525d291830c67000000.pdf
>
>
>
> Thank you,
> Saliya
> --
> Saliya Ekanayake
> Ph.D. Candidate | Research Assistant
> School of Informatics and Computing | Digital Science Center
> Indiana University, Bloomington
> Cell 812-391-4914
> http://saliya.org
>
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>
