I am copying Nick on this since he was the one who reported the PMR to IBM. My understanding of MPICH is that increasing the maximum communicator count is not easy. I recall reading an article by Cray on how they had to reduce the max to accomodate extra bits in order to run MPI at scale on Jaguar. I agree that this should not hang; rather, it should fail with an appropriate error message. On the other hand, I don't think that any effort should be put forth to accomodate users who want thousands of communicators. Jeff On Sat, Dec 15, 2012 at 11:39 PM, Jim Dinan <[email protected]> wrote:

Hi Bob,

I think I have a fix for detecting this error, and I should be able to send it along next week.

AFAIK, MPICH2 has never detected the particular context ID exhaustion scenario from the test case you sent. Is it possible that the size of the context ID space was increased on the BG/P until the error went away? If that was the case (would be in src/include/mpiimpl.h where MPIR_MAX_CONTEXT_MASK is defined), it may make sense to do the same for BG/Q, in case there are apps that need more communicators than the default limit.

Best, ~Jim.

On 12/15/12 9:23 AM, Bob Cernohous wrote:

...

I didn't try it myself on BG/P. All I really know is someone at ANL (possibly Nick?) reported (about ftmain.f90) :

This programs create as many communicators as there are MPI tasks. It is a bad program provided by a user. On BG/P, this type of program threw a proper warning. It should do the same on BG/Q.

Business impact ( BusImpact ) It is mostly a nuisance to those who don't understand the inherent limitations in MPICH2.

I don't have access to PMR's but it was *41473,122,000 . *The CPS issue

that was opened to me from that PMR had no other details.

Bob Cernohous: (T/L 553) 507-253-6093

[email protected] IBM Rochester, Building 030-2(C335), Department 61L 3605 Hwy 52 North, Rochester, MN 55901-7829

...

Chaos reigns within. Reflect, repent, and reboot. Order shall return.

From: Jim Dinan <[email protected]> To: [email protected], Date: 12/14/2012 11:52 PM Subject: Re: [mpich-devel] MPICH2 hang Sent by: [email protected] ------------------------------------------------------------------------

Hi Bob,

The ftmain2.f90 test fails on MPICH2 1.2.1p1, which was released on 2-22-2010, well before any of the MPI-3 changes. Could you provide some more information on when this test was reporting a failure instead of hanging?

It looks like this test case generates a context ID exhaustion pattern where context IDs are available at all processes, but the processes have no free context IDs in common. Because there is no common context ID available, allocation can't succeed and it loops indefinitely. This is a resource exhaustion pattern that AFAIK, MPICH has not detected in the past.

I attached for reference a C translation of this test that is a little easier to grok and also fails on MPICH, going back to MPICH2 1.2.1p1.

~Jim.

On 12/14/12 11:27 AM, Jim Dinan wrote:

...

Hi Bob,

Thanks for the detailed bug report and test cases. I confirmed that the failure you are seeing on the MPICH trunk. This is likely related to changes we made to support MPI-3 MPI_Comm_create_group(). I created a ticket to track this:

https://trac.mpich.org/projects/mpich/ticket/1768

~Jim.

On 12/12/12 5:38 PM, Bob Cernohous wrote:

...

I've had a hang reported on BG/Q after about 2K MPI_Comm_create's.

It hangs on the latest 2 releases (mpich2 v1.5.x and v1.4.x) on

BG/Q.

...

It also hangs on linux: 64bit (MPI over PAMI) MPICH2 library.

On older mpich 1.? (BG/P) it failed with 'too many communicators'

and

...

didn't hang, which is what they expected.

It seems like it's stuck in the while (*context_id == 0) loop repeatedly calling allreduce and never settling on a context id in commutil.c. I didn't do a lot of debug but seems like it's in vanilla mpich code, not something we modified.

ftmain.f90 fails if you run it on >2k ranks (creates one comm per rank). This was the original customer testcase.

ftmain2.f90 fails by looping so you can run on fewer ranks.

I just noticed that with --np 1, I get the 'too many communicators' from ftmain2. But --np 2 and up hangs.

stdout[0]: check_newcomm do-start 0 , repeat 2045 , total 2046 stderr[0]: Abort(1) on node 0 (rank 0 in comm 1140850688): Fatal error in PMPI_Comm_create: Other MPI error, error stack: stderr[0]: PMPI_Comm_create(609).........: MPI_Comm_create(MPI_COMM_WORLD, group=0xc80700f6, new_comm=0x1dbfffb520) failed stderr[0]: PMPI_Comm_create(590).........: stderr[0]: MPIR_Comm_create_intra(250)...: stderr[0]: MPIR_Get_contextid(521).......: stderr[0]: MPIR_Get_contextid_sparse(752): Too many communicators [attachment "too_many_comms3.c" deleted by Bob Cernohous/Rochester/IBM]

-- Jeff Hammond Argonne Leadership Computing Facility University of Chicago Computation Institute [email protected] / (630) 252-5381 http://www.linkedin.com/in/jeffhammond https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond