Specifications

VMware, Inc. 365
Chapter 12 VMware ESX Server Resource Management
The!dynamic!memory!allocation!for!a!virtual!machine!is!bounded!by!its!minimum!and!
maximum!size!parameters.!The!maximum!size!is!the!amount!of!memory!configured!for!
use!by!the!guest!operating!system!running!in!the!virtual!machine.!By!default,!virtual!
machines!operate!at!their!maximum!allocation,!unless!memory!is!overcommitted.
The!minimum!size!is!a!guaranteed!lower!bound!on!the!amount!of!memory!that!is!
allocated!to!the!virtual!machine,!even!when!memory!is!overcommitted.!Set!the!
minimum!size!to!a!level!that!ensures!the!virtual!machine!has!sufficient!memory!to!run!
efficiently,!without!excessive!paging.
You!can!set!the!maximum!
size!to!a!higher!level!to!allow!the!virtual!machine!to!take!
advantage!of!excess!memory,!when!available.
Overhead!memory!includes!space!reserved!for!the!virtual!machine!frame!buffer!and!
virtualization!data!structures.!A!virtual!machine!configured!with!less!than!512MB!of!
memory!requires!54MB!of!overhead!memory!for!a!single!
virtual!CPU!virtual!machine,!
and!64!MB!for!a!dual‐virtual!CPU!SMP!virtual!machine.!Larger!virtual!machines!
require!an!additional!32MB!of!overhead!memory!per!additional!gigabyte!of!configured!
main!memory.!For!example,!a!single!virtual!CPU!virtual!machine!with!a!configured!
maximum!memory!size!of!2GB!requires!102MB!of
!overhead!memory.
Memory Sharing
Many!workloads!present!opportunities!for!sharing!memory!across!virtual!machines.!
For!example,!several!virtual!machines!may!be!running!instances!of!the!same!guest!
operating!system,!have!the!same!applications!or!components!loaded!or!contain!
common!data.!ESX!Server!uses!a!proprietary!transparent!page!sharing!technique!to!
securely!eliminate!redundant!copies!of
!memory!pages.!
With!memory!sharing,!a!workload!consisting!of!multiple!virtual!machines!often!
consumes!less!memory!than!it!would!when!running!on!physical!machines.!As!a!result,!
the!system!can!support!higher!levels!of!overcommitment!efficiently.
The!amount!of!memory!saved!by!memory!sharing!is!highly!dependent!on!workload!
characteristics.!A
!workload!consisting!of!many!nearly‐identical!virtual!machines!may!
free!up!more!than!30!percent!of!memory,!while!a!more!diverse!workload!may!result!in!
savings!of!less!than!5!percent!of!memory.!
To!determine!the!effectiveness!of!memory!sharing!for!a!workload,!run!the!workload,!
and!observe!the!savings!
by!looking!at!the!output!of!the!/proc/vmware/mem!file.
ESX!Server!memory!sharing!runs!as!a!background!activity!that!scans!for!sharing!
opportunities!over!time.!The!amount!of!memory!saved!may!vary!over!time;!for!a!fairly!
constant!workload,!the!amount!generally!increases!slowly!until!all!sharing!
opportunities!are!exploited.