If a virtual machine is not actively using its currently allocated memory, ESX Server charges a
memory tax - more for idle memory than for memory that is in use. That is, the idle memory
counts more towards the share allocation than memory in use. The default tax rate is 75 percent,
that is, an idle page of memory costs as much as four active pages. This rate can be changed by
modifying a parameter setting.
Memory tax helps prevent virtual machines from hoarding idle memory. The approach in ESX
Server ensures that a virtual machine from which idle memory has been reclaimed can ramp up
quickly to its full share-based allocation once it starts using its memory more actively.
The balloon driver, also known as the vmmemctl driver, collaborates with the server to reclaim
pages that are considered least valuable by the guest operating system. It essentially acts like a
native program in the operating system that requires more and more memory. The driver uses a
proprietary ballooning technique that provides predictable performance that closely matches the
behavior of a native system under similar memory constraints. This technique effectively increases
or decreases memory pressure on the guest operating system, causing the guest to invoke its
own native memory management algorithms. When memory is tight, the guest operating system
decides which particular pages to reclaim and, if necessary, swaps them to its own virtual disk.
Hope that helps.
