Specifications

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.
D
U U U U U U U
U U U U U U U U U U U
U U U
D
U U
U U U U U
D D
U U U
U U
U U
D
U
U U
With
Active Memory
Deduplication
Mappings
AMS shared memory pool
LPAR1
Logical Memory
LPAR2
Logical Memory
LPAR3
Logical Memory
D
U
Duplicate pages
Unique pages
KEY:
Free