Red Hat LINUX VIRTUAL SERVER 4.7 - ADMINISTRATION Spécifications télécharger pdf (Page 16)

Installing and Configuring Linux Guest Operating Systems

Asynchronous I/O

SupportforasynchronousI/OinthekernelandcorrespondingsystemcallsareincludedinLinuxkernel

version2.6.ButtheAPIfortheappropriatesystemcallsdiffersfromtheoriginalfunctions,soasimple

recompilationofexistingapplicationsmightbeinsufficienttousethenewchange.Thechangerequiressource

code

modificationthathasoccurredindifferentapplicationsatdifferentrates.

BeawareofthisissuewhenconsideringapplicationsthatuseasynchronousI/O.Inbenchmarking,for

example,IometerisatoolthatcanbeusedforstoragesystemperformancemeasurementsonbothWindows

andLinux.However,becauseitssourcecodewasnever

updatedtousethenew,correctedasynchronousI/O

systemcallsonLinux,thenumbersitproducesonLinux(usingrawdevices)areartificiallyandunfairlylow.

ConsiderthiswhendoingLinux‐to‐WindowscomparisonsanduseasynchronousI/O‐awaretoolssuchas

aiostress,whenneeded.

Linux Timer Rates for Virtual Machines on VMware ESX

Modernsystemsuseahardwaretimerforavarietyoffine‐grainedoperationsattheoperatingsystemlevel.

VMware’svirtualizationplatformsvirtualizethistimerintheESXkernel.Becausethevirtualtimerprovided

tothevirtualmachineisactuallysoftware,itissubjecttothesameresourcerestrictionsasotherprocesses.

The

busierthesystem,themorethetimerexecutionmustcontendwithotherhypervisoractivities.Therearetwo

implicationsofthis:

 Whenthesystemisverybusy,thesoftwaretimermightnotexecuteasregularlyasitdoeswhenthe

systemislessbusyandvirtualtimecanfallbehind.

 Dependingonhowfrequentlytheoperatingsystemisinterruptedbythetimer,thehypervisormustdo

differentamountsofwork.

TheamountofworkrequiredtomanagethevirtualtimerisgreatestwithRedHatEnterpriseLinux5SMP

systems,whichuseaclockfrequencyof1000Hzandsufferfroma

multiplicativeamountofworkbecauseof

SMPsupport.Forinstance,thefollowingtableshowsthenumberoftimerinterruptsona1000HzRedHat

EnterpriseLinux5virtualmachine:

Theamountofworkthatneedstobedonebythehypervisorincreasesdramaticallywiththeadditionof

virtualCPUs.Inaddition,

decreasingthetimerinterruptrategreatlydecreasestheworkthatneedstobedone

bytheVMkerneltovirtualizethetimer.RedHatEnterpriseLinux5.1andRedHatEnterpriseLinux4.7offer

aLinuxkernelthatenablesreducingthetimerrate.Byaddingtheparameterdivider=10totheboot

parametersasdescribedin“LinuxTimeSynchronizationRecommendations”onpage 12,youcanreducethe

amountofworkrequiredoftheVMkerneltovirtualizethetimerbyanorderofmagnitude.

Table 6. Timer Interrupts for Multiple Virtual CPUs in Red Hat Enterprise Linux 5

Virtual CPU Count Timer Interrupts per Second per Virtual Machine

11000

26000

4 20000

8 72000

Table 7. Timer Interrupts for Selected Linux Distributions

Linux Version Timer Interrupts per Second per Virtual CPU

RedHatEnterpriseLinux51000

RedHatEnterpriseLinux4100

SUSELinuxEnterpriseServer10SP1 250

SUSELinuxEnterpriseServer91000

SUSELinuxEnterpriseServer8100

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