The size of POSIX shared memory (/dev/shm) is 50 % of physical RAM and mounted by default. It is only allocated as needed. There is no best practice required. However, most likely will not want to use Oracle AMM (Automatic Shared Memory) and use kernel Hugepages instead. The benefit of Hugepages comes from a larger page allocation size of 2 MB, vs. standard 4 KB, resulting in a much smaller pagetable and thereby using the TLB cache more efficiently. There are many resources available that go into more details.