1.2.1 Persistent Memory

The foundation of the hot restart mechanism is the use of persistent memory to store data which can persist across an actor or site restart. Persistent memory is used internally by the system, to store the actor image (text and data) from which a restartable actor can be reconstructed. Any actor can also allocate persistent memory to store data. This data could, for example, be used to checkpoint application execution.

At the lowest level, persistent memory is a bank of memory loaded by the ChorusOS kernel at cold boot. The content of this bank of memory is preserved across an actor or site restart. In the current implementation, the only supported medium for the persistent memory bank is RAM: in other words, persistent memory is simply a reserved area of physical memory. For this reason, persistent memory will resist a hot restart, but not a board reset. The size of the area of RAM reserved for persistent memory is governed by a tunable parameter.

The allocation and de-allocation (freeing) of persistent memory are managed by a ChorusOS actor known as the Persistent Memory Manager (PMM). The Persistent Memory Manager exports an API for this purpose. This API is distinct from the API used for allocating and de-allocating traditional ChorusOS memory regions (rgnAllocate(2K), rgnFree(2K), svPagesAllocate(2K), and svPagesFree(2K)).

The Persistent Memory Manager API is described in more detail in Chapter 3, Programming With Persistent Memory and in the pmmAllocate(2RESTART), pmmFree(2RESTART) and pmmFreeAll(2RESTART) man pages.