2019-05-01_Linux_Format

inusTorvaldsannouncedLinux5.0,

andwithittheopeningofthemerge

window(periodoftimeduringwhich

disruptivechangesareallowedaheadof5.1).

Inhisannouncement,hedetailedafewlast-

minutefixes,andthathewashappyoverall

withhavingextendedthe5.0cyclebyaweek,

followingtheatypical5.0-RC8.Thenewkernel

includesmanynewfeatures,amongthemthe

EnergyAwareSchedulercoveredinlastissue.

ItalsogainssupportintheAMDGPU

driversforFreeSync,atechnologypopular

withgamersthatsmoothsthedisplayby

aligningthetransitionoftheframedisplayed

withtheverticalblankingintervalofthe

display.Acompletesummaryofwhat’snew

in5.0isprovided(asusual)byKernelNewbies

athttps://kernelnewbies.org/Linux_5.0.

Physicallycontiguousmemory

Linuxgenerallydealswithtwodifferentviews

ofmemory:physicalandvirtual.Physical

memorydescribestheactualchipswithinthe

machine(thatis,theDRAM),andisusually

quitelimitedcomparedtosecondarystorage

suchasadiskorNVMe.Virtualmemory

representsatranslatedviewinwhichregions

ofmemorymayappearlargeandcontiguous

tosoftware,butinfactaremappedusing

OS-managed‘pagetables’intomanydifferent

physicallocationsbyhardware.Thetranslation

isfineformostpurposes,andinfactiscriticalto

ourabilitytowritemodernapplications.But

therearetimeswhencontiguousvirtualmemory

isnotenough.

Sometimes,hardwareneedstrulycontiguous

regionsofphysicalmemory.Thiscanbe

becausethehardwareislesscomplex–for

example,withembeddeddevicesthatarenot

capableof‘scattergather’memoryoperations

whenmovingdataaround.Butitcanalsobe

desirableforperformanceonmuch

biggermachines.Whena

sufficientlylargeregionof

contiguousphysicalmemoryexists,

aso-called‘huge’pagemapping

canbecreatedtorepresentthe

virtual-to-physicaltranslationofa

largeregionusingfewerentriesinpagetables,

andthehardwarestructuresthatcachethese.

Theproblemisthatwhenasystemis

running,physicalmemoryquicklybeginsto

fragmentasitisallocatedandfreed,so

eventuallyitcanbecomedifficulttofindlarge

regionsofcontiguousphysicalmemory.ZiYan’s

“Generatingphysicallycontiguousmemoryafter

pageallocation”pagesaimtohelpbyeffectively

defragmentingphysicalmemorytofreeuplarge

chunksofit.

opinion

I’velatelybeen

playingalotwithdifferent

computerarchitecturesthat

interestme.Anarchitecture

describesthekindof

fundamentalinstructions

thatareunderstoodbya

familyofcomputers;for

example‘x86’forInteland

AMD,‘arm’and‘riscv’.For

example,nearlyeveryone

readingthishasanx

laptopcomputer,and

maybeanArm-based

RaspberryPiorthreefor

variousprojects.

Butforotherkindsof

computer,it’smuchharder

togetoneathome.Insome

cases,thisisduetocost.

Fewpeoplehavethespace,

powerbudgetorbuilding

structuralsupport,letalone

themoney,forapersonal

mainframe.Inothercases,

scarcityisafactor.Older

computersarehardtocome

by,somearesonewthat

theyhaven’tbeenmadeyet.

Thelattercaseincludes

newdesignsthatarecoming

butarenotonthemarket

justyet.Inalmostallof

thesecases,thereare

readilyavailableemulators

thatletyoutestdrivejustas

easilyasrunningavirtual

machineonyourdesktop.

Agoodexampleisthe

RISC-VQEMUportand

associatedtools,whichyou

cangetholdofathttps://

github.com/riscv/riscv-

qemu/wiki.

JonMastersisakernelhacker
who’sbeeninvolvedwithLinuxfor
morethan 22 years,andworkson
energy-efficientARMservers.

just

emulate it

L

ongoingDeveLopment

DavidHowellspostedaRequestFor

Commentpatchseriestitled“Containers

andusingauthenticatedfilesystems”,

intendedtostartaconversationabout

containerisingthekernel’ssupportfor

cryptokeysthatitcanmanageonbehalfof

filesystemsandothersubsystems.The

patchesintroducedanew,explicitnotionof

acontainerinkernelspace,whichhasnot

previouslybeenthecase.

Theseideaswererejectedforavariety

ofreasons,buttheymayjuststartanew

dialogueaboutin-kernelcontainer

representation.SashaLevinproposeda

discussionattheupcomingLinuxStorage

FilesystemandMemoryManagement

Summitaboutthepreferredteststhat

shouldberunagainstpatchesdestinedfor

-stablekernels.

Stablekernelsarethosereleasedby

GregKroah-Hartmanandbasedupon

existingLinuskernelswithvariousfixesand

backports,intendedforthosewhowantto

runanupstreamkernelbutneedsome

levelofmaintainance.

Gregnotedthatvarioustestsagainst

stableprobablyexceededthosefor

mainlinereleases,whileotherspiled

onwithinputaroundthedifferenttests

thatcouldberun.

“physicalmemory quickly

begins to fragmentasit

isallocatedandfreed”

Kernel Watch

JonMasterssummarisesthelatesthappeningsinthe

Linuxkernelcommunity,foryourreadingpleasure.

10 LXF249May 2019 http://www.linuxformat.com

Newsdesk