[Gretl-devel] gretl <-> gretlmpi communication

OK, I should probably explain what gave rise to the problem noted by Ignacio in http://lists.wfu.edu/pipermail/gretl-devel/2017-December/008324.html It's to do with sending information between gretl and gretlmpi in connection with an "mpi block" in a hansl script. Say you want to send a matrix from gretl to gretlmpi, and get a processed matrix back again. You can of course use mwrite() and mread(), which work via writing and reading of files. But now, experimentally, you can use these two functions in a special mode: if you give the matrix filenames the suffix ".shm" we'll transfer the data via shared memory rather than regular files. Here's a trivial example: <hansl> set verbose off matrix m = mnormal(500,500) mwrite(m, "gmat.shm") mpi matrix m if $mpirank == 0 m = mread("gmat.shm") printf "mpi(root)\n" m *= 2 mwrite(m, "gmat2.shm") endif end mpi --np=4 m2 = mread("gmat2.shm") printf "Should be zero: %g\n", maxc(maxr(abs(m2 - 2*m))) </hansl> Using ".shm" the behavior is a bit different from usual: the shared memory block is freed on the first read (to avoid leaking memory). Also you shouldn't give a full path for the "file", it's automatically created under /dev/shm on Linux (or in "dotdir" on Windows, which doesn't support POSIX shared memory). Although this is quite cool, I'm not sure it's really worthwhile. In my testing I'm seeing only a small speed advantage over writing the matrix as a regular binary file, as in mwrite(m, "gmat.bin") But if anyone would like to test, please go ahead and tell me what you see. (The problem Ignacio found was due to the fact that some Linux variants require explicit linkage to librt when using shared memory functions while others such as Arch and Fedora do not. I was testing on Arch and Fedora and so didn't see a problem.) Allin