[Mpi-22] MPI_INIT assertions

[Richard Graham]

I think we are splitting hairs on wording here.  In one case the user is
saying ³I don¹t care about
 the performance of cancel² and in the other ³I promise not to call cancel².
Neither is changing the
 semantics of MPI.

Rich

On 5/15/08 5:44 PM, "Richard Treumann" <treumann_at_[hidden]> wrote:

> Hi Rich 
> 
> The difference between a hint and an assertion is that an hint cannot change
> semantic in any way.  An assertion will allow libmpi to relax a specific
> semantic guarantee.   Lets take a simple case:
> 
> The application wants to say it will not try to cancel any MPI_ISEND so it
> passes MPI_NO_SEND_CANCEL to MPI_INIT
> 
> If  MPI_INIT were passed MPI_NO_SEND_CANCEL as a hint and then the application
> went ahead and called MPI_CANCEL anyway, it would be fine for the MPI
> implementation to use a much slower MPI_CANCEL algorithm than if the hint had
> not been given.  It would not be OK for the implementation to hang or raise an
> error or cancel the wrong message.
> 
> If  MPI_Init were passed MPI_NO_SEND_CANCEL as an assertion and then the
> application went ahead and called MPI_CANCEL anyway, it would be OK for the
> MPI implementation to raise an error for the MPI_CANCEL call.
> 
> Every MPI Implementation must still be prepared to do MPI_CANCEL of an ISEND
> correctly if MPI_CANCEL is called by an application that did not provide the
> assertion. Adding assertion support in MPI 2.2 cannot break a correct MPI 2.1
> application.
> 
> The MPI_NO_SEND_CANCEL is only useful for cases where libmpi must devote extra
> memory, longer code paths or extra space in message headers across an entire
> run just to have the information it needs to do MPI_CANCEL correctly is
> somebody happens to call it. If a particular MPI implementation does not incur
> extra costs to be ready for a potential MPI_CANCEL then the implementation
> will ignore this assertion.
> 
>  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
> 
> 
> mpi-22-bounces_at_[hidden] wrote on 05/15/2008 01:33:02 PM:
> 
>> > Assertions should not change semantics ­ these are defined by the
>> > standard.   Assertions
>> >  may provide ³help² to the implementation, what ever that means.  I
>> > may be missing some thing,
>> >  but I really don¹t see a difference (aside  from name) between
>> > assertions an hints ­ both are
>> >  ways of the user conveying information to the library.
>> > 
>> > Rich
>> > 
> 
> 
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