Virtualizing Resources for the Cloud - Large Scale and Big Data: Processing and Management - page 518

16.7.3 m emory o ver -C ommitment
In system memory virtualization, the combined total size of real memories can grow
beyond the actual size of physical memory. This concept is typically referred to
as memory overcommitment [62]. Memory overcommitment ensures that physical
memory is highly utilized by active real memories (assuming multiple VMs run-
ning simultaneously). Indeed, without memory overcommitment, the hypervisor can
only run VMs with a total size of real memories that is less than that of the physical
memory. For instance, Figure 16.19 shows a hypervisor with 4-GB physical memory
and 3 VMs, each with 2-GB real memory. Without memory overcommitment, the
hypervisor can only run 1 VM, for the fact of not having enough physical memory
to assign to 2 VMs at once. Although each VM would require only 2 GB of memory
and the hypervisor has 4 GB of physical memory, this cannot be afforded because
the hypervisor generally requires overhead memories (e.g., to maintain various vir-
tualization data structures).
To this end, in practical situations, some VMs might be lightly loaded while oth-
ers might be heavily loaded. Lightly loaded VMs can cause some pages to sit idle,
while heavily loaded VMs can result in memory page thrashing. To deal with such
situations, the hypervisor can take (or steal) the inactive physical memory pages
away from idle VMs and provide them to heavily loaded VMs. As a side note, hyper-
visors usually write zeros to the stolen/reclaimed inactive physical memory pages to
avert information leaking among VMs.

16.7.4 r eClamation t eChniQues anD vmw are m emory b allooning
To maintain full isolation, guest OSs are kept unaware that they are running inside
VMs. VMs are also kept unaware of the states of other VMs running on the same
physical host. Furthermore, with multiple levels of page mapping, VMs remain
oblivious of any physical memory shortage. Therefore, when the hypervisor runs

VM0 (2 GB of real memory) VM1 (2 GB of real memory) VM2 (2 GB of real memory)

Hypervisor (4 GB of physical memory)

FIGURE 16.19 A hypervisor with 4 GB of physical memory enabling 3 VMs at once with
a total of 6 GB of real memory.