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Subject: Re: [OMPI devel] algorithm selection in open mpi
From: Rayson Ho (raysonlogin_at_[hidden])
Date: 2012-04-03 01:32:22


Performance depends on the network topology & node hardware, and the
benchmark - so we don't have enough information to determine the root
of the issue...

However, you can do some debugging on your end (once you master the
techniques you will be able to debug all sorts of performance problems
- not just those in Open MPI): Compile coll_tuned_decision_fixed.c
with debug on (-g), and remote log onto a node where one of the tasks
runs, and attach a debugger to see if the execution path changes. If
it does, then you next step will be to determine if the bottleneck of
the benchmark really is affected by the decisions made in
coll_tuned_decision_fixed.c .

Note that to attach a debugger (just gdb will do for this case), you
will need to put a sleep after main (ideally before MPI_Init() so that
you can attach the debugger. You can even use all sorts of hacks like
using shell script wrappers to echo the PID of the task and then sleep
in the script before starting the real MPI task so that you don't need
to recompile the benchmark...

Rayson

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On Mon, Apr 2, 2012 at 11:10 PM, roswan ismail <iewanis_at_[hidden]> wrote:
> Hi all..
>
> I am Roswan Ismail from Malaysia. I am focusing on MPI communication
> performance on quad-core cluster at my university. I used Open MPI-1.4.3 and
> measurements were done using scampi benchmark.
>
> As I know, open MPI used multiple algorithms to broadcast data (MPI_BCAST)
> such as binomial, pipeline, binary tree, basic linear and split binary tree.
> All these algorithms will be used based on message size and communicator
> size. For example, binomial is used when message size to be broadcasted is
> small while pipeline used for broadcasting a large message.
>
> What I want to do now is, to use fixed algorithm i.e binomial for all
> message size. I want to see and compare the results with the default
> results. So, I was modified coll_tuned_decision_fixed.c which is located in
> open mpi-1.4.3/ompi/mca/coll/tuned by returning binomial algorithm for all
> condition. Then I recompile the files but the problem is, the results
> obtained is same as default. It seems I do not do any changes to the codes.
>
> So could you guys tell me the right way to do that.
>
> Many thanks
>
> Roswan Binti Ismail,
> FTMK,
> Univ. Pend. Sultan Idris,
> Tg Malim, Perak.
> Pej: 05-4505173
> H/P: 0123588047
> iewanis_at_[hidden]
> roswan_at_[hidden]
>
> _______________________________________________
> devel mailing list
> devel_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/devel

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