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Subject: Re: [OMPI users] openmpi credits for eager messages
From: Richard Treumann (treumann_at_[hidden])
Date: 2008-02-04 13:29:29


Hi Ron -

I am well aware of the scaling problems related to the standard send
requirements in MPI. I t is a very difficult issue.

However, here is what the standard says: MPI 1.2, page 32 lines 29-37

=======
a standard send operation that cannot complete because of lack of buffer
space will merely block, waiting for buffer space to become available or
for a matching receive to be posted. This behavior is preferable in many
situations. Consider a situation where a producer repeatedly produces new
values and sends them to a consumer. Assume that the producer produces new
values faster than the consumer can consume them. If buffered sends are
used, then a buffer overflow will result. Additional synchronization has to
be added to the program so as to prevent this from occurring. If standard
sends are used, then the producer will be
automatically throttled, as its send operations will block when buffer
space is unavailable.
========

If there are people who want to argue that this is unclear or that it
should be changed, the MPI Forum can and should take up the discussion. I
think this particular wording is pretty clear.

The piece of MPI standard wording you quote is somewhat ambiguous:
============
The amount
of space available for buffering will be much smaller than program data
memory on many systems. Then, it will be easy to write programs that
overrun available buffer space.
============
But note that this wording mentions a problem that an application can
create but does not say the MPI implementation can fail the job. The
language I have pointed to is where the standard says what the MPI
implementation must do.

The "lack of resource" statement is more about send and receive descriptors
than buffer space. If I write a program with an infinite loop of MPI_IRECV
postings the standard allows that to fail.

                Dick

Dick Treumann - MPI Team/TCEM
IBM Systems & Technology Group
Dept 0lva / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846 Fax (845) 433-8363

users-bounces_at_[hidden] wrote on 02/04/2008 12:24:11 PM:

>
> > Is what George says accurate? If so, it sounds to me like OpenMPI
> > does not comply with the MPI standard on the behavior of eager
> > protocol. MPICH is getting dinged in this discussion because they
> > have complied with the requirements of the MPI standard. IBM MPI
> > also complies with the standard.
> >
> > If there is any debate about whether the MPI standard does (or
> > should) require the behavior I describe below then we should move
> > the discussion to the MPI 2.1 Forum and get a clarification.
> > [...]
>
> The MPI Standard also says the following about resource limitations:
>
> Any pending communication operation consumes system resources that are
> limited. Errors may occur when lack of resources prevent the execution
> of an MPI call. A quality implementation will use a (small) fixed
amount
> of resources for each pending send in the ready or synchronous mode and
> for each pending receive. However, buffer space may be consumed to
store
> messages sent in standard mode, and must be consumed to store messages
> sent in buffered mode, when no matching receive is available. The
amount
> of space available for buffering will be much smaller than program data
> memory on many systems. Then, it will be easy to write programs that
> overrun available buffer space.
>
> Since I work on MPI implementations that are expected to allow
applications
> to scale to tens of thousands of processes, I don't want the overhead of
> a user-level flow control protocol that penalizes scalable applications
in
> favor of non-scalable ones. I also don't want an MPI implementation that
> hides such non-scalable application behavior, but rather exposes it at
lower
> processor counts -- preferably in a way that makes the application
developer
> aware of the resources requirements of their code and allows them to make
> the appropriate choice regarding the structure of their code, the
underlying
> protocols, and the amount of buffer resources.
>
> But I work in a place where codes are expected to scale and not just
work.
> Most of the vendors aren't allowed to have this perspective....
>
> -Ron
>
>
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