8.4.3. Complications


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Up: Examples Next: Multiple counting Previous: Systems without weak symbols


8.4.3.1. Multiple counting


Up: Complications Next: Linker oddities Previous: Complications

Since parts of the MPI library may themselves be implemented using more basic MPI functions (e.g. a portable implementation of the collective operations implemented using point to point communications), there is potential for profiling functions to be called from within an MPI function which was called from a profiling function. This could lead to ``double counting'' of the time spent in the inner routine. Since this effect could actually be useful under some circumstances (e.g. it might allow one to answer the question ``How much time is spent in the point to point routines when they're called from collective functions ?''), we have decided not to enforce any restrictions on the author of the MPI library which would overcome this. Therefore the author of the profiling library should be aware of this problem, and guard against it herself. In a single threaded world this is easily achieved through use of a static variable in the profiling code which remembers if you are already inside a profiling routine. It becomes more complex in a multi-threaded environment (as does the meaning of the times recorded !)



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8.4.3.2. Linker oddities


Up: Complications Next: Multiple levels of interception Previous: Multiple counting

The Unix linker traditionally operates in one pass : the effect of this is that functions from libraries are only included in the image if they are needed at the time the library is scanned. When combined with weak symbols, or multiple definitions of the same function, this can cause odd (and unexpected) effects.

Consider, for instance, an implementation of MPI in which the Fortran binding is achieved by using wrapper functions on top of the C implementation. The author of the profile library then assumes that it is reasonable only to provide profile functions for the C binding, since Fortran will eventually call these, and the cost of the wrappers is assumed to be small. However, if the wrapper functions are not in the profiling library, then none of the profiled entry points will be undefined when the profiling library is called. Therefore none of the profiling code will be included in the image. When the standard MPI library is scanned, the Fortran wrappers will be resolved, and will also pull in the base versions of the MPI functions. The overall effect is that the code will link successfully, but will not be profiled.

To overcome this we must ensure that the Fortran wrapper functions are included in the profiling version of the library. We ensure that this is possible by requiring that these be separable from the rest of the base MPI library. This allows them to be ared out of the base library and into the profiling one.



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