This module provides a shared memory cache accessed as a tied hash.
Shared memory is an area of memory that is available to all processes.
It is accessed by choosing a key, the ipc_key argument to tie. Every
process that accesses shared memory with the same key gets access to
the same region of memory. In some ways it resembles a file system,
but it is not hierarchical and it is resident in memory. This makes
it harder to use than a filesystem but much faster. The data in
shared memory persists until the machine is rebooted or it is