Linux Format - UK (2019-12)

inus Torvalds announced the release

of Linux 5.4-rc5, which should have

been smaller than it was given that

the current development cycle is coming to a

close. Still, he wasn’t too concerned about the

slight bloat. If things continue to plan, the final

release of 5.4 should be featured in our

summary next issue.

Is hibernation usable?

Linux has supported hibernation (suspending

to disk) for many years. This differs from the

typical ‘suspend’ (closing the lid on your

laptop) in that the latter relies upon special

hardware support maintaining the contents

of RAM while effectively shutting down almost

everything else. This allows for a very fast

suspend, while limiting power draw

sufficiently to last a few days.

With hibernation, the machine is actually

fully powered-down and can maintain a

suspended state almost indefinitely. But a

hibernation to disk typically takes much

longer than a simple suspend. And it turns out

that there are some little-documented issues

with hibernation.

These issues include a restriction on the

system being able to use more than half of

available RAM during the process – the other

half is used to store the image being prepared

to be written to system swap space. As a

result, the current hibernate logic will free up

memory in the process, dropping cached state

and other unnecessary memory use. It mostly

works, but as Luigi Semenzato noted, it may not

always. These and other limitations caused him

to ask aloud whether hibernation is useful, and

whether it needs to be fixed.

Thermal pressure

Thara Gopinath posted a patch series titled

Introduce Thermal Pressure, which aims to

teach the kernel’s scheduler about the impact

of thermals upon maximum CPU frequency.

The kernel currently understands that

processors implement DVFS (Dynamic Voltage

and Frequency Scaling), in which the operating

frequency of cores on the same die (chip) are

adjusted continually in accordance with the load.

If a machine is not too busy, there are ‘governor’

algorithms that automatically throttle down the

speed to save power.

Until now, the kernel did not understand that

there is another interaction between frequency

and temperature. Under sufficient thermal

‘pressure’, the processor may automatically

throttle back the maximum operating frequency

in order to avoid damage to the hardware, or

because the cooling solution is not sufficient to

maintain normal operation – in a phone, for

example. The new patches aim to teach the

scheduler how to incorporate thermals too.

OPINION

Recently, there was

a post on the Linux

Kernel Mailing List

pondering the future of

packet-writing support.

This is a mechanism,

implemented by a driver

known as ‘pktcdvd’, used

when writing data to

rewriteable DVD and Blu-ray

media in such a way that the

disks can be treated as

mountable filesystems. The

poster was concerned that

the driver was going away

and was keen to point out

that optical drives are still

on sale and even used.

My first reaction upon

reading this was twofold.

First, the inevitable question

of when had I last used a

physical CD, DVD or Blu-ray

disk. That I couldn’t really

remember, and that it

wasn’t a big deal, alarmed

me a little, in the same way

that progress sometimes

does. Secondly – and even

though I had just

established I didn’t care

about DVDs much any more

• I assumed there was no
  way that support for
  rewriteable disks was
  now obsolete.
  It turns out that I was
  wrong. The driver was,
  in fact, marked as
  ‘deprecated’ and ‘orphaned’
  back in 2016. In theory this
  means it could just go away.
  And few people seem to
  even care or notice. If that
  isn’t an interesting sign of
  the changing times in which
  we live, then I’m not
  sure what is.

Jon Masters is a kernel hacker

who’s been involved with Linux for

more than 22 years, and works on

energy-efficient Arm servers.

WHAT DVD?

L

ONGOING DEVELOPMENT

Dmitry Safonov posted ‘kernel: Introduce

Time Namespace’, a patch series that adds

a new namespace for managing the

perceived time as seen by processes.

A typical use case will be to apply a time

namespace to a cgroup containing the

running tasks of a container, allowing a

system administrator to alter time seen by

applications. More importantly, it will allow

container orchestration tooling to adjust

the time seen by applications, particularly

during migration.

When moving containers from one host

to another, the two hosts may not agree or

be sychronised to the same time, which

could otherwise have a devastating impact

on applications not expecting discontinuity.

Jessica Yu (maintainer of in-kernel

module loading support) let everyone know

that a new feature merged in Linux 5.3 may

be the cause of some problems seen when

trying to load drivers. The new feature is

known as ‘symbol namespaces’, and is

intended to allow limits to be placed upon

modules in the kernel that may use

symbols (functions and data) provided by

other modules, or by core components of

the main kernel image. But it also has the

unintended consequence of breaking some

soft dependencies between modules.

Fancis Deslauriers announced that

videos from the Tracing Summit 2019

talks are now available at:

http://bit.ly/lxf257kernel.

Kernel Watch

Jon Masters summarises the latest happenings

in the Linux kernel, so that you don’t have to.

10 LXF257 December 2019 http://www.linuxformat.com

NEWSDESK

inusTorvaldsannouncedtherelease

ofLinux5.4-rc5,whichshouldhave

beensmallerthanitwasgiventhat

thecurrentdevelopmentcycleiscomingtoa

close.Still,hewasn’ttooconcernedaboutthe

slightbloat.Ifthingscontinuetoplan,thefinal

releaseof5.4shouldbefeaturedinour

summarynextissue.

Ishibernationusable?

Linuxhassupportedhibernation(suspending

todisk)formanyyears.Thisdiffersfromthe

typical‘suspend’(closingthelidonyour

laptop)inthatthelatterreliesuponspecial

hardwaresupportmaintainingthecontents

ofRAMwhileeffectivelyshuttingdownalmost

everythingelse.Thisallowsforaveryfast

suspend,whilelimitingpowerdraw

sufficientlytolastafewdays.

Withhibernation,themachineisactually

fullypowered-downandcanmaintaina

suspendedstatealmostindefinitely.Buta

hibernationtodisktypicallytakesmuch

longerthanasimplesuspend.Anditturnsout

thattherearesomelittle-documentedissues

withhibernation.

Theseissuesincludearestrictiononthe

systembeingabletousemorethanhalfof

availableRAMduringtheprocess–theother

halfisusedtostoretheimagebeingprepared

tobewrittentosystemswapspace.Asa

result,thecurrenthibernatelogicwillfreeup

memoryintheprocess,droppingcachedstate

andotherunnecessarymemoryuse.Itmostly

works,butasLuigiSemenzatonoted,itmaynot

always.Theseandotherlimitationscausedhim

toaskaloudwhetherhibernationisuseful,and

whetheritneedstobefixed.

Thermalpressure

TharaGopinathpostedapatchseriestitled

IntroduceThermalPressure,whichaimsto

teachthekernel’sschedulerabouttheimpact

ofthermalsuponmaximumCPUfrequency.

Thekernelcurrentlyunderstandsthat

processorsimplementDVFS(DynamicVoltage

andFrequencyScaling),inwhichtheoperating

frequencyofcoresonthesamedie(chip)are

adjustedcontinuallyinaccordancewiththeload.

Ifamachineisnottoobusy,thereare‘governor’

algorithmsthatautomaticallythrottledownthe

speedtosavepower.

Untilnow,thekerneldidnotunderstandthat

thereisanotherinteractionbetweenfrequency

andtemperature.Undersufficientthermal

‘pressure’,theprocessormayautomatically

throttlebackthemaximumoperatingfrequency

inordertoavoiddamagetothehardware,or

becausethecoolingsolutionisnotsufficientto

maintainnormaloperation–inaphone,for

example.Thenewpatchesaimtoteachthe

schedulerhowtoincorporatethermalstoo.

OPINION

Recently,therewas

apostontheLinux

KernelMailingList

ponderingthefutureof

packet-writingsupport.

Thisisamechanism,

implementedbyadriver

knownas‘pktcdvd’,used

whenwritingdatato

rewriteableDVDandBlu-ray

mediainsuchawaythatthe

diskscanbetreatedas

mountablefilesystems.The

posterwasconcernedthat

thedriverwasgoingaway

andwaskeentopointout

thatopticaldrivesarestill

onsaleandevenused.

Myfirstreactionupon

readingthiswastwofold.

First,theinevitablequestion

ofwhenhadIlastuseda

physicalCD,DVDorBlu-ray

disk.ThatIcouldn’treally

remember,andthatit

wasn’tabigdeal,alarmed

mealittle,inthesameway

thatprogresssometimes

does.Secondly–andeven

thoughIhadjust

establishedIdidn’tcare

aboutDVDsmuchanymore

• Iassumedtherewasno
  waythatsupportfor
  rewriteablediskswas
  nowobsolete.
  ItturnsoutthatIwas
  wrong.Thedriverwas,
  infact,markedas
  ‘deprecated’and‘orphaned’
  backin2016.Intheorythis
  meansitcouldjustgoaway.
  Andfewpeopleseemto
  evencareornotice.Ifthat
  isn’taninterestingsignof
  thechangingtimesinwhich
  welive,thenI’mnot
  surewhatis.

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

WHAT DVD?

L

ONGOINGDEVELOPMENT

DmitrySafonovposted‘kernel:Introduce

TimeNamespace’,apatchseriesthatadds

anewnamespaceformanagingthe

perceivedtimeasseenbyprocesses.

Atypicalusecasewillbetoapplyatime

namespacetoacgroupcontainingthe

runningtasksofacontainer,allowinga

systemadministratortoaltertimeseenby

applications.Moreimportantly,itwillallow

containerorchestrationtoolingtoadjust

thetimeseenbyapplications,particularly

duringmigration.

Whenmovingcontainersfromonehost

toanother,thetwohostsmaynotagreeor

besychronisedtothesametime,which

couldotherwisehaveadevastatingimpact

onapplicationsnotexpectingdiscontinuity.

JessicaYu(maintainerofin-kernel

moduleloadingsupport)leteveryoneknow

thatanewfeaturemergedinLinux5.3may

bethecauseofsomeproblemsseenwhen

tryingtoloaddrivers.Thenewfeatureis

knownas‘symbolnamespaces’,andis

intendedtoallowlimitstobeplacedupon

modulesinthekernelthatmayuse

symbols(functionsanddata)providedby

othermodules,orbycorecomponentsof

themainkernelimage.Butitalsohasthe

unintendedconsequenceofbreakingsome

softdependenciesbetweenmodules.

FancisDeslauriersannouncedthat

videosfromtheTracingSummit 2019

talksarenowavailableat:

http://bit.ly/lxf257kernel.

Kernel Watch

JonMasterssummarisesthelatesthappenings

intheLinuxkernel,sothatyoudon’thaveto.

10 LXF257December 2019 http://www.linuxformat.com

NEWSDESK