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From: Andrew Friedley (afriedle_at_[hidden])
Date: 2007-09-11 12:37:30

First off, I've managed to reproduce this with nbcbench using only 16
procs (two per node), and setting btl_ofud_sd_num to 12 -- eases
debugging with fewer procs to look at.

ompi_coll_tuned_alltoall_intra_basic_linear is the alltoall routine that
is being called. What I'm seeing from totalview is that some random
number of procs (1-5 usually, varies from run to run) are sitting with a
send and a recv outstanding to every other proc. The other procs
however have moved on to the next collective. This is hard to see with
the default nbcbench code since it calls only alltoall repeatedly --
adding a barrier after the MPI_Alltoall() call makes it easier to see,
as the barrier has a different tag number and communication pattern. So
what I see is a few procs stuck in alltoall, while the rest are waiting
in the following barrier.

I've also verified with totalview that there are no outstanding send
wqe's at the UD BTL, and all procs are polling progress. The procs in
the alltoall are polling in the opal_condition_wait() called from

Not sure what to ask or where to look further other than, what should I
look at to see what requests are outstanding in the PML?


George Bosilca wrote:
> The first step will be to figure out which version of the alltoall
> you're using. I suppose you use the default parameters, and then the
> decision function in the tuned component say it is using the linear
> all to all. As the name state it, this means that every node will
> post one receive from any other node and then will start sending to
> every other node the respective fragment. This will lead to a lot of
> outstanding sends and receives. I doubt that the receive can cause a
> problem, so I expect the problem is coming from the send side.
> Do you have TotalView installed on your odin ? If yes there is a
> simple way to see how many sends are pending and where ... That might
> pinpoint [at least] the process where you should look to see what'
> wrong.
> george.
> On Aug 29, 2007, at 12:37 AM, Andrew Friedley wrote:
>> I'm having a problem with the UD BTL and hoping someone might have
>> some
>> input to help solve it.
>> What I'm seeing is hangs when running alltoall benchmarks with
>> nbcbench
>> or an LLNL program called mpiBench -- both hang exactly the same way.
>> With the code on the trunk running nbcbench on IU's odin using 32
>> nodes
>> and a command line like this:
>> mpirun -np 128 -mca btl ofud,self ./nbcbench -t MPI_Alltoall -p
>> 128-128
>> -s 1-262144
>> hangs consistently when testing 256-byte messages. There are two
>> things
>> I can do to make the hang go away until running at larger scale.
>> First
>> is to increase the 'btl_ofud_sd_num' MCA param from its default
>> value of
>> 128. This allows you to run with more procs/nodes before hitting the
>> hang, but AFAICT doesn't fix the actual problem. What this parameter
>> does is control the maximum number of outstanding send WQEs posted at
>> the IB level -- when the limit is reached, frags are queued on an
>> opal_list_t and later sent by progress as IB sends complete.
>> The other way I've found is to play games with calling
>> mca_btl_ud_component_progress() in mca_btl_ud_endpoint_post_send
>> (). In
>> fact I replaced the CHECK_FRAG_QUEUES() macro used around
>> btl_ofud_endpoint.c:77 with a version that loops on progress until a
>> send WQE slot is available (as opposed to queueing). Same result -- I
>> can run at larger scale, but still hit the hang eventually.
>> It appears that when the job hangs, progress is being polled very
>> quickly, and after spinning for a while there are no outstanding send
>> WQEs or queued sends in the BTL. I'm not sure where further up things
>> are spinning/blocking, as I can't produce the hang at less than 32
>> nodes
>> / 128 procs and don't have a good way of debugging that (suggestions
>> appreciated).
>> Furthermore, both ob1 and dr PMLs result in the same behavior, except
>> that DR eventually trips a watchdog timeout, fails the BTL, and
>> terminates the job.
>> Other collectives such as allreduce and allgather do not hang -- only
>> alltoall. I can also reproduce the hang on LLNL's Atlas machine.
>> Can anyone else reproduce this (Torsten might have to make a copy of
>> nbcbench available)? Anyone have any ideas as to what's wrong?
>> Andrew
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