On the Difference Between Shared Memory and Shared Address Space in HPC Communication.

Shared memory mechanisms, e.g., POSIX shmem or XPMEM, are widely used to implement efficient intra-node communication among processes running on the same node. While POSIX shmem allows other processes to access only newly allocated memory, XPMEM allows accessing any existing data and thus enables more efficient communication because the send buffer content can directly be copied to the receive buffer. Recently, the shared address space model has been proposed, where processes on the same node are mapped into the same address space at the time of process creation, allowing processes to access any data in the shared address space. Process-in-Process (PiP) is an implementation of such mechanism. The functionalities of shared memory mechanisms and the shared address space model look very similar both allow accessing the data of other processes -, however, the shared address space model includes the shared memory model. Their internal mechanisms are also notably different. This paper clarifies the differences between the shared memory and the shared address space models, both qualitatively and quantitatively. This paper is not to showcase applications of the shared address space model, but through minimal modifications to an existing MPI implementation it highlights the basic differences between the two models. The following four MPI configurations are evaluated and compared; 1) POSIX Shmem, 2) XPMEM, 3) PiP-Shmem, where intra-node communication is implemented to utilize POSIX shmem but MPI processes share the same address space, and 4) PiP-XPMEM, where XPMEM functions are implemented by the PiP library (without the need for linking to XPMEM library). Evaluation is done using the Intel MPI benchmark suite and six HPC benchmarks (HPCCG, miniGhost, LULESH2.0, miniMD, miniAMR and mpiGraph). Most notably, mpiGraph performance of PiP-XPMEM outperforms the XPMEM implementation by almost 1.5x. The performance numbers of HPCCG, miniGhost, miniMD, LULESH2.0 running with PiP-Shmem and PiP-XPMEM are comparable with those of POSIX Shmem and XPMEM. PiP is not only a practical implementation of the shared address space model, but it also provides opportunities for developing new optimization techniques, which the paper further elaborates on.