Overview of shared memory

Shared memory is physical memory that is assigned to the shared memory pool and shared among multiple logical partitions. The shared memory pool is a defined collection of physical memory blocks that are managed as a single memory pool by the hypervisor. Logical partitions that you configure to use shared memory, share the memory in the pool with other shared memory partitions.

For example, you create a shared memory pool with 16 GB of physical memory. You then create three logical partitions, configure them to use shared memory, and activate the shared memory partitions. Each shared memory partition can use the 16 GB that are in the shared memory pool.

The hypervisor determines the amount of memory that is allocated from the shared memory pool to each shared memory partition based on the workload and memory configuration of each shared memory partition. When allocating the physical memory to the shared memory partitions, the hypervisor ensures that each shared memory partition can access only the memory that is allocated to the shared memory partition at any given time. A shared memory partition cannot access the physical memory that is allocated to another shared memory partition.

The amount of memory that you assign to the shared memory partitions can be greater than the amount of memory in the shared memory pool. For example, you can assign 12 GB to shared memory partition 1, 8 GB to shared memory partition 2, and 4 GB to shared memory partition 3. Together, the shared memory partitions use 24 GB of memory, but the shared memory pool has only 16 GB of memory. In this situation, the memory configuration is considered over committed.

Over committed memory configurations are possible because the hypervisor virtualizes and manages all of the memory for the shared memory partitions in the shared memory pool as follows:

1. When shared memory partitions are not actively using their memory pages, the hypervisor allocates those unused memory pages to shared memory partitions that currently need them. When the sum of the physical memory that is currently used by the shared memory partitions is less than or equal to the amount of memory in the shared memory pool, the memory configuration is logically over committed. In a logically over committed memory configuration, the shared memory pool has enough physical memory to contain the memory that is used by all shared memory partitions at one point in time. The hypervisor need not store any data in auxiliary storage.
2. When a shared memory partition requires more memory than the hypervisor can provide to it by allocating unused portions of the shared memory pool, the hypervisor stores some of the memory that belongs to a shared memory partition in the shared memory pool and stores the remainder of the memory that belongs to the shared memory partition in auxiliary storage. When the sum of the physical memory that is currently used by the shared memory partitions is greater than the amount of memory in the shared memory pool, the memory configuration is physically over committed. In a physically over committed memory configuration, the shared memory pool does not have enough physical memory to contain the memory that is used by all the shared memory partitions at one point in time. The hypervisor stores the difference of the physical and shared memory in the auxiliary storage. When the operating system attempts to access the data, the hypervisor might need to retrieve the data from auxiliary storage before the operating system can access it.

Because the memory that you assign to a shared memory partition might not always reside in the shared memory pool, the memory that you assign to a shared memory partition is logical memory. Logical memory is the address space assigned to a logical partition, that the operating system perceives as its main storage. For a shared memory partition, a subset of the logical memory is backed up by physical main storage (or physical memory from the shared memory pool) and the remaining logical memory is kept in auxiliary storage.

A Virtual I/O Server logical partition provides access to the auxiliary storage, or paging space devices, which are required for shared memory partitions in an over committed memory configuration. A paging space device is a physical or logical device that is used by a Virtual I/O Server to provide the paging space for a shared memory partition. The paging space is an area of nonvolatile storage that is used to hold portions of a shared memory partition's logical memory that does not reside in the shared memory pool. When the operating system that runs in a shared memory partition attempts to access data, and the data is located in the paging space device that is assigned to the shared memory partition, the hypervisor sends a request to a Virtual I/O Server to retrieve the data and write it to the shared memory pool so that the operating system can access it.

On systems that are managed by a Hardware Management Console (HMC), you can assign up to two Virtual I/O Server (VIOS) logical partitions to the shared memory pool at a time. When you assign two paging VIOS partitions to the shared memory pool, you can configure the paging space devices such that both paging VIOS partitions have access to the same paging space devices. When one paging VIOS partition becomes unavailable, the hypervisor sends a request to the other paging VIOS partition to retrieve the data on the paging space device.

You cannot configure paging VIOS partitions to use shared memory. Paging VIOS partitions do not use the memory in the shared memory pool. You assign paging VIOS partitions to the shared memory pool so that they can provide access to the paging space devices for the shared memory partitions that are assigned to the shared memory pool.

Driven by workload demands from the shared memory partitions, the hypervisor manages over committed memory configurations by continually performing the following tasks:

• Allocating portions of physical memory from the shared memory pool to the shared memory partitions as needed.
• Requesting a paging VIOS partition to read and write data between the shared memory pool and the paging space devices as needed.

The ability to share memory among multiple logical partitions is known as the PowerVM® Active Memory Sharing technology. The PowerVM Active Memory Sharing technology is available with the PowerVM Enterprise Edition for which you must obtain and enter a PowerVM Editions activation code. Only 512 byte block devices are supported for PowerVM Active Memory Sharing.