Thank you Ashley - This is exactly the distinction between assertions and hints and the reason this narrow proposal advocates assertions and does not advocate hints. An MPI_NO_SEND_CANCEL hint still requires the MPI implementation to invest whatever it takes, across the entire application life to do MPI_CANCEL correctly. An MPI_NO_SEND_CANCEL assertion allows the MPI implementation to forgo those costs and just refuse to try to MPI_CANCEL if the application has an MPI_CANCEL call. The assertion does relax a semantic guarentee (I say relax rather than change because "change" has broader implications but an assertion does change semantics in a narrow and well defined way). An MPI Implementation that is prepared to do MPI_CANCEL correctly has a different semantic than one that will issue an error message if MPI_CANCEL is called. As a footnote - MPI 2 defined some hints and MPI 3 may define more hints. That is OK. There are cases where a hint will allow an MPI implementation to give better performance without changing its semantic. In a situation where a hint is enough, the MPI standard should avoid defining an 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 05:59:51 PM: <snip>

To me I see little benefit in offering MPI_NO_SEND_CANCEL as a hint, all that does is say to the library that a cancel is unlikely to happen and we all know that anyway. As a cancel is still possible I see no opportunity for optimisation of any kind making itself possible, all the extra space in headers and longer code paths would still be needed anyway.

As an assertion it would offer some scope for library writers to make simplifications leading to better performance, as a hint I can't see how it would change anything.

Ashley,

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