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Subject: [OMPI devel] (no subject)
From: ananda.mudar_at_[hidden]
Date: 2010-11-30 00:07:12


Jeff, George, Ralph

Thanks a lot for your clarifications!!

- Ananda

----------- PREVIOUS MESSAGE -----------------------

Subject: Re: [OMPI devel] Warning on fork() disappears if I use MPI
threads!!
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-11-29 19:50:33

* Previous message: Ralph Castain: "Re: [OMPI devel] Warning on
fork() disappears if I use MPI threads!!"
<http://www.open-mpi.org/community/lists/devel/2010/11/8735.php>
* In reply to: George Bosilca: "Re: [OMPI devel] Warning on fork()
disappears if I use MPI threads!!"
<http://www.open-mpi.org/community/lists/devel/2010/11/8734.php>
* Next in thread: Ralph Castain: "Re: [OMPI devel] Warning on
fork() disappears if I use MPI threads!!"
<http://www.open-mpi.org/community/lists/devel/2010/11/8735.php>

________________________________

On Nov 29, 2010, at 6:25 PM, George Bosilca wrote:

> The main problem is that openib require to pin memory pages in order
to take advantage of RMA features. There is a major issues with these
pinned pages and fork, leading to segmentation fault in some specific
cases. However, we only pin the pages on the MPI calls related to data
transfers. Therefore, if you call fork __before__ any other MPI data
transfer function (but after MPI_Init as you use the process rank), your
application should be safe.

Note that Open MPI also pins some internal memory during MPI_INIT, but
that memory is totally internal to libmpi, so you should be safe (i.e.,
you should never be able to find it and therefore never be able to try
to touch it).

>> How can one be sure that the disabling the warning is ok? Could you
please elaborate on what makes forks vulnerable? May be that will guide
the developers to make an informed decision on whether to disable them
or find another alternative.
>
> No way to know at 100%. Now for an elaborate answer: Once upon a time
... The fork story is a long and boring one, we would all have preferred
to never heard about it (believe me). A quick and compressed version can
be found on the QLogic download page
(http://filedownloads.qlogic.com/files/driver/70277/release_QLogicIB-Bas
ic_4400_Rev_A.html).

That's a good summary. The issue is with OFED itself, not with Open MPI.

Note, too, that calling popen() should also be safe (even though we'll
warn about it -- our atfork hook has no way of knowing whether you're
calling system, popen, or something else).

-- 
Jeff Squyres
jsquyres_at_[hidden]
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________________________________
 
 
Ananda B Mudar, PMP
Senior Technical Architect
Wipro Technologies
Ph: 972 765 8093
ananda.mudar_at_[hidden]
 
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