Product : Citrix, XenServer/7.1, Standard
Feature : Dynamic / Over-Commit, Memory, Hypervisor
Content Owner:  Roman Macek
Summary
Yes (DMC)
Details
XenServer 5.6 introduced Dynamic Memory Control (DMC) that enables dynamic reallocation of memory between VMs. This capability is maintained in 7.x.
XenServer DMC (sometimes known as "dynamic memory optimization", "memory overcommit" or "memory ballooning") works by automatically adjusting the memory of running VMs, keeping the amount of memory allocated to each VM between specified minimum and maximum memory values, guaranteeing performance and permitting greater density of VMs per server. Without DMC, when a server is full, starting further VMs will fail with "out of memory" errors: to reduce the existing VM memory allocation and make room for more VMs you must edit each VMs memory allocation and then reboot the VM. With DMC enabled, even when the server is full, XenServer will attempt to reclaim memory by automatically reducing the current memory allocation of running VMs within their defined memory ranges.