6.7

To improve memory utilization, the ESXi host transfers memory from idle virtual machines to virtual
machines that need more memory. Use the Reservation or Shares parameter to preferentially allocate
memory to important virtual machines. This memory remains available to other virtual machines if it is not
in use. ESXi implements various mechanisms such as ballooning, memory sharing, memory compression
and swapping to provide reasonable performance even if the host is not heavily memory overcommitted.
An ESXi host can run out of memory if virtual machines consume all reservable memory in a memory
overcommitted environment. Although the powered on virtual machines are not affected, a new virtual
machine might fail to power on due to lack of memory.
Note All virtual machine memory overhead is also considered reserved.
In addition, memory compression is enabled by default on ESXi hosts to improve virtual machine
performance when memory is overcommitted as described in Memory Compression.
Memory Sharing
Memory sharing is a proprietary ESXi technique that can help achieve greater memory density on a host.
Memory sharing relies on the observation that several virtual machines might be running instances of the
same guest operating system. These virtual machines might have the same applications or components
loaded, or contain common data. In such cases, a host uses a proprietary Transparent Page Sharing
(TPS) technique to eliminate redundant copies of memory pages. With memory sharing, a workload
running on a virtual machine often consumes less memory than it might when running on physical
machines. As a result, higher levels of overcommitment can be supported efficiently. The amount of
memory saved by memory sharing depends on whether the workload consists of nearly identical
machines which might free up more memory. A more diverse workload might result in a lower percentage
of memory savings.
Note Due to security concerns, inter-virtual machine transparent page sharing is disabled by default and
page sharing is being restricted to intra-virtual machine memory sharing. Page sharing does not occur
across virtual machines and only occurs inside a virtual machine. See Sharing Memory Across Virtual
Machines for more information.
Memory Virtualization
Because of the extra level of memory mapping introduced by virtualization, ESXi can effectively manage
memory across all virtual machines.
Some of the physical memory of a virtual machine might be mapped to shared pages or to pages that are
unmapped, or swapped out.
A host performs virtual memory management without the knowledge of the guest operating system and
without interfering with the guest operating system’s own memory management subsystem.
vSphere Resource Management
VMware, Inc. 30