Shared memory

VASP is mainly parallelized using MPI, and as much as practically feasible the computational work and storage demands are distributed over the MPI ranks. Unavoidably, however, some data structures are duplicated across all MPI ranks. For some of these data structures, VASP offers the option to reduce the memory consumption by putting them into shared-memory segments. That is segments of memory shared between the MPI-ranks that reside on the same compute-node and hence have access to the same physical memory.

Whether to use shared memory or not has to be decided when compiling the code. It is controlled by the precompiler options: -Duse_shmem, -Dshmem_bcast_buffer, und -Dshmem_rproj.

The allocation and handling of shared memory segments has been implemented in two different ways:

• Using the MPI-3 shared memory capabilities (default).

Warning: Per default, VASP uses MPI-3 calls to allocate and manage shared memory segments. Unfortunately, we have observed that for some MPI implementations an abnormal termination of the code (e.g. segfaults or user initiated abort) does not free these shared memory segments. This is not a VASP related error. It is caused by the way these shared memory segments are handled by the operating system and MPI. Without explicit clean-up this leads to a "memory leakage" that persists until the compute node is rebooted. Obviously this is very problematic at high-performance-computing centers. For this reason we do not recommend using shared memory indiscriminately (i.e., without explicit need).

• Using "icps" shared memory segments and "system-V" semaphores (add precompiler option -Dsysv).

Tip: Using "icps" shared memory segments and "system-V" semaphores (add precompiler option -Dsysv) rarely leads to memory leakage. However when it does it is guaranteed to persist until reboot of the node, and no manner of other clean-up will be effective.