[mpiwg-tools] PMPI and sessions init

Marc-André Hermanns hermanns at itc.rwth-aachen.de
Mon Nov 9 04:00:46 CST 2020

Hi Joachim,

> if I understood the discussion about MPI_Session_init and PMPI right, I
> don't think there exists an issue.
> By ld-preloading the PMPI tool, the tool can also intercept a call to
> MPI_Session_init, which might come from a library constructor before
> main. Even a function called from the library constructor of a static
> library can be intercepted by a ld-preloaded tool.
> For libraries, which are explicitly loaded by the MPI runtime during
> initialization, this would also work. For others, it depends on the
> ordering of calling the library constructor (might work by chance :).

Wasn't the problem the fact not necessarily that the tools can not
intercept MPI_Session_init in time, but our current decision to
disallow tool *registration* after MPI was initialized, a library
using session init inside a constructor would cut off registration for
any tool not registered at the time.

Maybe I am missing something here or misunderstood something in the

> This was one of my main motivation of having such an environmental
> variable.

I think the environment variable is separate (or orthogonal) to the
problem above. It's not so much that MPI won't eventually know all the
tools and their order, but that a "rogue" library would just use (and
with that initialize) MPI at a time out of control of the tool
registration process.

At least that was my understanding of the discussion.


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Dr. rer. nat. Marc-André Hermanns

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