Thanks George. I figured out the problem (two of them actually) based
on a pointer from Gleb (thanks Gleb). I have two types of send queues
on the UD BTL -- one is per-module, and the other is per-endpoint. I
had missed looking for stuck frags on the per-endpoint queues.
So something is wrong with the per-endpoint queues and their interaction
with the per-module queue. Disabling the per-endpoint queue makes the
problem go away, and I'm not sure I liked having them in the first place.
But this still left a similar problem at 2kb messages. I had static
limits set for free list lengths based on the btl_ofud_sd_num MCA
parameter. Switching the max to unlimited makes this problem go away
too. Good enough to get some runs through for now :)
Andrew
George Bosilca wrote:
Andrew,
There is an option on the message queue stuff, that allow you to see all
internal pending requests. On the current trunk, edit the file
ompi/debuggers/ompi_dll.s at line 736 and set the
p_info->show_internal_requests to 1. Now compile and install it, and
then restart totalview. You should be able to get access to all pending
requests, even those created by the collective modules.
Moreover, the missing sends should be somewhere. If they are not in the
BTL, and i they are not completed, then hopefully they are in the PML in
the send_pending list. As the collective works on all other BTL I
suppose the communication pattern is correct, so there is something
happening with the requests when using the UD BTL.
If the requests are not in the PML send_pending queue, the next thing
you can do is to modify the receive handles in the OB1 PML, and print
all incoming match header. You will have to somehow sort the output, but
at least you can figure out, what is happening with the missing messages.
george.
On Sep 11, 2007, at 12:37 PM, Andrew Friedley wrote:
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
ompi_request_wait_all().
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?
Andrew
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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