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<p>Thank you Ashley -<br>
<br>
This is exactly the distinction between <b>assertions </b>and <b>hints</b> and the reason this narrow proposal advocates <b>assertions</b> and does not advocate <b>hints</b>.<br>
<br>
An <tt>MPI_NO_SEND_CANCEL</tt> <b>hint</b> still requires the MPI implementation to invest whatever it takes, across the entire application life to do MPI_CANCEL correctly. <br>
<br>
An <tt>MPI_NO_SEND_CANCEL</tt> <b>assertion</b> allows the MPI implementation to forgo those costs and just refuse to try to MPI_CANCEL if the application has an MPI_CANCEL call.<br>
<br>
The assertion does <b>relax</b> 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.<br>
<br>
As a footnote -<br>
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.<br>
<br>
Dick <br>
<br>
Dick Treumann - MPI Team/TCEM <br>
IBM Systems & Technology Group<br>
Dept 0lva / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601<br>
Tele (845) 433-7846 Fax (845) 433-8363<br>
<br>
<br>
<tt>mpi-22-bounces@lists.mpi-forum.org wrote on 05/15/2008 05:59:51 PM:<br>
<br>
<snip><br>
> <br>
> To me I see little benefit in offering MPI_NO_SEND_CANCEL as a hint, all<br>
> that does is say to the library that a cancel is unlikely to happen and<br>
> we all know that anyway. As a cancel is still possible I see no<br>
> opportunity for optimisation of any kind making itself possible, all the<br>
> extra space in headers and longer code paths would still be needed<br>
> anyway.<br>
> <br>
> As an assertion it would offer some scope for library writers to make<br>
> simplifications leading to better performance, as a hint I can't see how<br>
> it would change anything.<br>
> <br>
> Ashley,<br>
> <br>
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