Right, by default all connections will be handled on the fly. So as an
MPI_Send is executed to a process that there is not a connection to then
a dance happens between the sender and the receiver. So why this
happens with np > 60 may have to do with how many connections are
happening at the same time or if the destination of one connection
request is not in the MPI library.
It would be interesting to figure out when in the timeline of the job
that such requests are are being delayed. You can get such a timeline
by using a tool like Solaris Studio collector/analyzer (which actually
has a Linux version).
--td
Steve Wise wrote:
Yes it does. With mpi_preconnect_mpi to 1, NP64 doesn't stall. So
its not the algorithm in and of itself, but rather some interplay
between the algorithm and connection setup I guess.
On 9/17/2010 5:24 AM, Terry Dontje wrote:
Does setting mca parameter mpi_preconnect_mpi to 1 help at all. This
might be able to help determine if it is the actually connection set
up between processes that are out of sync as oppose to something in
the actual gather algorithm.
--td
Steve Wise wrote:
Here's a clue: ompi_coll_tuned_gather_intra_dec_fixed() changes its
algorithm for job sizes > 60 to some binomial method. I changed the
threshold to 100 and my NP64 jobs run fine. Now to try and
understand what about ompi_coll_tuned_gather_intra_binomial() is
causing these connect delays...
On 9/16/2010 1:01 PM, Steve Wise wrote:
Oops. One key typo here: This is the IMB-MPI1 gather test, not
barrier. :(
On 9/16/2010 12:05 PM, Steve Wise wrote:
Hi,
I'm debugging a performance problem with running IMB-MP1/barrier
in an NP64 cluster (8 nodes, 8 cores each). I'm using
openmpi-1.4.1 from the OFED-1.5.1 distribution. The BTL is
openib/iWARP via Chelsio's T3 RNIC. In short, a NP60 and smaller
run completes in a timely manner as expected, but NP61 and larger
runs come to a crawl at the 8KB IO size and take ~5-10min to
complete. It does complete though. It behaves this way even if I
run on > 8 nodes so there are available cores. IE a NP64 on a 16
node cluster still behaves the same way even though there are only
4 ranks on each node. So its apparently not a thread starvation
issue due to lack of cores. When in the stalled state, I see on
the order of 100 or so established iwarp connections on each
node. And the connection count increases VERY slowly and
sporadically (at its peak there are around 800 connections for a
NP64 gather operation). In comparison, when I run the <= NP60
runs, the connections quickly ramp up to the expected amount. I
added hooks in the openib BTL to track the time it takes to setup
each connection. In all runs, both <= NP60 and > NP60, the
average connection setup time is around 200ms. And the max setup
time seen is never much above this value. That tells me that its
not individual connection setup that is the issue. I then added
printfs/fflushes in librdmacm to visually see when a connection is
attempted and when it is accepted. When I run with these printfs,
I see the connections get setup quickly and evently in the <= NP60
case. Initially when the job is started, I see a small flurry of
connections getting setup, then the run begins and at around 1KB
IO size I see a 2nd large flurry of connection setups. Then the
test continues and completes. With the >NP60 case, this second
round of connection setups is very sporadic and slow. Very slow!
I'll see little bursts of ~10-20 connections setup, then long
random pauses. The net is that full connection setup for the job
takes 5-10min. During this time the ranks are basically spinning
idle awaiting the connections to get setup. So I'm concluding
that something above the BTL layer isn't issuing the endpoint
connect requests in a timely manner.
Attached are 3 padb dumps during the stall. Anybody see anything
interesting in these?
Any ideas how I can further debug this? Once I get above the
openib BTL layer my eyes glaze over and I get lost quickly. :) I
would greatly appreciate any ideas from the OpenMPI experts!
Thanks in advance,
Steve.
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Oracle
Terry D. Dontje | Principal Software Engineer
Developer Tools Engineering | +1.781.442.2631
Oracle * - Performance Technologies*
95 Network Drive, Burlington, MA 01803
Email terry.don...@oracle.com <mailto:terry.don...@oracle.com>
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Oracle
Terry D. Dontje | Principal Software Engineer
Developer Tools Engineering | +1.781.442.2631
Oracle * - Performance Technologies*
95 Network Drive, Burlington, MA 01803
Email terry.don...@oracle.com <mailto:terry.don...@oracle.com>
