[Mpi-22] MPI_INIT assertions

Richard Graham rlgraham at [hidden]
Thu May 15 10:25:36 CDT 2008

We should be careful about making a change in MPI 2.2, knowing that we will
likely turn around again
 in MPI 3.0 and change things again.  If we are talking about changing the
interface in 2.2, and
 then extending the assertions/hints in 3.0, this seems fine, but if we may
want to change the
 interface yet again in 3.0, we should rethink things.
I will add that if we are going to add some sort of  ³info² argument to
ŒMPI_Init()¹, we should deprecate
 MPI_Init() and MPI_Init_thread(), and include the threading specification
in the ³info² object.
Finally, before we decide on how to pass hints/assertions (if we do) to
MPI_Init(), we should
 define a consistent way across the standard for passing information between
the application and
 the library, as this is not the only instance where this is useful, and a
uniform way of doing this
 makes things much easier on users.


On 5/14/08 9:14 AM, "Richard Treumann" <treumann_at_[hidden]> wrote:

> MPI_Init time assertions must be few and each must be valuable or the concept
> will fall like a house of cards.
> There is nothing in my proposal for MPI_Init time assertions that rules out
> providing other mechanisms in MPI 3 for giving guidance to the MPI
> implementation.  In MPI 3 we can consider more hints and we can add the
> abiility to give stronger direction to MPI or provide it on a more granular
> basis - If it makes sense. These extra mechanisms are far to complex to
> consider as part of MPI 2.2.
> I would not use the phrase that Dries does when he says "Assertions are bad"
> but I agree with the the sentiment behind his statement. I think there should
> be a very small number of assertions defined in the standard and for each
> there should be a good rationale.  For MPI 2.2 there should be a great
> rationale because we can come back in MPI 3 and add more assertions if we miss
> some important ones. It is much harder to remove one that turns out to be real
> trouble.
> Each assertion the the MPI Standard defines has the potential to break some
> piece of code that is valid MPI but that depends on semantic the assertion
> says is optional. The author of the routine that calls MPI_Init has the power
> to declare assertions and the authors of other parts of an applicaton must
> either live within the rules or explicitly shield against them.
> For project teams that develop complete applications, the decision to use an
> assertion belongs to the team,  team leader or architect.  If it is decided
> that an application will use a specific assertion it is the team lead  who
> must make sure all developers understand the decision and write appropriate
> code.  All testing will be done with the assertion in place.
> For third part libraries, the only option is to either forbid all assertions
> or explicitly pick some to allow. If there are  4 potential assertions, it is
> not very hard to decide for each one - "Will the library tolerate it?".  If
> there are 50 assertions, library authors will seldom allow them all and will
> be more tempted to just say "No assertions allowed" because making judgements
> about each of 50 is too difficult.
> For Community developed code where people contribute source but are not under
> direct control of an architect or team lead, reviewing each submission for
> compliance with one or two assertions may be acceptable but reviewing for 50
> each time somebody contributes new code is not.
> 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
> mpi-22-bounces_at_[hidden] wrote on 05/14/2008 07:21:41 AM:
>> > * Terry Jones <trj_at_[hidden]> [2008-05-13 15:19:07]:
>> > 
>>> > > You can also imagine other possibilities to provide helpful context. For
>>> > > instance, perhaps the user could provide Assertions that would help MPI
>>> > > with read-ahead prefetching or write-behind, or even meta-data
>>> operations 
>>> > > (e.g., later I will be creating one file per MPI task).
>> > 
>> > Things like read-ahead or write-behind clearly shouldn't be assertions but
>> > hints. (And, probably 'one file per MPI task' too -- if this is still going
>> to
>> > be needed in 2 years)
>> > 
>> > MPI already has hints that can capture some of the things mentioned
>> > above. 
>> > 
>> > access_style: (read_once, write_once, read_mostly, write_mostly,
>> sequential,
>> > reverse_sequential, and random)
>> >     sequential -> this can easily be used to turn on read-ahead
>> >                   IF THE MPI LIBRARY decides this is useful
>> > 
>> > Assertions are bad -- they break compatibility -- and should only be
>> > tolerated if they provide real benefits and if the same cannot be obtained
>> > through existing mechanisms (hints, ...).
>> > 
>> > In the examples mentioned, this is not the case.
>> > 
>> >    Dries
>> > 
>> > [attachment "attia6gr.dat" deleted by Richard
>> > Treumann/Poughkeepsie/IBM] _______________________________________________
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