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Chapter 3. Virtualization 145

Figure 3-14 shows the behavior of a system with Active Memory Deduplication enabled on its

AMS shared memory pool. Duplicated pages from different LPARs are stored just once,

providing the AMS pool with more free memory.

Figure 3-14 Identical memory pages mapped to a single physical memory page with Active Memory

Duplication enabled

Active Memory Deduplication (AMD) depends on the Active Memory Sharing (AMS) feature

to be available, and consumes CPU cycles donated by the AMS pool's VIOS partitions to

identify deduplicated pages. The operating systems running on the AMS partitions can hint to

the PowerVM Hypervisor that some pages (such as frequently referenced read-only code

pages) are particularly good for deduplication.

To perform deduplication, the hypervisor cannot compare every memory page in the AMS

pool with every other page. Instead, it computes a small signature for each page that it visits

and stores the signatures in an internal table. Each time that a page is inspected, its signature

is looked up against the known signatures in the table. If a match is found, the memory pages

are compared to be sure that the pages are really duplicates. When a duplicate is found, the

hypervisor remaps the partition memory to the existing memory page and returns the

duplicate page to the AMS pool.

U U U U U U U

U U U U U U U U U U U

U U U

U U

U U U U U

D D

U U U

U U

With

Active Memory

Deduplication

Mappings

AMS shared memory pool

LPAR1

Logical Memory

LPAR2

Logical Memory

LPAR3

Logical Memory

Duplicate pages

Unique pages

KEY:

Free

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