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, 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 in virtual machines often consumes less memory than it would 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, while a more diverse workload might result in a signifigantly lower percentage of memory savings.
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. This means page sharing does not occur across virtual machines and only occurs inside of a virtual machine. See Sharing Memory Across Virtual Machines for more information.