On Feb 20, 2014, at 9:19 AM, Jeff Hammond <[email protected]> wrote:
On Thu, Feb 20, 2014 at 9:16 AM, Rolf Rabenseifner <[email protected]> wrote:
Most users don't understand this and write code that is incorrect.
Therefore such a clarification is essential.
I dispute the need for the MPI standard to educate programmers about the pitfalls of shared memory programming. The standard is not a programmer manual. Does C99 standard have text on how shared memory works?
This argument is bogus. C99 takes the "ostrich approach" to discussing threads, multiprocessing, and shared memory (much to the world's detriment, IMO). The MPI-3 standard explicitly supports shared memory, making us at least partly responsible for clarifying which parts of MPI are expected to interact with normal shared memory programming practices. The real takeaway from this thread should be that: 1) Rolf is a smart guy who knows much more about Fortran and reading the MPI standard than a typical skilled scientific programmer. I.e., Rolf is one of the people who can reasonably be expected to help translate the MPI Standard into useful interpretations for users. 2) His reading of the current text did not lead to a correct toy program for using shared memory windows. It required several interactions with the RMA working group. This should concern us. If only the members of the MPI RMA working group can decipher it, then we have failed to make a standard that will be useful to the rest of the world. I don't think we should turn the standard into a textbook or manual, but we can still do our best to communicate clearly to sufficiently qualified readers by making modest improvements to the text. -Dave