On 5 August 2008 at 21:58, Tilman Koschnick wrote: | On Tue, 2008-08-05 at 12:52 -0500, Dirk Eddelbuettel wrote: | > In which case we wouldn't need alternative's for mpiexec? Sounds too good to | > be true. Maybe all MPI variants just handle it via alternatives. Manuel may | > know this best. | | I was looking into this some time ago, and gave up in despair... | | Alternatives work well if all participating packages agree on the same | 'master' alternative and exactly the same 'slaves'. As soon as a package | deviates from this, things get messy, e.g. it depends on in which order | you add, remove or change alternatives. | | One example: package A has files a.foo, a.bar and a.baz, and B has | b.foo, b.bar (but not b.baz). Both agree to handle these files via | alternatives, using foo as the master link. | | While A is active, we have: | | foo -> a.foo | bar -> a.bar | baz -> a.baz | | Now activate B. The result is: | | foo -> b.foo | bar -> b.bar | baz -> a.baz | | IIRC, if both packages are removed, we are left with a dangling baz -> | a.baz... They other way round, going from B to A, would be fine. | | Across MPICH, MPICH2 (never made it into the archive), LAM and OpenMPI, | a variety of files is handled via alternatives: mpiexec & Co, mpicc & | Co, headers, man pages... Some spread the files across two packages | (-bin, -dev), other have only one. Some have mpirun, some mpiexec, some | both. Some have mpic++, some mpicxx, some mpiCC. Some have a full set of | man pages, some not. | | I think alternatives would need a thorough redesign to handle things | like this sanely. Until then, they are only good for very close packages | (one binary + man page, doing exactly the same in both packages), and | it's probably not worth the bother banging all MPI implementations into | the same shape. | | As for the problem with migration brought up by the original poster: In | a similar situation, we found it was easiest to ignore alternatives | altogether and ask all users to use the "qualified" binary names, e.g. | mpicc.openmpi. This way, every user could switch implementations | conciously, at any time, without being dependent on whatever the admin | thought the default should be at the time.
Excellent post, thanks Til. So to mark a corner case: maybe we should just stick this into a README and be done with it? Dirk -- Three out of two people have difficulties with fractions. -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
