TPS de-duplicates pages of memory both within a virtual machine (Intra-VM) and across virtual machines (Inter-VM). By default, Inter-VM TPS is disabled due to security concerns about sharing memory pages between virtual machines that cross security boundaries (for example, virtual machine guests within the DMZ and virtual machine guests in the production environment). TPS can be configured to allow for Inter-VM sharing between all virtual machines, or only across certain groups of virtual machines, by adding a salt value to the virtual machines. The VMware KB article at http://kb.vmware.com/kb/2097593 provides more information about changes and enhancements to TPS.
The host advanced configuration option, Mem.ShareForceSalting, can be set to configure how TPS will be used. This setting is configured per ESXi host, as shown in the following screenshot:
Mem.ShareForceSalting can be set to a value of 0, 1, or 2. Add the configuration parameter sched.mem.pshare.salt to a virtual machine to set the salt value. Page sharing can be configured to only share pages between virtual machines with the same salt values.
The following table outlines how Intra-VM and Inter-VM page sharing is impacted based on the Mem.ShareForceSalting setting:
| Mem.ShareForceSalting settings | Inter-VM sharing | Intra-VM sharing |
| 0 | Yes, between all virtual machines on the host. Virtual machine salt value is ignored. | Yes |
| 1 | Sharing between virtual machines with the same sched.mem.pshare.salt setting. Sharing between virtual machines where sched.mem.pshare.salt is not present. | Yes |
| 2 (default) | Only among virtual machines with the same sched.mem.pshare.salt setting. The virtual machine vc.uuid is used as the salt value by default. |
Yes |
The following screenshot displays the memory utilization for a specific virtual machine, showing the amount of shared memory:
Notice the savings that the shared memory provides compared to the VM consumed memory, which is the amount of physical memory consumed on the host. The following screenshot shows the TPS savings across a vSphere cluster:
