VMware vSphere vCenter Server 4.0 - Manual (Page 31)

An ESX/ESXi host determines allocations for each virtual machine based on the number of shares allocated to
it and an estimate of its recent working set size.
n Shares — ESX/ESXi hosts use a modified proportional-share memory allocation policy. Memory shares
entitle a virtual machine to a fraction of available physical memory.
n Working set size —ESX/ESXi hosts estimate the working set for a virtual machine by monitoring memory
activity over successive periods of virtual machine execution time. Estimates are smoothed over several
time periods using techniques that respond rapidly to increases in working set size and more slowly to
decreases in working set size.
This approach ensures that a virtual machine from which idle memory is reclaimed can ramp up quickly
to its full share-based allocation when it starts using its memory more actively.
Memory activity is monitored to estimate the working set sizes for a default period of 60 seconds. To
modify this default , adjust the Mem.SamplePeriod advanced setting. See “Set Advanced Host
Attributes,” on page 99.
Memory Tax for Idle Virtual Machines
If a virtual machine is not actively using all of its currently allocated memory, ESX/ESXi charges more for idle
memory than for memory that is in use. This is done to help prevent virtual machines from hoarding idle
memory.
The idle memory tax is applied in a progressive fashion. The effective tax rate increases as the ratio of idle
memory to active memory for the virtual machine rises. (In earlier versions of ESX that did not support
hierarchical resource pools, all idle memory for a virtual machine was taxed equally.)
You can modify the idle memory tax rate with the
Mem.IdleTax option. Use this option, together with the
Mem.SamplePeriod advanced attribute, to control how the system determines target memory allocations for
virtual machines. See “Set Advanced Host Attributes,” on page 99.
N
OTE
In most cases, changes to Mem.IdleTax are not necessary nor appropriate.
Memory Reclamation
ESX/ESXi hosts can reclaim memory from virtual machines.
An ESX/ESXi host allocates the amount of memory specified by a reservation directly to a virtual machine.
Anything beyond the reservation is allocated using the host’s physical resources or, when physical resources
are not available, handled using special techniques such as ballooning or swapping. Hosts can use two
techniques for dynamically expanding or contracting the amount of memory allocated to virtual machines.
n ESX/ESXi systems use a memory balloon driver ( vmmemctl ), loaded into the guest operating system
running in a virtual machine. See “Memory Balloon Driver,” on page 31.
n ESX/ESXi systems page from a virtual machine to a server swap file without any involvement by the guest
operating system. Each virtual machine has its own swap file.
Memory Balloon Driver
The memory balloon driver ( vmmemctl ) collaborates with the server to reclaim pages that are considered least
valuable by the guest operating system.
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 increases or decreases
memory pressure on the guest operating system, causing the guest to use its own native memory management
algorithms. When memory is tight, the guest operating system determines which pages to reclaim and, if
necessary, swaps them to its own virtual disk. See Figure 3-2 .
Chapter 3 Managing Memory Resources
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