Red Hat NETSCAPE MANAGEMENT SYSTEM 4.5 User Manual download pdf (Page 241)

Resource Management

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example, a virtual machine that has twice as many shares as another is entitled to

consume twice as much memory, subject to their respective minimum and maximum

constraints, provided that they are both actively using the memory they have been

allocated. In general, a virtual machine with S memory shares in a system with an

overall total of T shares is entitled to receive at least a fraction S/T of physical memory.

However, virtual machines that are not actively using their currently allocated

memory automatically have their effective number of shares reduced, in order to

prevent virtual machines from unproductively hoarding idle memory. This is achieved

by levying a tax on idle memory. A virtual machine is charged more for an idle page

than for one that it is actively using.

The MemIdleTax configuration option provides explicit control over the policy for

reclaiming idle memory. A tax rate of x percent means that up to x percent of a virtual

machine's idle memory may be reclaimed. A low tax rate mostly ignores working sets

and allocate memory based on shares. A high tax rate allows most idle memory to be

reallocated away from virtual machines that are unproductively hoarding it, regardless

of shares.

ESX Server estimates the working set for a virtual machine automatically by

monitoring memory activity over successive periods of virtual machine virtual time.

Estimates are smoothed over several 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 has been reclaimed

is be able to ramp up quickly to its full share-based allocation once it starts using its

memory more actively. The default monitoring period may be modified via the

MemSamplePeriod configuration option.

Memory Reclamation

ESX Server employs two distinct techniques for dynamically expanding or contracting

the amount of memory allocated to virtual machines — a VMware-supplied

vmmemctl module that is loaded into the guest operating system running in a

virtual machine and swapping pages from a virtual machine to a server swap file

without any involvement by the guest operating system.

The preferred mechanism is the vmmemctl driver, which cooperates with the server

to reclaim those pages that are considered least valuable by the guest operating

system. This proprietary technique provides predictable performance that closely

matches the behavior of a native system under similar memory constraints. It

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

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Red Hat NETSCAPE MANAGEMENT SYSTEM 4.5 User Manual Page 241