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Virtualization with PowerVM > Practical Introduction > Memory Virtualization > Active Memory Sharing (AMS)

6.2.5. Logical Memory and Paging

In the case of LPARs with dedicated memory, the memory allocated to an LPAR is physical memory. Memory can only be allocated in integer multiples of the Logical Memory Block (LMB) size. Figure 6.4 shows an LPAR with 1536 MB of dedicated memory. With an LMB size of 256 MB, there are 6 memory blocks. Each block of dedicated memory with a size of 256 MB is uniquely assigned to a block of physical memory with 256 MB in size.

Allocation of physical memory for LPARs with dedicated memory in units of the LMB size (here 256 MB).
Figure 6.4: Allocation of physical memory for LPARs with dedicated memory in units of the LMB size (here 256 MB).

In the case of shared memory LPARs, the assigned memory is generally composed of physical memory and memory on paging devices. The memory configured and assigned by desired_mem is then referred to as logical memory, since, similar to the virtual memory in an operating system, it is not only composed of physical memory. Again, the memory is allocated in multiples of the LMB size. The size of the shared memory pool must also be specified in multiples of the LMB size.

However as shown in figure 6.5, there are two important differences in the case of shared memory compared to the allocation in the case of dedicated memory. On the one hand, instead of physical memory, memory can also be assigned from a paging device; on the other hand, physical memory (and paging device memory) are not allocated in multiples of the LMB size, but in units of memory pages (with POWER 4 KB ). So physical memory is assigned to LPARs in smaller units, which very likely provides better utilization.

Allocation of logical memory of an LPAR with shared memory in units of the page size of 4 KB to physical memory and paging device.
Figure 6.5: Allocation of logical memory of an LPAR with shared memory in units of the page size of 4 KB to physical memory and paging device.

Physical memory can therefore be assigned (or deassigned) in units of 4 KB. Paging in units of 256 MB would take too long for an I/O and would lead to noticeable delays in the shared memory LPAR.