Memory Requirements and Design Constraints

The HOT_RESTART feature implements persistent memory as a portion of the random access memory (RAM) on the target device. Although the persistent memory bank does not use virtual memory or swapping, HOT_RESTART is compatible with all three main memory models: flat, protected, and virtual.

The size of the persistent memory bank is defined in bytes by the system tunable parameter, pmm.rambankSize. The value of this parameter is static and cannot be modified while the system is running. In addition, because the RAM persistent memory bank does not use virtual memory or swapping, objects in persistent memory are locked in memory until they are freed. For these two reasons, it is important to ensure that pmm.rambankSize is set to a value realistic for the amount of data likely to be stored in persistent memory at any given time.

A portion of space reserved for an object in the persistent memory bank is known as a persistent memory block. A block is a contiguous set of memory pages, which means that the size of a block is always a multiple of the page size. For more information on the page size relevant to your platform, see the vmPageSize(2K) man page.

For each restartable process that is running, the system stores the following data in persistent memory:

• The text and initialized data which were loaded into memory from stable storage. This is known as the process image. The process image occupies a single block of persistent memory.

• The executed text, initialized data, and BSS (data initialized to zero), from which the process is running. This is known as the executing image of the process. The executing image occupies two blocks of persistent memory: one block for the text and one block for the data. The heap and stack for the executing process are stored in non-persistent memory.

The persistent memory blocks used to store the process image and executing image are freed only when the originating group terminates cleanly. Note that this may be some time after the process itself has terminated. The process can also allocate its own blocks of persistent memory to store run-time data while executing.

Although it can be difficult to predict the likely required value of pmm.rambankSize in the early stages of the development cycle, the following rule of thumb, derived from the previously mentioned statements, may be of use to developers at the system design stage:

• Restartable processes require an absolute minimum of twice their size in persistent memory. This minimum memory portion will accommodate the process's process image and executing image (although it does not enable rounding of memory block sizes to the nearest page).

• The process can also allocate additional memory. Therefore, pmm.rambankSize should be greater than twice the combined size of the restartable processes expected to run simultaneously.

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Note -

Sharing persistent memory blocks between user processes, or between user and supervisor processes is not supported. Persistent memory blocks can only be shared between supervisor processes.

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The default value of the pmm.rambankSize tunable parameter is one megabyte.