Red Hat LINUX VIRTUAL SERVER 4.7 - ADMINISTRATION Specifications download pdf (Page 17)

Installing and Configuring Linux Guest Operating Systems

ThemainlineLinuxkernelhasmovedtoaticklesstimekeepingmodelinwhich,insteadofcountingperiodic

interruptstotellthetime,theLinuxkernelqueriesahardwaretimesourcewhenitneedstoknowthetime.

ThesechangesmakeLinuxamuchbetter‐behavedoperatingsysteminphysicalaswell

asvirtual

environments.Ticklesskernelskeepbettertimeandreducepowerutilizationonlaptopsandothermobile

deviceswherebatterylifeisimportant.Invirtualmachines,ticklesskernelsareidealbecausetheyimposeless

overheadonthehypervisorandkeepveryaccuratetime.Toserveourcustomersbetter,VMwarehas

worked

directlywiththemainlineLinuxkernelaswellasCanonical,thepublisheroftheUbuntudistribution,to

implementoperatingsystemchangestosupportticklesstimekeeping.Becausethemigrationtoatickless

kernelwascompletedrecently,onlyrecentUbuntuLinuxreleasesincorporatethisfunctionality.

Accurateticklesstimekeepingcanalsobeachieved

throughparavirtualization,inwhichtheoperatingsystem

ismodifiedtobeawarethatitisrunningonahypervisor.TheVirtualMachineInterface(VMI),developedby

VMwareinconjunctionwiththeLinuxcommunityandnowpartofthemainlineLinuxkernel,isa

paravirtualizationsolutionthat,inadditiontoprovidinghigher

performance,ensuresaccuratetimekeeping

invirtualmachines.VMwareworkedwithNovellandCanonicaltosupportVMI‐enabledguestoperating

systems,suchas32‐bitSUSELinuxEnterpriseServer10SP2andUbuntu8.04LTS,forESX3.5Update2and

later.The32‐bitversionsofUbuntu7.10and7.04

supportVMIonESX3.5andlater.VMwarehasencouraged

RedHattoincorporatesupportforVMIintoitsreleases.OutsideofVMI,VMwarealsoworkedwithNovell

toimplementticklesstimekeepingthroughparavirtualizing64‐bitSUSELinuxEnterpriseServer10SP2.

Linux Swappiness

Inavirtualizedenvironment,thefrequencyofswappingapplicationsinandoutofmemorycanhaveadverse

effectsonaLinuxvirtualmachine’sresponsivenessafterlongidleperiods.Youcanadjustthisfrequency,or

“swappiness,”bysetting/proc/sys/vm/swappinesstoarangebetween0and100.TuneLinuxswappiness

downto

makeLinuxvirtualmachinesmoreresponsiveusingacommandinsidetheLinuxvirtualmachine

similartothefollowing:

echo 0 > /proc/sys/vm/swappiness

Ifyouwantthissettingtopersist,addthefollowinglineto/etc/sysctl.confintheLinuxvirtualmachine:

vm.swappiness=0

Whenyoulowertheswappinesssetting,idleprocessesinthevirtualmachinearenotpenalizedasLinux,by

default,triestoswapthemout.Swappingandpagefaultsinavirtualmachinearemoreexpensivethanina

nativeenvironment.Forexample,alarge,idleapplicationwithitsownlargecache

wouldnotbenefitfrom

beingswappedoutofmemory.Ifitisswappedout,ittakesalongtimeforitspagestoswapbackintomemory

whentheapplicationisaccessed.Theimpactofchangestotheswappinesssettingismorepronouncedin

LinuxvirtualmachinesthaninLinux

distributionsrunningnatively.

Paravirtualization and Virtual Machine Interface

ESX3.5introducessupportforguestoperatingsystemsthatuseVMware’sparavirtualizationstandard,

VirtualMachineInterface.

Twotechniquesforvirtualizingunmodifiedguestoperatingsystemkernelsarebinarytranslationand

hardwarevirtualization.Acomparisonofthese“fullvirtualization”techniquescanbefoundinthepaper“A 

ComparisonofSoftwareandHardwareTechniques

forx86Virtualization.”Adifferenttechniqueis

paravirtualization,whichrequiresmodificationstotheguestoperatingsystemandcanbeusedinconjunction

withthefirsttwotechniquesoronitsown.Foradditionaldetailsonthistechnique,seethepaper

“PerformanceofVMwareVMI.”Forlinkstothesepapers,see

“Resources”onpage 19.

Paravirtualization

Paravirtualizationisatechniqueinwhichamodifiedguestoperatingsystemkernelcommunicatestothe

hypervisoritsintenttoperformprivilegedCPUandmemoryoperations.Thistechniquereducesthework

requiredofthehypervisor,thusmakingitsimplerthanabinarytranslationhypervisor.Although

paravirtualizationdoesnoteliminatevirtualizationoverhead,it

canimproveguestoperatingsystem

performance.

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Red Hat LINUX VIRTUAL SERVER 4.7 - ADMINISTRATION Specifications Page 17

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